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A Guidebook on Soil Fertility and Plant Nutrient Management
Research · January 2020
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A Guidebook on
Soil Fertility and Plant Nutrient Management
Getachew Agegnehu (PhD)
Ethiopian Institute of Agricultural Research
Addis Ababa, Ethiopia
January 2020
Contents
1. General Introduction.............................................................................................................................1
2. Essential Characteristics of a Soil for Suitable Crop Production...........................................................3

i
2.1. Edaphic factor ...............................................................................................................................3
2.2. Soil moisture .................................................................................................................................4
3. Inorganic Fertilizer Management..........................................................................................................6
3.1. Essential plant nutrients ...............................................................................................................7
3.2. Role of plant nutrients and their deficiency symptoms................................................................9
3.3. Choice of fertilizer products........................................................................................................13
3.4. Movement of nutrient element ions in soil and their uptake by plants.....................................14
3.5. Determination of the fertilizer rates and levels..........................................................................15
3.6. Soil nutrient evaluation...............................................................................................................16
3.7. Plant analysis...............................................................................................................................19
4. Integrated Soil Fertility Management.................................................................................................20
4.1. Introduction ................................................................................................................................20
4.2. Nutrient flows and balances .......................................................................................................21
4.3. Organic resources use and management ...................................................................................22
4.4. Combined application of organic and mineral inputs.................................................................24
4.5. Nitrogen fixing legumes and crop yield ......................................................................................28
5. Acid Soil Management........................................................................................................................32
5.1. Extent and distribution of soil acidity .........................................................................................32
5.2. Causes of soil acidity...................................................................................................................33
5.3. Types of acidity ...........................................................................................................................35
5.4. Effect of soil acidity on nutrient availability and crop yield........................................................36
5.4.1. Soil acidity and nutrient availability....................................................................................36
5.4.2. Soil acidity and crop performance......................................................................................37
5.5. Management of acid soils...........................................................................................................39
5.5.1. Liming and lime requirement..............................................................................................39
5.5.2. Effect of lime on soil acidity and crop yields.......................................................................41
5.5.3. Integrated soil fertility management..................................................................................43
5.5.4. Choice of acid tolerant crops and development of varieties..............................................44
6. Soil Salinity and Sodicity......................................................................................................................45
6.1. Extent and distribution ...............................................................................................................45
6.2. Sources and causes of salinity and sodicity ................................................................................46
6.3. Effects of salinity/sodcity on plant growth.................................................................................47

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6.4. Reclamation and management of salt affected soils..................................................................48
7. Composting and Vermicomposting.....................................................................................................49
7.1. Compost reparation....................................................................................................................49
7.2. Characteristics of finished compost............................................................................................52
7.3. Application of compost on soils..................................................................................................52
7.4. What to compost ........................................................................................................................53
7.5. What not to compost..................................................................................................................55
7.6. Composting systems ...................................................................................................................56
7.6.1. Composting bins and systems.............................................................................................56
7.7. Vermiculture and vermicomposting ...........................................................................................58
7.7.1. Comparison of composting.................................................................................................58
7.7.2. Worm bin composting ........................................................................................................59
8. Utilization of Bio-fertilizers for the Production of Pulses in Ethiopia.................................................63
8.1. Introduction ................................................................................................................................63
8.2. Bio-fertilizer development and application................................................................................65
8.2.1. Highlights of rhizbial biofertilizer development process ....................................................65
8.2.2. Handling and storage of biofertilizer ..................................................................................65
8.3. Compatibility of rhizobial inoculants with agricultural inputs....................................................66
8.3.1. Evidence of inoculants effectiveness under field condition ...............................................67
8.4. Risk mitigation actions in storage, handling and application .....................................................69
9. Application of Biochar in Agriculture..................................................................................................71
9.1. Introduction ................................................................................................................................71
9.2. Carbon and nutrient content of biochars ...................................................................................72
9.3. Biochar on soil physical, chemical and biological properties......................................................73
9.4. Effects of biochar on plant growth and yield..............................................................................76
10. Climate-Smart Agriculture and development.................................................................................77
10.1. Introduction ............................................................................................................................77
10.2. Adaptation ..............................................................................................................................77
10.3. Mitigation................................................................................................................................81
Selected References....................................................................................................................................84
Appendix .....................................................................................................................................................86

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1. General Introduction
With the world population increasing rapidly, and projected to do so for some time, and with
improved plant nutrition remaining as one of the major factors increasing crop yields, use of our
knowledge of plant nutrition to maximize agricultural yields grows in importance. However,
public interest in minimizing the use of chemical inputs in agriculture is also increasing with
emphasis on less use of chemical fertilizers and more use of alternative fertilizers. Attention to
precision agriculture, in which plant nutrition is controlled or monitored carefully, has grown in
research and practice (Alemaw and Agegnehu 2019; Goulding et al. 2008). All these situations
require knowledge of plant nutrition.
A plant nutrient is a chemical element that is essential for plant growth and reproduction. Essential
element is a term often used to identify a plant nutrient. The term nutrient implies essentiality, so
it is redundant to call these elements essential nutrients. Normally, for an element to be a nutrient,
it must fit certain criteria; the first main criterion is that the element must be required for a plant to
complete its life cycle (Marschner 2011; Barker and Pilbeam 2015). The second criterion is that
no other element substitutes fully for the element being considered as a nutrient. The third criterion
is that all plants require the element. However, all the elements that have been identified as plant
nutrients do not fully meet these criteria, so, some debate occurs regarding the standards for
classifying an element as a plant nutrient (Marschner 2011; Barker and Pilbeam 2015).
The first criterion, that the element is essential for a plant to complete its life cycle, has historically
been the one with which essentiality is established. This criterion includes the property that the
element has a direct effect on plant growth and reproduction. In the absence of the essential
element or with severe deficiency, the plant will die before it completes the cycle from seed to
seed. This requirement acknowledges that the element has a function in plant metabolism; meaning
that with short supply of the nutrient, abnormal growth or deficiency symptoms will develop
because of the disrupted metabolism; and that the plant may be able to complete its life cycle with
restricted growth and abnormal appearance. This criterion also notes that the occurrence of an
element in a plant is not evidence of essentiality. Plants will accumulate elements that are in
solution without regard to the elements having any essential role in plant metabolism or physiology
(Marschner 2011; Barker and Pilbeam 2015).
The second criterion states that the role of the element must be unique in plant metabolism or
physiology, which means that no other element will substitute fully for this function. A partial
substitution might be possible. For example, a substitution of manganese for magnesium in
enzymatic reactions may occur, but no other element will substitute for magnesium in its role as a
constituent of chlorophyll. Some scientists believe that this criterion is included in the context of
the first criterion (Barker and Pilbeam 2015).
The third criterion requires that the essentiality be universal among plants. Elements can affect
plant growth without being considered as essential elements. Enhancement of growth is not a

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defining characteristic of a plant nutrient, since although growth might be stimulated by an
element, the element is not absolutely required for the plant to complete its life cycle. Some plants
may respond to certain elements by exhibiting enhanced growth or higher yields, such as that
which occurs with the supply of sodium to some crops. Also, some elements may appear to be
required by some plants because the elements have functions in metabolic processes in the plants,
such as in the case of cobalt being required for nitrogen-fixing plants. Nitrogen fixation, however,
is not vital for these plants since they will grow well on mineral or inorganic supplies of nitrogen.
Also, plants that do not fix nitrogen do not have any known need for cobalt. Elements that might
enhance growth or that have a function in some plants but not in all plants are referred to as
beneficial elements (Marschner 2011; Barker and Pilbeam 2015).
Seventeen elements are considered to have met the criteria for designation as plant nutrients.
Carbon, hydrogen, and oxygen are derived from air or water. The other 14 are obtained from soil
or nutrient solutions. It is difficult to assign a precise date or a specific researcher to the discovery
of the essentiality of an element. For all the nutrients, their roles in agriculture were the subjects
of careful investigations long before the elements were accepted as nutrients. Many individuals
contributed to the discovery of the essentiality of elements in plant nutrition. Much of the early
research focused on the beneficial effects or sometimes on the toxic effects of the elements.
Generally, an element was accepted as a plant nutrient after the body of evidence suggested that
the element was essential for plant growth and reproduction, leading to the assignment of certain
times and individuals to the discovery of its essentiality (Barker and Pilbeam 2015).
Table 1. List of essential elements, their date of acceptance as essential, and discoverers of essentiality
Element Date of Essentiality Discoverer of essentiality
Carbon 1804 de Saussure
Hydrogen 1804 de Saussure
Oxygen 1804 de Saussure
Nitrogen 1804 de Saussure
Phosphorus 1839 Liebig
Potassium 1866 Birner and Lucanus
Calcium 1862 Stohmann
Magnesium 1875 Boehm
Sulfur 1866 Birner and Lucanus
Iron 1843 Gris
Manganese 1922 McHargue
Copper 1925 McHargue
Boron 1926 Sommer and Lipman
Zinc 1926 Sommer and Lipman
Molybdenum 1939 Arnon and Stout
Chlorine 1954 Broyer, Carlton, Johnson, and Stout
Nickel 1987 Brown, Welch, and Cary

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2. Essential Characteristics of a Soil for Suitable Crop Production
2.1. Edaphic factor
If the soil is to meet the conditions necessary for suitable crop production, it must have the
following characteristics.
▪ It must be suitable for working.
▪ It should resist destructive soil erosion and depletion of nutrients under the cropping system
practiced.
▪ It should hold sufficient moisture for the crop under normal precipitation.
▪ It must be adequately aerated to a depth that allows proper root development.
▪ It should have available plant nutrients in the quantity needed for profitable crop production.
▪ It must be free from harmful concentrations of constituents and other conditions that favor the
development of organisms damaging to crops.
Studies have shown that certain crops are adapted to certain kinds of soil. The physical and
chemical properties of the soil will largely determine the vigor of the growth of plants, which also
affect the capacity of the soil to hold moisture. As a rule, when taken on a volumetric basis, an
average soil in good tillage consists of 50% of soil minerals including humus, and air and water in
equal proportions of 25% each. Such soil is composed of half solids and half pore spaces in
conditions of sufficient soil moisture. The pores are filled half with air and half with water. As the
soil moisture is depleted the proportion of air to water increases. It should be noted that sufficient
air is required for normal root development and functioning. In certain soils, such as heavy clays,
which may have a larger total pore space may be so small and filled with water. This causes
insufficient air (oxygen) for the roots to function normally, and anaerobic conditions may develop.
It should be noted that low oxygen concentrations at the root zone would affect the ability of plants
to absorb an adequate supply of nutrients from the soil.
Textural classes: The 12 textural classes (sand, loamy sand, sandy loam, sandy clay loam, sandy
clay, loam, silt loam, silt, clay loam, silty clay loam, silty clay, clay) of soils can be determined by
knowing the relative percentages of the three major soil separates (sand, silt, and clay) as shown
in Figure 1.

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Figure 1. Soil textural classes based on percentage content of sand, silt, and clay.
Nutrient deficiencies: The supply of soil nutrients is important since it affects the productivity of
a soil. Nutrient deficiencies in soils are often shown by plant symptoms. Soil and plant analysis is
the other method to detect the deficiency of nutrients in the soil.
Soil acidity: In humid tropical soils, the soils are usually acidic, the soil sorption complexes are
mainly occupied Al and Fe, high rainfall and leaching conditions are dominant. Soil acidity must
first be neutralized to an acceptable degree to provide a medium where fertilizer can provide a
basis for crop production. Aluminum and Fe activity must be reduced to control fixation of
phosphate and molybdate and boron as well as Al and Fe toxicity to plants.
2.2. Soil moisture
Soil moisture can be classed as gravitational water, capillary water and hygroscopic water.
Gravitational water: occupies the larger pore spaces and drains away under the influence of
gravity. Its upper limit is when the pores are filled completely with water, and the soil is saturated.
The saturation capacity is then equal to the porosity of the soil. Gravitational water drains

5
downward from the root zone unless prevented by an impervious layer of soil, rock or a high water
table. The rate at which this takes place varies with the soil type, taking less than a day in coarse
sands to more than three days in heavy clay soils.
Capillary water: is water, which is held by surface tension forces in the pore spaces between the
soil particles. Its upper limit is when all the gravitational water has drained away; and the soil is
said to be at Field Capacity (F.C). This is the main source of water to the plant.
Hygroscopic water: This is held as a very thin film round the particles of soil being held so firmly
and, in most cases, it is not available to plants. Soils vary in their capacity to hold soil moisture
according to their texture and physical structure with the fine soils like clays being able to store
much more water than coarser textured soils, such as sands. Table 2 shows the various ranges of
soil moisture expressed in mm of water per meter of soil for various soil types.
Table 2. Range of soil moisture for various soil types
Soil Type
Water potential in bars
- 0.2 (F.C) - 0.5 - 2.5 -15 (W.P)
Available soil moisture in mm/m (Sa)
Heavy clay 180 150 80 0
Silty clay 190 170 100 0
Loam 200 150 70 0
Silt loam 250 190 50 0
Silty clay loam 160 120 70 0
Fine textured soils 200 150 70 0
Sandy clay loam 140 110 60 0
Sandy loam 130 80 30 0
Loamy fine sand 140 110 50 0
Medium textured soils 140 100 50 0
Medium fine sand 60 30 20 0
Coarse textured soils 60 30 20 0
F.C. = Field capacity; W.P. = Wilting Point
The removal of soil moisture by plants: The amount of water that is available for plant growth
is that which is between the field capacity (when all the gravitational water has drained away) and
the amount of soil moisture held mainly as hygroscopic water which the plant is unable to utilize
quickly enough to maintain normal growth (Table 3). This lower limit is known as the permanent
wilting point, because under such moisture conditions, plant leaves become permanently wilted.
This usually occurs at a suction pressure of 15 bars (atmospheres) for most plants. In extreme cases
it is as high as 40 bars. Generally, water is necessary when the soil moisture is about halfway
between field capacity and permanent wilting point. However, in the case of heavy clay soils,
such as Vertisols, which swell and crack and have a low infiltration rate after they have swelled

6
the movement of water through them is largely facilitated by their shrinkage, forming large deep
cracks for the water to infiltrate. This only occurs when most of the soil moisture has been
depleted. To summarize, plants extract water from soils within their rooting zone when it is held
between a suction pressure of 0.3 bar at field capacity and 15 bars at permanent wilting point.
Table 3. Judging removal of available moisture
Remaining
available soil
moisture
Feel or Appearance of Soil
Coarse texture Moderately coarse
texture
Medium Texture Fine and very fine
Texture
Permanent
wilting to 25%
Dry, loose, single
grained, flows through
fingers
Dry, loose, flows through
finger
Powdery dry, sometimes
slightly crusted but easily
broken down into powdery
condition.
Hard, baked cracked,
sometimes has loose
crumbs on surface
25 to 50% Appears to be dry, will
not form a ball with
pressure
Appears to be dry, will
not form a ball
Somewhat crumble but
holds together from
pressure.
Somewhat pliable, will
ball under pressure.
50 to 75% Appears to be dry, will
not form a ball with
pressure
Tends to ball under
pressure but seldom holds
together
Forms a ball somewhat
plastic, will sometimes
stick slightly with pressure
Forms a ball, ribbons
out between thumb and
forefinger.
75% to field
capacity
(100%)
Tends to stick together
slightly, sometimes
forms a very weak ball
under pressure
Forms weak ball, breaks
easily, will not stick
Forms a ball, is very
pliable, sticks readily if
relatively high in clay
Forms a ball, easily
ribbons cut between
fingers, has stick
feeling.
At field
capacity
(100%)
Upon squeezing, no
free water appears on
soil but wet outline of
ball is left on land.
Upon squeezing, no free
water appears on soil, but
wet outline of ball is left
on hand.
Upon squeezing, no free
water appears on soil, but
wet outline of ball is left on
hand.
Upon squeezing, no
free water appears on
soil, but wet outline of
ball is left on hand
Note: Ball is formed by squeezing a handful of soil very firmly
3. Inorganic Fertilizer Management
With increasing population, the question of food for the future is an important consideration. In
order to produce crops profitably and at the same time furnish them to consumer at a reasonable
price, there are several factors to consider. Among these the following are mentioned:
Maintenance of the productivity of the soil: A productive soil is of first importance in the
economic production of crops. The farmer who farms highly productive land is less affected by
extreme fluctuations in crop values and by variations of seasons than who farms less productive

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land. Even under very adverse climatic conditions, a fair crop may be secured from very productive
soil.
Improved varieties: The use of good seeds of high-yielding, adapted varieties is one of the best
means of producing large crops. It is evident that the quality and preparation of the soil cannot
overcome the serious consequences arising from the use of poor seeds. The differences in yield
between varieties are often sufficiently great to determine whether the crop will be produced at a
profit or at a loss.
Moisture: water supply is the most important factor in determining the productivity of crops.
Improved cultural practices: Good cultural methods are necessary for high production. Proper
culture aids the liberation of plant nutrients, the aeration of the soil, the conservation of soil
moisture, the control of weeds, and the prevention of erosion. Good methods should be used from
the time the preparation of the land is begun until the crop no longer needs cultivation.
Control of crop enemies: The damage done to crops by insect pests, diseases, weeds, and other
crop enemies is enormous.
3.1. Essential plant nutrients
Plants need food for their growth and development. Of the large numbers of elements, identified
in plant tissue, sixteen have been found indispensable for their growth, development and
reproduction. To be categorized as essential, an element should meet the following three criteria:
i. A deficiency of the element makes it impossible for the plant to complete the vegetative
or reproductive stage of its life.
ii. The deficiency symptoms of the element in question can be prevented or corrected
only by supplying the element.
iii. The element is directly involved in the nutrition of the plant quite apart from its possible
effect in correcting some micro-biological or chemical condition in the soil or culture
medium.
The essential elements include C, H2 and O2 from air and soil water, and N2, P, K, Ca, Mg, S, Fe,
Zn, Mn, Cu, B, Mo and Cl supplied from the reserves in the soil or through application of
manures and fertilizers. Nutrient element requirements for several agronomic crops can be
classified based on crop removal and/or growth response (table 4.)

8
Table 4. Nutrient requirements of crops based on their removal and/or growth response.
Crop Scientific name Major Element Requirement Levels
N P K Ca Mg S
Barley Hordeum vulgare M L L L L L
Cassava Manihot esculenta L L H L L M
Corn, grain Zea mays H M M M M M
Oat Avena sativa L L L L L L
Peanut Arachis hypogaea M L L H L M
Rice Oryza sativa L L L L VL L
Rye Secale cereale L VL VL (M) (M) L
Sorghum Sorghum bicolor H M M L M M
Soybean Glycine max VH M M M L M
Sugar beet Beta vulgaris H L VH (M) H H
Wheat Triticum aestivum L L L L L L
Crop Scientific name Micronutrient Requirement Levels
B Cu Fe Mn Zn
Barley Hordeum vulgare L M (M) M H
Cassava Manihot esculenta M L M M H
Corn, grain Zea mays L (M) H (M) (M)
Oat Avena sativa L M M H M
Peanut Arachis hypogaea L H H M M
Rice Oryza sativa L L M H M
Rye Secale cereale L L M L L
Sorghum Sorghum bicolor (L) (M) H M H
Soybean Glycine max L M M M H
Sugar beet Beta vulgaris M L M H M
Wheat Triticum aestivum L M L M L
VL = very low; L = low; M = medium; H = high; VH = very high. Requirements shown in parentheses
designate levels assumed and not clearly known. If crop removal and/or growth response to an element
are unknown, a medium (M) requirement is assumed. The only exception is boron (B), for which a low
(L) requirement is always assumed.

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Table 5. Indicator plant species for nutrient deficiencies
Deficient
element
Indicator plants
N Cereals, mustard, apple, citrus
P Maize, barley, lettuce, tomato
K Potato, clover, lucerne, bean, tobacco, cucurbits, cotton, tomato, maize
Ca Lucerne, other legumes
Mg Potato, cauliflower, sugar-beet
S Lucerne, clover, rape-seed
Fe Sorghum, barley, citrus, peach, cauliflower
Zn Maize, onion, citrus, peach
Cu Apple, citrus, barley, maize, lettuce, oats, onion, tobacco, tomato
Mn Apple, apricot, bean, cherry, citrus, cereals, pea, raddish
B Lucerne, turnip, cauliflower, apple, peach
Mo Cauliflower, other brassica spp., citrus, legumes, oats, spinach
Cl Lettuce
3.2. Role of plant nutrients and their deficiency symptoms
Functions Deficiency symptoms
Nitrogen
• An important constituent of chlorophyll,
protoplasm, protein and nucleic acids.
• Increase growth and development of all
living tissues.
• Improves the quality of leafy vegetables
and fodders and the protein content of
food grains.
• Stunted growth
• Appearance of light green to pale-yellow color on
old leaves, starting from the tips. This is followed
by death and/or dropping of the older leaves
depending on the degree of deficiency.
• flowering is greatly reduced .in acute deficiency,
• Lower protein content.
Phosphorous
• Constituent of phosphatides, nucleic acids,
proteins, phospholipids, and co-enzymes
NAD, NADP and ATP.
• Constituent of certain amino acids.
• Necessary for cell division, constituent of
chromosome; stimulates root devt.
• Necessary for meristematic growth; seed
and fruit development; stimulates
flowering.
• Stunted appearance, mature leaves have
characteristic dark to blue-green coloration,
restricted root development.
• In acute deficiency, occasional purpling of leaves
and stems; spindly growth
• Delayed maturity and lack of or poor seed and
fruit development.
Potassium

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• An activator of enzyme involved in
photosynthesis and protein and
carbohydrate metabolism.
• Assists carbohydrate translocation,
synthesis of protein and maintenance of its
stability; membrane permeability and pH
control; water utilization by stomatal
regulation.
• Improves utilization of light during cool and
cloudy weather and thereby enhances plant
ability to resist cold and other adverse
conditions.
• Enhances the plant’s ability to resist
diseases.
• Increases size of grains or seeds and
improves the quality of fruits and
vegetables.
• Chlorosis along the leaf margins followed by
scorching and browning of tips of older leaves;
these symptoms then gradually progress inwards.
• Slow and stunted growth of plants.
• Weak stalks and plants lodge easily.
• Shriveled seeds or fruits.
Calcium
• Constituent of cell walls in the form of
calcium pectate; necessary for normal
mitosis (cell division).
• Helps in membrane stability, maintenance
of chromosome structure.
• Activator of enzymes (phospholipase,
argine kinase, adenosine triphosphates).
• Acts as a detoxifying agent by neutralizing
organic acids in plants.
• Ca deficiencies are not often seen in the field
because secondary effects associated with high
acidity limit growth.
• Young leaves of new plants are affected first.
These are often distorted, small and abnormally
dark green.
• Leaves may be cup-shaped and crinkled and the
terminal buds deteriorate with some breakdown of
petioles.
• Root growth is markedly impaired; rotting of roots
occurs.
• Desiccation of growing points (terminal buds) of
plants under severe deficiency.
• Buds and blossoms shed prematurely.
• Stem structure weakened.
Magnesium
• Constituent of chlorophyll molecule thereby
essential for photosynthesis.
• Activator of many enzyme systems
involved in carbohydrate metabolism,
synthesis of nucleic acids, etc.
• Promotes uptake and translocation of P.
• Helps in movement of sugars within plant.
• Interveinal chlorosis, mainly of older leaves,
producing streaked or patchy effect; with acute
deficiency the affected tissue may dry up and die.
• Leaves small, brittle in final stages and curve
upwards at margin.

11
• In some vegetable plants, chlorotic spots between
veins, and marbling with tints of orange, red and
purple.
• Twigs are weak and prone to fungus attack,
usually premature leaf drop.
Sulfur
• Constituent of sulfur-bearing amino-acids
• Involved in the metabolic activities of
vitamins, biotin, thiamine, and coenzyme A.
• Aids stabilization of protein structure.
• Younger leaves turn uniformly yellowish green or
chlorotic.
• Shoot growth is restricted, flower production often
indeterminate.
• Stems are stiff, woody and small in diameter.
Zinc
• Involved in the biosynthesis of indole acetic
acid.
• Essential component of variety of metallo-
enzymes-carbonic anhydrase, alcohol
dehydrogenase, etc.
• Plays a role in nucleic acid and protein
synthesis.
• Assists the utilization of P and N in plants.
• Deficiency symptoms mostly appear on the 2nd or
3rd fully mature leaves from the top of plants.
• In maize, from light yellow striping to abroad
band of white or yellow tissue with reddish purple
veins between the midrib and edges of the leaf,
occurring mainly in the lower half of the leaf.
• In wheat, a longitudinal band of white or yellow
leaf tissue leaf tissue, followed by interveinal
chlorotic mottling and white to brown necrotic
lesions in the middle of the leaf blades, eventual
collapse of the affected leaves near the middle.
Copper
• Constituent of cytochrome oxidase and
component of many enzymes – ascorbic
acid oxidase, phenolase, lactase, etc.
• Promotes formation of vitamin A in plants.
• In cereals, yellowing and curling of the leaf blade,
restricted ear production and poor grain set,
indeterminate tillering.
• In citrus, dieback of new growth; exanthema
pockets of gum develop between the bark and the
wood; the fruit shows brown excrescences.
Iron
• Necessary for the synthesis and
maintenance of chlorophyll in plants.
• Essential component of many enzymes.
• Plays an essential role in nucleic acid
metabolism – affects RNA metabolism or
chloroplasts.
• Typical interveinal chlorosis: youngest leaves first
affected, points and margins of leaves keeps their
green color longest.
• In severe cases, the entire leaf, veins and
interveinal areas turn yellow and may eventually
become bleached.
Manganese

12
• Catalyst in several enzymatic and
physiological reactions in plants; a
constituent of a pyruvate carboxylase
• Involved in the plant’s respiratory process.
• Activates enzymes concerned with the
metabolism of N and synthesis of
chlorophyll.
• Controls the redox potential in plant cells
during the phases of light and darkness.
• Chlorosis between the veins of young and leaves,
characterized by the appearance of chlorotic and
necrotic spots in the interveinal areas.
• Grayish areas appear near the base of the youngest
leaves and become yellowish to yellow orange.
• Symptoms of deficiency popularly known in oats
as “grey speck”, in field peas as “marsh spot”, in
sugarcane as “streak disease”.
Boron
• Affects the activities of certain enzymes.
• Ability to complex with various poly-
hydroxy-compounds.
• Increases permeability in membrane and
thereby facilitates carbohydrate transport.
• Involved in lignin synthesis and other
reactions.
• Essential for cell division.
• Associated with the uptake of Ca and its
utilization by plants.
• Regulates potassium/calcium ratio in plants.
• Essential for protein synthesis.
• Death of growing plants (shoot tips).
• The leaves have a thick texture, sometimes curling
and becoming brittle.
• Flowers do not form, and root growth is stunted.
• “Brown heart” in root crops characterized by dark
spots on the thickest part of the root or splitting at
center.
• Fruits such as apples develop “internal and
external cork” symptoms.
Molybdenum
• Associated with N utilization and N
fixation.
• Constituent of nitrate reductase and
nitrogenase.
• Required by Rhizobia for N fixation.
• Chlorotic interveinal mottling of the lower leaves
followed by marginal necrosis and in-folding of
the leaves.
• In cauliflower, the leaf tissues wither leaving only
the midrib and a few small pieces of leaf blade
(“whiptail”).
• Molybdenum deficiency is markedly evident in
leguminous plants.
Chlorine
• Constituent of auxin-chloro-indole-3-acetic
acid which in immature seeds takes the
place of indole acetic acid.
• Constituent of many compounds found in
fungi and bacteria.
• Stimulates activity of some enzymes and
influences carbohydrate metabolism &
water holding capacity of plant tissue.
• Wilting of leaflet tips, chlorosis of leaves and
finally bronzing and drying.

13
Table 6. Agronomic crop species sensitive to deficient or excessive levels of micronutrients
Micronutrient Sensitive to deficiency Sensitive to Excess
Boron Legumes, cotton, and sugar beet Cereals
Chlorine Cereals and sugar beet
Copper Cereals (oats) and alfalfa Cereals and legumes
Iron Sorghum, soybean, and clover Rice and tobacco
Manganese Cereals (oats), legumes,
soybean, sugar beet
Cereals and legumes
Molybdenum Legumes Cereals
Zinc Cereals (corn), legumes, grasses Cereals and soybean
3.3. Choice of fertilizer products
The choice of the right fertilizer types and grades is very important for the achievement of
maximum yield and good quality agricultural output. Hence, much consideration will be given for
the following fertilizer products.
1 Nitrogen fertilizers
2 Phosphate fertilizers
3 Secondary nutrient fertilizers
4 Micronutrient fertilizers
The following are examples of fertilizer products:
▪ Various grades /analyses/ of NP /NPK /NPK complex fertilizers.
▪ Calcium Nitrate (CN)
▪ Calcium Ammonium Nitrate (CAN)
▪ Other fertilizer products such as Ammonium Sulfate (AS) and Ammonium Nitrate (AN).

14
Table 7. Properties of major element fertilizer formulations
Fertilizer
N
(%)
P2O5
(%)
K2O
(%)
Soil
reactiona
lb
Lime/100
lb Nb
Salt
Indexc
Ammonium nitrate 33.5 0 0 A 180 105
Monoammonium phosphate 11 48 0 A 180 30
Diammonium phosphate 18 46 0 A 180 3
Ammonium sulfate (23.7% S) 21 0 0 A 538 69
Ammonium polyphosphate 10 34 0 A 180 -
Urea 46 0 0 A 180 75
Liquid nitrogen 30 0 0 A 180 -
Calcium nitrate 15 0 0 N 0 65
Potassium nitrate 13 0 44 N 0 74
Muriate of potash 0 0 60 N 0 116
Potassium sulfate (18% S) 0 0 50 N 0 46
Sodium nitrate 16 0 0 B 0 100
Magnesium sulfate (10% Mg; 13% S) 0 0 0 N 0 44
Sulfate of potash-magnesia (11% Mg; 22% S) 0 0 22 N 0 43
Nitrate of soda-potash 15 0 14 N 0 19
Normal superphosphate (12% S) 0 20 0 N 0 10
Triple superphosphate 0 46 0 N 0 10
Gypsum (19% S; 22% Ca) 0 0 0 N 0 8
a
A = acid; B = basic; N = neutral.
b
Lb lime required to neutralize acid from 100 lb of nitrogen.
c
Salt index for equal weights of materials, NaNO3 = 100.
3.4. Movement of nutrient element ions in soil and their uptake by plants
Mass flow: Ion movement with and in water because of rainfall, applied irrigation water, or water
movement because of evapotranspiration (e.g., nitrate-NO3
-
, chloride-Cl-
, calcium-Ca2+
, sulfate-
SO4
2-
).
Diffusion: Diffusion Ion movement in soil solution driven by concentration gradients (e.g.,
phosphate-PO4
3-
, potassium-K+
).
Root interception: Absorption of ions from the soil solution by plant root contact because of root
movement (growth) through soil; extent of uptake depends on amount of root occupation and root
characteristics.

15
Table 8. Relative significance of movements of ions from soil to corn roots
Element Amount (lbs)
required for
150 Bu/Acre corn
% Supplied by
Root
interception
Mass flow Diffusion
Nitrogen 170 1 99 0
Phosphorus 35 3 6 94
Potassium 175 2 20 78
Calcium 35 71 29 0
Magnesium 40 38 50 12
Sulfur 20 5 95 0
Copper 0.1 10 40 50
Zinc 0.3 33 33 33
Boron 0.2 10 35 55
Iron 1.9 11 53 37
Manganese 0.3 33 33 33
Molybdenum 0.01 10 20 70
Source: Soil Nutrient Bioavailability: A Mechanistic Approach, 1984
3.5. Determination of the fertilizer rates and levels
In countries where some research information is available and where large zones of uniform
climate and soil types are found, simple fertilizer trials (e.g., basic eight-plot design) can be carried
out in farmers' fields, which will give detailed results on fertilizer response under farm conditions
in the shortest possible time. After three cropping seasons some preliminary fertilizer
recommendations can be made (FAO, 1985).
The amount of fertilizer to apply per hectare depends on the amount of nutrient needed and the
fertilizer grades available. Crop requirements, nutrient supply from soils as determined by soil
analysis, residues from past cropping, manure application and local soil and climatic conditions
are all important in estimating the fertilizer rate. The analysis of fertilizer is obtained by
determining chemically the percentage of nutrient present:
Percentage of nutrient =
Nutrient content × 100
Total weight of fertilizer
They are commonly expressed as percent N-P2O5-K2O (sometimes with the addition of Mg-S-
trace elements).
There is a general formula to get the required dose of fertilizer. The basic formula is:

16
Quantity of fertilizers required =
Quantity of nutrients
Percentage of nutrients × 100
For example, to get 60 kg of N from a fertilizer containing 15% N, you would need 60/15x100 =
400 kg of product.
How much increase in crop yield results due to the application of fertilizer?
It is estimated that approximately 50% of the increase in yield is due to fertilizer use.
Which other new farming techniques lead to increased yields?
Modern farming includes the use of high-yielding varieties and the correct application of water
and agro-pesticides. Also important are seedbed preparation methods, time of sowing and weed
control. Bigger and better crops will result from the proper use of modern farm inputs.
3.6. Soil nutrient evaluation
The type and amount of fertilizer to be applied depends on the crop to be grown and the nutrient-
supplying power of the soils. Determination of the level of soil nutrients allows deficiencies to be
detected and suitable rates of fertilizer to be recommended there are a number of methods for
determining the nutrient status of soils.
Visual diagnosis: Plants exhibit characteristic symptoms when a nutrient is present in insufficient
quantity for normal growth and development. The method is rapid and no elaborate apparatus is
required.
Plant analysis: Plant analysis may be semi-quantitative, as in rapid tissue tests, or fully
quantitative.
Biological tests: Biological tests are carried out using microorganisms, e.g. Azotobacter.
Soil testing: The main objective of soil testing is to evaluate the fertility status of the soil. It
provides a basis for the recommendation of fertilizer and soil amendments such as lime and
gypsum. Soil testing is a better method than deficiency symptoms and plant and tissue analysis,
because it helps in determining the nutrient need of the plant before the crop is planted. It is simpler
and less time consuming.
Soil sampling procedures: Soil samples should be properly collected and should be representative
of the area to be tested. Soil analysis and its interpretation are as reliable as the soil sample drawn.
Important points to note are:

17
a. Each field should be sampled separately. When areas within a field differ distinctly in crop
growth, appearance of the soil, elevation or known cropping or manuring history, the field
should be divided suitably, and each area sampled separately.
b. Drawing samples from areas which do not represent the field should be avoided. Such areas
may be marshy spots, hedges, areas previously occupied by compost heaps, etc. Sampling
should not be done in a field within 3 months after application of fertilizer or lime.
c. Samples should be taken with a soil corer or an auger, or in very friable soils a large spoon
or trowel can be used.
d. A composite sample may be taken from each area. After scrapping the surface free of litter,
a uniform core or a thin slice of soil from the surface to plough depth (15 to 25 cm) should
be taken at 15 to 20 sampling points well distributed over the area to be sampled. In a hard
soil, a small pit of about 15 cm 15 cm and of about 15 cm in depth should be made, and
a V-shaped slice taken from one of the sides.
e. Where crops have been planted in rows, sampling may be done between the rows.
f. Individual cores or slices should be collected in clean containers. All lumps should be
broken and mixed well in the container or on a clean cloth. The size of the composite
sample should be reduced by successive quartering to about 0.5 kg.
Sample preparation: The preparation of a bulk soil sample is necessary to make it suitable for
analysis. Soil sample coming to the laboratory must first of all be arranged and registered into a
“soil laboratory ledger” and the given identification numbers, which will be used as a reference
during the analysis. Sample preparation entails the crushing of the clods by hand, drying, and
reduction of the aggregate’s size to < 2mm, and separation of the coarse fraction from the fine ones
by sieving.
Registration: Register samples in the laboratory ledger book, immediately after arrival. Delays in
registration easily cause some mistakes.
Information sheets
i. Each sample should be identified by name or number, and by the farmer’s name and
address.
ii. The information sheet furnished by the soil testing laboratory should be filled up
completely as it will help the analyst to provide an accurate fertilizer recommendation.
The information sheet and the soil sample in its container should be sent to the soil
testing laboratory, following prescribed procedures.
The information usually consists of:
• Name of the sampling area or experiment
• Profile core and horizon
• Depth (cm)
• Date of sampling

18
a. Add to this information:
• Date received by the laboratory.
• Person who brought the samples
• Lab number must be assigned to each sample.
b. Date of expected comp lesion of analysis
c. Drying
Allow the samples to air-dry on shelves.
d. Grinding, sieving and storage
Aggregates > 2mm should be ground and sieved through a 2mm sieve.
If the standard information sheets are not available, information may be given on the following
points:
i. field identification, farmer’s name and address;
ii. crops grown in the last two to three years;
iii. date of last plowing of the field;
iv. quantity of fertilizer, gypsum and lime used and when;
v. whether green manuring practiced, and when;
vi. topography, degree of erosion, drainage, crop growth, etc.;
vii. Crops proposed for the next year.
If the sample is very wet, it may be dried in shade for an hour or two before bagging and
dispatching it to the nearest soil testing laboratory. Plastic or cloth bags are suitable and should be
available from the soil testing laboratory. Soil samples are analyzed for the following soil
properties:
• soil reaction (pH);
• total soluble salts (indicate whether the soil is alkaline or normal);
• organic carbon (measure of organic matter status and available N);
• available P;
• available K;
Where the need and facilities exist, soils are also analyzed for secondary and micronutrients.
Soil nutrient classification and interpretation of soil tests
The status of nutrients can be classified based on the response of crops to applied nutrients (Tables
1 and 2). The results of the analyses are reported to the extension worker together with
recommendations on fertilizers and amendments. The reports include:
i. A statement of analytical results, including both the numerical result and a rating
interpretation of this result. The nutrient categories may be rated for example as “ low,
medium, high” or “very low, low, medium, high, very high”.

19
ii. Fertilizer recommendations for the proposed crop are given based on:
✓ soil analysis,
✓ past and future cropping pattern, and
✓ manures and fertilizers recently applied.
iii. The recommendations state the quantities of N, P, K, and micronutrients (where
appropriate) and also of soil amendments (gypsum and lime) to be applied.
Table 9. Classification of soil nutrients
Classification Interpretation
Very low to low Very high probability of achieving a response to applied nutrient;
unlikely probability of achieving a response to applied ameliorant
for toxicity.
Moderately low High probability of achieving a response to applied nutrient;
possible or low probability of achieving a response to applied
ameliorant for toxicity.
Marginal Possible or low probability of achieving a response to applied
nutrient; high probability of achieving ameliorant for toxicity.
Adequate to high Unlikely probability of achieving a response to applied nutrient;
very high probability of achieving ameliorant for toxicity.
Source: Soil analysis- An interpretation manual, 1999, CSIRO Publishing, Collingwood, Australia
Table 10. Soil analysis interpretation by test rating
Rating Symbol Interpretation
Low L Profitable response in almost all cases
Medium M Profitable response in most cases
High H Profitable response rare
Very high VH Not profitable to apply fertilizer
Excessive E Application may lower crop yield or quality
Source: Soil Fertility Handbook, 1998, Toronto, Canada
3.7. Plant analysis
Plant analysis, also called leaf analysis, is a technique for determining the elemental contents of
tissues of particular plant parts. It plays a major role in diagnosing mineral nutrition problems in
the field and involves a series of steps.
• Sampling
• Sample preparation
• Laboratory analysis
• Interpretation

20
Principles and practices: The concentration of a nutrient within a plant is an integral value of all
factors affecting it. Analyses of plants normally deal with all essential nutrients, toxic or beneficial
ones, except C, H2 and O2.
Plant sampling: Sampling is the first step in the plant analysis. There are large variations in
nutrient concentration in different parts of some plants. Careful selection of the appropriate part is
important in sampling. Confining sampling to a particular organ also increases the accuracy of
estimation. The general rule for most plants is to sample the upper, recently matured leaves, just
prior to the beginning of the reproductive stage when nutrient disorders suspected, sampling may
be done at the time when the symptoms are observed. Old and new leaves on the same plant have
different nutrient compositions. In sample collection, dead plants or stressed ones should be
avoided.
4. Integrated Soil Fertility Management
4.1. Introduction
The adoption of climate-smart agriculture would enhance productivity and incomes of farmers
while contributing to overcome the negative effects of climate change. Food insecurity is
becoming a recurrent challenge affecting livelihoods and socio-economic developments in
Ethiopia. Increasing climate variability, accompanied with soil fertility decline, decreasing land
holdings and low crop and livestock productivity have amplified national concerns about the
ability of the Ethiopian agricultural sector to feed the ever-growing population.
Land degradation and associated soil fertility decline is considered as the major bio-physical root
cause for the decline in per-capita food production in sub-Saharan Africa in general and in Ethiopia
in particular (. The traditional soil fertility management practices, including long term fallows and
crop rotations have been diminishing over time due to population pressure and other external
drivers. The amount of N and P applied in Ethiopia has been one of the lowest (below 20 kg ha-1
)
in SSA and hence the traditional agriculture has been mining the inherent soil fertility over
centuries. Inherent soil fertility is commonly the major source of N for crops in the region until the
labile soil organic fraction (N-capital) is depleted. The consequence is decline in crop yield, with
average yield of major cereals in Ethiopia is 2.0 t ha-1
, while the global average is beyond 3.5 t
ha-1
. The low crop productivity, even in relatively high rainfall areas, has also prompted the farmers
to expand their farming into marginal, non-cultivable lands, including steep landscapes and semi-
arid rangelands. On the other hand, crop yield in research fields within Ethiopia could reach up to
3 times more than in farmers’ fields, among others, due to improved agronomic practices and
application of organic and inorganic fertilizers.

21
4.2. Nutrient flows and balances
Many tropical soils are poor in inorganic nutrients and rely on the recycling of nutrients from soil
organic matter (SOM) to maintain fertility. More than half of all African area are affected by land
degradation, making this one of the continent’s urgent development issues. For example, an
estimated US $42 billion in income and 6 million ha of productive land are lost every year due to
land degradation and declining agricultural productivity. Moreover, Africa is saddled with a US
$9.3 billion annual cost of desertification. Assessments have shown that nutrient losses are only
partially compensated by natural and synthetic inputs, thus the nutrient balance for the total of
Sub-Saharan Africa appears to be negative, by 26 kg N, 3 kg P, and 19 kg K ha-1
yr-1
, while in
Ethiopia the loses are larger, amounting to minus122, 13 and 82 kg ha-1
yr-1
. In addition to the
limited use of fertilizers among smallholder farmers, the nutrient loss due to erosion, leaching and
crop residue removal depletes nutrients from the agricultural system at over 60-100 kg ha-1
yr-1
of
N, P and K in Eastern Africa and is commonly reflected by low crop and livestock productivity
(Mulatu et al. 2007; Tsige et al. 2012). Similarly, the Ethiopian farming systems are operating
under imbalanced nutrient status.
Farmers commonly set priorities in applying fertilizers in terms of crop types, market
opportunities, farm locations, distance from homestead and other socio-economic conditions. The
differential application of organic and chemical fertilizers within a farm over years, aggravated by
erosion, commonly creates a clear soil fertility gradient from the homestead to the outfield. For
instance, in southern Ethiopian farming systems, where perennial crops are grown around the
homesteads, soil nutrient status commonly decreases from the homestead to the outfields,
regardless of resource endowment categories. A detailed nutrient flow analysis in southern
Ethiopia revealed that nutrient distribution also varies among landscapes, households, farms, and
farm subunits. In these systems, high concentration of nutrients in the homestead is created
because nutrients move from the house to the home garden in the form of household refusal,
chemical fertilizer, animal manure, and others. It also moves from the far away fields to the
homestead fields in the form of grain, crop residue for feed, mulch, fuel wood and other uses
(Mulatu et al. 2007; Tsgie et al. 2012). In general, the home garden fields had a positive nutrient
balance while the outfields had a strong negative nutrient balance (Table 11).
Table 11. Nutrient balances at farm level in relatively rich or poor households in Areka, south Ethiopia
Farm units
Rich farmers Poor farmers
N P N P
Enset (Ensete ventricosum) garden 12 11 -12 6
Midfield -3 8 -5 4
Outfield -95 7 -54 3
Country wide, nutrient loses under cereals and other annual crops were predominantly due to
erosion (Table 12). Of the total nutrients, removal from cereal cropping, about 70% of N, 80% of

22
P and 63% of K were removed by erosion. Countrywide analysis of nutrient balance indicated a
depletion rate of 122, 13 and 82 kg K ha-1
yr-1
.
Table 12. Determinants of nutrient depletion under different cropping systems in Ethiopian smallholders’
mixed farming (% share in depletion)
Description
Harvested products Residue removal Leaching DNa
Erosion
N P K N P K N K N N P K
Cereals 10.0 19.4 6.0 4.0 5.0 11.0 9.0 17.0 3.0 74.0 80.0 66.0
Pulses 14.0 16.8 13.0 4.0 3.0 8.0 8.0 17.0 2.0 72.0 84.0 62.0
Oilseeds 1.0 1.0 2.0 1.0 4.0 5.0 8.0 20.0 2.0 88.0 96.0 73.0
Vegetables 21.0 30.4 25.0 19.0 31.0 34.0 22.0 22.0 12.0 24.0 44.0 19.0
Permanent 16.0 24.9 14.0 40.0 67.0 70.0 27.0 13.0 14.0 3.0 10.0 2.0
DN = Denitrification
4.3. Organic resources use and management
Organic resources are the major nutrient sources for smallholders’ Agriculture. However, the
nutrient contents of organic materials, ranging from crop residues through manure to agro-
industrial wastes widely vary. A 2% O.M. content is considered normal for Montana soils. Soils
that contain greater amounts of O.M. will mineralize more N and soils testing lower in O.M. will
mineralize less N. General guidelines are to reduce fertilizer N recommendations by 20 lb/acre for
soils with > 3% O.M., and for soils with < 1% O.M., to increase fertilizer N recommendations by
20 lb/acre. Soils that are low in O.M. typically have poor soil structure which reduces water
holding capacity and increases the risk of soil loss from erosion.
Table 13 compares the nutrient contents of a variety of organic materials with the nutrients required
to produce a modest 2 t ha -1
crop of maize grain. Although all the nutrients in organic sources will
not be available for crops, the information could be used for designing a soil fertility management
strategy that would consider organic resources as part of the nutrient budget in a given cropping
system and yield goal. These estimates could be adjusted, knowing that crop recovery of N
supplied by high-quality organic resources (e.g., green manures) is rarely more than 20%, while
that recovered from lower quality cereal stovers is even generally much lower. Some organic
materials such as poultry manures contain sufficient nutrients, with about 2 t of manure good
enough to fertilize a 2-t maize, while other organic resources such as crop residues may require up
to 10 t ha-1
to match the requirements of a 2-t maize crop. Cattle manure also varies in its quality
and fertilizer value tremendously. Nutrient contents of commercial dairy farms have been
significantly higher than smallholders’ farms.

23
Table 13. Average nutrient contents on a dry matter basis of selected plant materials and manures
Material
N P K
kg t-1
Crop residues
Maize Stover 6  1 7
Bean trash 7  1 14
Banana leaves 19 2 22
Sweet potato leaves 23 3.6 -
Sugarcane trash 8  1 10
Coffee husks 16 4 -
Refuse compost 20 7 20
Animal manures
Cattle
High quality 23 11 6
Low quality 7 1 8
Chicken 48 18 18
Farmyard chicken 24 7 14
Leguminous tree species (leaves)
Calliandra calothyrsus 34 2 11
Gliricidia sepium 33 15 21
Leucaena leucocephala 34 15 21
Sesbania sesban 34 15 11
Senna spectabilis (non-N2 -fixing) 33 2 16
Nonleguminous tree and shrubs (leaves)
Chromolaena ordorata 38 2.4 15
Grevillea robusta 14  1 6
Lantana camara 27 2.4 21
Tithonia diversifolia 36 2.7 43
Leguminous cover crops
Crotalaria ochroleuca 42 16 9
Dolichos lablab 41 2.2 13
Mucuna pruriens 35 2.0 7
Nutrients required by 2 t maize grain + 3 t Stover 80 18 60
Most farmers focused on cattle feeding at the expense of soil fertility. There are few studies in
Ethiopia that assessed the effect of crop residue management on soil properties, crop growth and
yield under field conditions. There is a strong competition for biomass in Ethiopia, with about 63,
20, 10 and 7% of cereal straws used for feed, fuel, construction, and bedding purposes, respectively.
The application of 3 t ha-1
of tef straw increased grain yield of sorghum by 70% in conventional
tillage and by 46% in zero tillage (Table 14), probably through reducing unproductive water losses.
In their experiment, mean soil water content throughout the season was 16% more with 3 t ha-1
application of straw compared to plots without straw application. They concluded that ground

24
cover with crop residues is necessary to achieve acceptable yield along with minimum tillage
particularly in low-moisture-stressed areas of the country.
Table 14. Effect of tef crop residue application on sorghum grain, stover and biomass yields, harvest index
and seasonal water use efficiency (WUE) at Melkassa, Ethiopia
Mulch rate
(t ha-1
)
Grain yield
(kg ha-1
)
Biomass yield
(kg ha-1
)
Seasonal water
use (mm)
WUE for grain yield
(kg ha-1
mm-1
)
0 2916 9614 595 4.85
3 3591 14322 618 5.73
6 4138 14710 614 6.55
LSD0.05 924 1241 16.9 1.32
4.4. Combined application of organic and mineral inputs
World average grain yields have almost doubled since the early 1960s. It is estimated that some 70-
80% of future increases in crop production in developing countries will have to come from
intensification, i.e. higher yields. The use of fertilizers is indispensable to alleviate the existing crop
nutrient deficiencies as it was also recognized by the African heads of states. In June 2006 in Abuja,
Nigeria, the African Union (AU) Special Summit of the Heads of State adopted the 12-Resolution
“Abuja Declaration on Fertilizer for African Green Revolution.” At the end of the Summit, the AU
Member States resolved to increase fertilizer use from 8.0 kg ha-1
at the time to 50 kg ha-1
by 2015,
which was coincidentally the International Year of Soils. African leaders declared fertilizer, from
both inorganic and organic sources, “a strategic commodity without borders” and resolved that “the
AU Member States will accelerate the timely access of farmers to fertilizers”. Reports as of March
2015, have shown that average fertilizer use in Africa was still only 11 kg ha-1
in 2014, equivalent
to one tenth of the world average.
At the same time, the recent status of the world’s soil resources report established that 40% of
African soils were subject to moderate to severe degradation. Despite the recognition for the need
to increase fertilizer use in Ethiopia, fertilizer consumption is still below 20 kg NPK ha-1
. Several
studies have examined the responses of various crops to applied fertilizer in Africa. Results from
the FAO fertilizer program, for instance, have indicated an average increase in yield of 64% after
application of NPK fertilizer across SSA. Other experiences with the African Millennium Villages
project also showed an average threefold increase in maize yield with fertilizer application.
Moreover, because of the inconsistent use of chemical fertilizers and the very limited returns of
crop residues to the soil, most of the internal N cycling in smallholder systems results from
mineralization of soil organic N. The addition of farmyard manure (FYM) at the rate of 4 and 8 t
ha-1
and 50% of the recommended NP fertilizer on dila (moderately fertile soil) and dimile (poorly
fertile soil) resulted in wheat yields similar to that of the recommended rate of 60/20 kg N/P
fertilizer ha-1
(Table 15).

25
Table 15. Effects of nitrogen (N) and phosphorus (P) fertilizers and farmyard manure (FYM) on wheat yield
on Nitisols, central Ethiopian highland
Medium soil (dila) Poor soil (dimile)
Treatment Grain yield
(t ha-1
)
Total biomass
(t ha-1
)
Grain yield
(t ha-1
)
Total biomass
(t ha-1
)
N/P kg ha-1
/FYM (t ha-1
)
9/10/0 2.63c 7.10c 1.63c 5.06c
9/10/8 3.05b 8.56b 2.15b 6.23b
32/10/4 3.27ab 9.18ab 2.29b 6.37b
32/10/8 3.44a 9.77ab 2.59a 7.45a
64/20/0 3.46a 10.06a 2.78a 8.18a
LSD0.05 0.34 1.38 0.23 0.96
Soil management practices for sustainable use can be best practiced through the adoption of an
ISFM practice. “ISFM is an integrated approach that seeks to enhance agricultural productivity and
improve ecosystem services for sustainable future use through combined application of soil fertility
management practices, and the knowledge to adapt these to local conditions to maximize fertilizer
and water use efficiency” (Vanlauwe et al. 2010). “ISFM definition is extended as a set of soil
fertility management practices that necessarily include use of chemical fertilizer, organic inputs and
improved crop varieties, combined with the knowledge on how to adapt these practices to local
conditions, aimed at maximizing agronomic efficiency (AE) of the applied nutrients and improving
crop productivity”. All inputs need to be managed in line with sound agronomic principles. It
incorporates both organic and inorganic nutrient sources to attain higher yield, prevent soil
degradation, improve soil water infiltration thereby help meet future food supply needs. ISFM also
promotes the dissemination of knowledge among farmers, extension personnel and researchers
(Vanlauwe et al. 2010).
Building sustainable soil fertility management is a long-term process that would require a system
approach integrating various components. These include the combination of judicious use of
inorganic fertilizers, improved organic residue management through composting and application of
FYM, deliberate crop rotations, cereal-legume intercropping and integration of green manures
(Abraham et al. 2011; Agegnehu and Amede 2017; Agegnehu et al. 2018). It also demands building
a strong local capacity and market incentives for farmers to experiment, innovate and adopt suitable
ISFM practices. As the current use of inorganic fertilizer in Ethiopia is one of the lowest, and it is
also neither crop nor soil specific, the limited availability of fertilizers may affect considerably the
application of ISFM approaches in the country (Agegnehu and Amede 2017; Girm et al. 2020).
The other key influence of ISFM is on developing strategies to enhance fertilizer use efficiency,
which should focus on factors affecting nutrient availability and use, including choice of crop
varieties, soil moisture status and appropriate agronomic practices. In most Ethiopian farming
systems, the nutrient use efficiency (NUE, kg yield per kg nutrient applied) is remarkably low

26
compared to other African countries, which is probably caused by interactions between soil erosion,
improper crop management and limited use of inputs (Jones 2008; Zeleke et al. 2010; Agegnehu et
al. 2013). For instance, the NUE of maize in Ethiopia, Kenya and Tanzania is 9-17, 7-36 and 18-
43 kg grain kg-1
of N applied, respectively. This may partly be because nutrients applied to the soil
are exposed to complex chemical and biological interaction but also competition between erosion,
soil microorganisms and plant roots. In contrast, mixing fertilizer with manure or compost resulted
in the highest AE of N (36 kg maize grain kg-1
N), while organic inputs of medium quality also
showed significantly higher AE of N compared with fertilizer alone but only at low organic input
application rates (40 and 23 kg maize grain kg N−1
, respectively) (Zeleke et al. 2013).
Agricultural soils could be divided into two groups based on their response to management: (1)
soils that are high responsive to application of external input of fertilizers (line A, Figure 2) and (2)
soils that are low responsive to external inputs due to other constraints in addition to the nutrients
contained in the fertilizer (line B, Figure 2). The above soils are ‘responsive soils’ and ‘poor, less
responsive soils’. For instance, N use efficiency by maize varied from > 50 kg grain kg–1
of N
applied on the fertile fields close to homesteads to less than 5 kg grain kg–1
N applied in degraded
outfields. In some cases, where fields are close to homesteads and receive large amounts of organic
inputs each year, or where land is newly opened, a third class of soil (line C, Figure 2) exists where
crops respond little to fertilizer as the soils are already fertile. These soils need only maintenance
fertilization and are termed “fertile, less responsive soils”. On the other hand, soils become non-
responsive when other inputs, beyond the supplied nutrient, are limiting plant growth and
productivity following the well-established principle ‘the law of the minimum’.
Use of fertilizer and improved seed on responsive soils will enhance yield and improve the AE
relative to current farming practices, characterized by local cultivars receiving very little and sub-
optimally managed nutrient inputs (line A, Figure 2). For example, recent experiences with the
Millennium Villages project showed an average threefold increase in maize yield with fertilizer
application. On the other hand, the return from low yielding local crop cultivars is expected to be
modest compared to high yielding improved varieties even under favourable conditions, though the
level of risk of crop failure due to extreme conditions is less with local cultivars. Major requirements
for achieving production gains on ‘responsive fields’ within line A include (1) the use of high
yielding crop varieties, (2) appropriate soil fertility and plant nutrient management practices, with
the right fertilizer formulation and rates, and (3) suitable crop management practices. However,
gains from a combined use of improved seed and soil fertility management could be reversed unless
other constraints including disease and pest management practices are in place.

27
Figure 2. The relationship between fertilizers and organic resource and the implementation of various
components of ISFM developing into complete ISFM towards the right side of the graph. Soils that are
medium in fertility and responsive to NP fertilizer and those that are low in fertility and less responsive are
clearly observed in field research.
Despite the production of substantial quantities of crop residues and manures in the country as a
source of organic soil amendments, they are not returned to soil due to competing utilization.
Composting of both crop residues and manures together or carbonization of part of them to biochar
can reduce the volume of organic resources, which means less labor/cost to transport them back to
the field. Application of compost to the soil may enable farmers to get the benefit of fairly resistant
organic matter and nutrients, provided that they have not been leached out or denitrified during the
composting process, but the labile C may be lost to the atmosphere. The labile C would benefit the
soil because it can feed soil organisms, which are responsible for several beneficial processes in
the soil (Agegnehu et al. 2017). Large-scale dissemination of ISFM practices will strategically
help to intensify agriculture in Africa in general and in Ethiopia in particular. While ISFM is an
important strategy, its implementation demands the deliberate integration of various soil fertility
management interventions along with incentives for farmers to adopt and implement these
strategies.
To facilitate proper management and use of nutrient resources, there is also a need to create strong
collective action at national, regional, and local levels that may address the following challenges:
1000
2000
3000
4000
5000
6000
FP RF BC + RF Com + RF BC + Com + RF
Barley
grain
yield
(kg
ha
-1
)
Medium fertility Low fertility
A
C
B
Farmer’s
practice
Recommended
NP fertilizer
Biochar +
Fertilizer
Compost +
fertilizer
Biochar + compost +
Fertilizer
Increase in knowledge and progress towards “complete ISFM”

28
• Minimizing major agents of nutrient movement, mainly soil erosion through improved
management of upper watersheds. In this case, there could be a need for integrated
application of soil and water conservation, afforestation, establishing waterways and other
practices through enhancing collective action and farmer innovation.
• Producing sufficient organic matter within the cropping systems that would satisfy the
competing demands of animal feed, household energy and soil fertility management. While
increasing biomass through application of chemical fertilizers to crop and forage fields is
possible, this challenge may require solutions that could be beyond soil management
practices. For instance, introducing fuel-efficient stoves and introducing alternatives
energy sources that would minimize competition and spare more organic matter for soil
fertility improvement.
• Enabling effective policy strategies that would induce communities to recycle organic
resources to valuable nutrients in homesteads and farm niches at household and community
levels may also demand collective action to collecting, processing and market organic
resources, particularly in peri-urban settings.
• Sustaining crop yields by soil ameliorating materials, particularly on highly weathered
acidic soils is the best approach for achieving higher crop yield, higher fertilizer use
efficiency, and economic feasibility.
• Establishing crops to fertilizer responses that would consider economic returns and socio-
economic requirements. Thus, sound soil-test crop response and balanced use of fertilizers
based on soil test fertilization is essential for successful fertilizer promotion and increased
crop production. This will also help to inform farmers on the use of correct and balanced
use of fertilizers for maximum efficiency and profitability.
• Facilitating by various associated measures the adoption of ISFM practices in priority
farming systems. Widespread adoption of ISFM has the potential not only to improve farm
productivity and farmers’ livelihoods but also to bring about environmental benefits.
• Engaging agricultural development personnel with the training of farmers to grow more
grain and forage legumes and trees in watersheds, to make compost and to recycle
nutrients, thus creating an opportunity for farmers to learn and adopt these practices.
4.5. Nitrogen fixing legumes and crop yield
Integration of multipurpose N-fixing legumes into farming systems commonly improves soil
fertility and agricultural productivity through symbiotic associations between leguminous crops
and Rhizobium. However, the contribution of N-fixation to soil fertility varies with the types of
legumes grown, the characteristics of the soils, and the availability of key micronutrients in soil to
facilitate fixation and the frequency of growing legumes in the cropping system. Although
perennial legumes are known to fix more N than annual legumes, the most prominent ones
contributing to the N enrichment of soils in Ethiopia are annual legumes, including faba beans,
peas and chickpeas. Some food legumes (e.g. Phaseolus vulgaris) are known to fix N below their

29
own N demands and may not contribute much to replenish the soil with additional nutrients. On
the other hand, perennial legumes, including those refereed as legume cover crops, could produce
up to 10 t ha-1
dry matter and fix up to 120 kg N ha-1
per season. Other studies conducted to evaluate
effective rhizobial isolates and strains for different agroecologies in Ethiopia indicated that
biological N fixation (BNF) could play an important role in increasing food production through
increasing yield of crops and forages. Crop yield increases of 51-158% were reported on Nitisols
at Holleta, Ethiopia due to the combined application of 20 kg P ha-1
with strain over non-inoculated
ones (Table 16).
Table 16. Grain yield and plant height of faba bean as influenced by phosphorus and Rhizobium inoculation
at Holetta
Treatment
Plant height
(cm)
Grain yield
(kg ha-1
)
N0P0 42.5 680
N0+20 kg P ha-1
51.0 1540
Strain#18+20 kg P ha-1
88.6 3980
56.5 2320
57.5 2740
23 kg N/ha+20 kg P ha-1
61.7 2050
20 kg N/ha+20 kg P ha-1
66.9 2240
LSD0.05 10.8 2980
Wheat grain yield was enhanced by dicot-rotations compared to cereal rotations. Long-term
experiment indicated that faba bean as a precursor crop increased mean grain yield of wheat by
660 -1210 kg ha-1
at Kulumsa and 350 - 970 kg ha-1
at Asassa compared to continuous wheat
(Table 17). The highest wheat grain yield was recorded after faba bean in two-course rotation
(FbW) and in first wheat after faba bean in three-course rotation (FbWW). From economic point
of view, a three-course rotation with either faba bean or rapeseed was found as an appropriate
cropping sequence in a wheat-based cropping system. Incorporation of vetch in the crop rotation
significantly increased wheat grain yield after vetch by 98-202% compared to wheat after wheat.
The efficiency of applied NP fertilizer was also enhanced in the field rotated with vetch. In an
experiment conducted to determine N2 fixation in three sites in Arsi highlands, the amount of N
fixed by faba bean ranged from 139-210 kg ha-1
. This, in turn, resulted in substantial mean soil N
balance that ranged from 12 - 58 kg N ha-1
after the seed had been removed but all faba bean
residues were incorporated in the soil. In contrast, the mean soil N balance in wheat after wheat
was at deficit (-9 to -44 kg ha-1
N) indicating nutrient mining and hence the need for higher rate of
fertilizer N application in a continuous wheat production system.

30
Table 17. Mean yield increment of wheat (kg ha-1
) in two-year and three-year rotation and higher level of
N and P as affected by crop rotation at Kulumsa and Asassa in Ethiopia
Cropping sequence
Yield increments due to
rotation over continuous
wheat
Yield increases due to higher N and P
rates
N P
Kulumsa Asassa
Kulumsa
a
Asassa Kulumsa Asassa
Wheat after faba bean 1370 860 40 -60 380 580
First wheat after faba
bean
1300 1050 250 0 140 360
First wheat after faba
bean
620 380 310 260 160 420
Wheat after rapeseed 670 600 730 240 650 320
First wheat after rapeseed 640 470 850 420 550 280
First wheat after rapeseed 310 80 780 390 -30 610
Wheat after barley 200 230 850 140 270 460
First wheat after barley 120 220 660 390 0 330
First wheat after barley 100 10 550 610 110 250
Continuous wheat 3130 2400 620 450 150 230
In Ethiopia, where demographic and economic pressures are intense, monocropping is a common
practice, soil fertility depletion is severe, and use of external inputs is very low. Rotating to
different crops on barley plots resulted in higher grain yields than continuous cropping of barley.
Planting barley after faba bean, field pea and rapeseed increased grain yield by 93, 67 and 78%,
respectively at Holetta and 47, 26 and 34%, respectively at Jeldu compared to planting barley after
barley (Figure 3). The highest yield increment due to break crops at Holetta in comparison to Jeldu
has been an indication of the low fertility status of soils at Holetta which was proved by soil
analysis. Field pea and faba bean significantly increased grain and straw yields of barley by about
20-117% and 34-102% at different locations in the highlands of Ethiopia, respectively compared
to continuous barley. Generally, higher yields after faba bean and field pea versus barley could be
due to the result of additional N release from the residues of these preceding crops. Crop rotations
have the ability to provide succeeding crops with N and reduce disease incidence and weed
populations. Inclusion of legumes in rotation can also bring about changes in soil fertility, soil
microorganisms, soil organic matter, soil water and crop responses. It is assumed that N-fixation
is largely responsible for the yield increment compared to cereal after cereal. Barley after legume,
without any N fertilization, yielded as much as continuously cropped barley supplied with 60 kg
N ha-1
.

31
Figure 3. Preceding crops effects on grain yields of barley at two locations
In addition to food legumes, other N-fixing forage legumes and cover crops that could be integrated
into the Ethiopian highlands include Tephrosia, Mucuna, Crotalaria, Canavalia, and vetch. A
study conducted in western Ethiopia showed that the integrated use of improved fallow using
Mucuna with low dose of NP fertilizers or FYM significantly increased maize grain yield. The
three years’ average maize grain yield showed that Mucuna fallow produced double maize yield
compared to the control treatment (Figure 4). Supplementing the improved fallow with low doses
of NP fertilizers or FYM further increased grain yield, ranging between 5.9 and 6.1 t ha-1
. Another
study conducted at Melkassa, central Rift Valley of Ethiopia, on selected leguminous shrubs and
their suitability for alley cropping with food crops, such as sorghum and maize, indicated that grain
yield increased by 4.2 and 13% for maize and 38.3 and 8% for sorghum, when maize and sorghum
were alley cropped with Sesbania Leucaena and Cajanus spp. compared to sole maize and
sorghum, respectively. Nitrogen fixation could be improved by improved agronomic and
nutritional management of the host plant. For instance, P nutrition increased symbiotic N fixation
in legumes by stimulating host plant growth. Application of micronutrients such as Mo, Mn, Fe
and Zn could stimulate symbiotic N fixation. In some cases, the contribution of legumes could be
beyond N fixation. For example, some legumes (e.g., chickpea) could modify the soil climate and
increase the availability of major nutrients, such as P and K, particularly in acidic soils where P
fixation occurs.
a
a
a
b
a
b
ab
c
500
1000
1500
2000
2500
3000
3500
4000
Faba bean Field pea Rape seed Barley
Grain
yield
(kg
ha
-1
)
Preceding crops
Holetta Jeldu

32
Figure 4. Effects of improved fallow with Mucuna alone, Mucuna + 55/10 kg N/P ha-1
, Mucuna +37/7 kg
N/P ha-1
, Mucuna + 4 t FYM ha-1
, Mucuna + 2.7 t FYM ha-1
and 110/20 kg N/P ha-1
on maize grain yield (t
ha-1
) in western Ethiopia.
5. Acid Soil Management
5.1. Extent and distribution of soil acidity
Soil acidity is among the major land degradation problems, which affects ~50% of the world’s
potentially arable soils. Naturally, soil acidification takes place due to carbonic acid triggered
leaching of basic cations and soil acidity increases with rainfall. Leaching of basic cations due to
high rainfall, weathering of acidic parent materials, organic matter decay, and removal of the
cations with harvest of high yielding crops are the major causes of soil acidification. Acidification
continues until a balance is reached between removal and replacement of the basic cations such as
Ca and Mg that are removed through leaching and crop harvest and replaced due to organic matter
decomposition and from weathering of minerals. Continuous application of acid forming fertilizers
and contact exchange between exchangeable hydrogen on root surfaces and the bases in
exchangeable form on soils, microbial production of nitric and sulfuric acids can also contribute
to soil acidity (Mesfin 2007; Agegnehu et al. 2019). Acidic soils contain high concentration of Al,
manganese (Mn) and iron (Fe). At pH below 5.0, Al is soluble in water and becomes the dominant
ion in the soil solution. In acid soils, excess Al primarily injures the root apex and inhibits root
elongation. The poor root growth leads to reduced water and nutrient uptake, and as a result crops
grown on acid soils are constrained with poor nutrients and water availability leading to reduced
growth and yield of crops (Agegnehu and Girma 2003; Fageriaand Nascente 2014).
Soil acidity is expanding both in area and level of acidity in Ethiopia; it extends from south-west
to north-west with east-west distribution. Acid Nitisols (pH < 5.5) occur widely in Ethiopian
c
b
a a
a a
b
1
2
3
4
5
6
7
Control Mucuna
alone
Mucuna +
55/10 kg N/P
Mucuna +
37/7 kg N/P
Mucuna + 4 t
FYM
Mucuna +
2.7 t FYM
110/20 kg
N/P
Maize
grain
yield
(t
ha
-1)

33
highlands where the rainfall intensity is high and crop cultivation has gone for many years, where
~80% of acidic soils emanate from the Nitisol areas of the country. In most cases, soils found in
high-altitude areas of the country are acidic in reaction, poor in exchangeable cations and low in
base saturation. Some of the areas severely affected by soil acidity include Ghembi, Nedjo,
Hossana, Sodo, Endibir, Chencha, Hagere-Mariam and Awi. As shown in Figure 5, approximately
43% of the Ethiopian cultivated land is affected by soil acidity. About 28.1% of these soils are
dominated by strong acid soils (pH 4.1-5.5). Strongly acidic soils are usually infertile because of
the possible Al and Mn toxicities, and Ca, Mg, P, and molybdenum (Mo) deficiencies.
Figure 5. Extent and distribution of soil acidity in Ethiopia (ATA, 2014)
5.2. Causes of soil acidity
Soil acidification is a complex set of process resulting in the formation of an acid soil. It has been
recognized that there are several causes for soils to become acidic. In the broadest sense, it can be
considered as the summation of natural and anthropogenic processes that lower down the pH of
soil solution. Inefficient use of nitrogen is another causes of soil acidification, followed by the
export of alkalinity (Mesfin 2007). Ammonium based fertilizers are major contributors to soil
acidification. Ammonium nitrogen is readily converted to nitrate and hydrogen ions in the soil.
Soil forming factors including parent materials, climate and anthropogenic factors explain the
acidity of soils.
Climate
It has been well recognized that in soils of dry region a large supply of bases is usually present,
as leaching is limited. With an increase in rainfall, the contents of soluble salts such calcium
carbonate and gypsum are removed by leaching or runoff. With further increase in rainfall, a point
is reached at which the rate of removal of bases exceeds the rate of their liberation from non-

34
exchangeable forms. Hence, wet climates have a greater potential for acidic soils. Over time,
excessive rainfall leaches soluble nutrients such as Ca, Mg and K that prevent soil acidity which
are specifically replaced by Al from the exchange sites.
Parent materials and organic matter decomposition
Acidic parent materials: Rocks containing an excess of quartz or of silica as compared to their
content of basic materials or of basic elements are categorized as acid rocks (e.g., granite and
rhyolite). When rocks which are deficient in bases are weathered the product is acidic. Soils that
develop from weathered granite are likely to be more acidic than those developed from shale or
limestone. The inherent fertility of Ethiopian soils developed from varied parent materials and
climate varies depending on the origin and composition of the materials. For instance, soils
developed from sandstones are infertile sandy soils, whereas the inherent soil fertility developed
over basic parent materials is relatively high (Abebe 1998). In alluvium plains, alluvium becomes
rich and fertile if it originates from relatively young materials, and less fertile if it originates from
highly weathered surfaces.
Decomposition of organic matter: The decomposition of organic matter produces H+
ions,
which are responsible for acidity. The development of soil acidity from the decomposition of
organic matter is insignificant in the short term. Large quantities of carbonic acid produced by
microorganisms and higher plants including through other physicochemical and biological
processes are the causes of soil acidity although the effect from its dissociation is relatively small
as most of it is lost to the atmosphere as CO2.
Low buffer capacity from little clay and organic matter
Another source of soil acidity is contact exchange between exchangeable hydrogen on root
surfaces and the bases in exchangeable form on soils. Where leaching is limited, microbial
production of nitric and sulfuric acids also occurs. The lime requirement of acid soil is related not
only to the soil pH but also to the buffer or CEC. The buffering or CEC is related to the amount of
clay and organic matter present, the larger the amount, the greater the buffer capacity. Soils with
higher buffer capacity (clayey, peats), if acid, have high lime requirement. Coarse textured soils
with little or no organic matter will have low buffer capacity and, even if acid, will have low lime
requirement.
Alumino-silicate minerals
The principal hydrous oxides of the soils are Al and Fe which occur in amorphous, crystalline or
colloidal forms as coating on other mineral particles or as inter-layers in clay mineral structures.
When the pH of the soil decreases, these oxides get into solution and through stepwise hydrolysis
release H+
ions resulting into further acidification. Soil acidity limits plant growth not only because
of the deficiencies of P, Mo, Ca, Mg, etc. but also due to toxicities of Al, Mn and H ions. Toxicities
of these elements have been recognized as one of the most common cause of yield reduction in
acid soils.

35
Anthropogenic factors
Application of ammonium fertilizers: Continuous application of inorganic fertilizer without soil
test, in the long run, can increase soil acidity. The use of N fertilizers in ammonia form is a source
of acidification. When ammonium fertilizers are applied to the soil, acidity is produced, but the
form of N removed by the crop is similar to that found in fertilizer. Hydrogen is added in the form
of ammonia-based fertilizers (NH4), urea-based fertilizers CO(NH2)2, and as proteins (amino acid)
in organic fertilizers. Transformation of such sources of N fertilizers into nitrate (NO3) releases
hydrogen ions (H+
) to create soil acidity. In reality, N fertilizer increases soil acidity by increasing
crop yields, thereby increasing the amount of basic elements being removed. Hence, application
of fertilizers containing NH4 or even adding large quantities of organic matter to a soil can
ultimately increase soil acidity and lower pH.
Removal of elements through crop harvest: Removal of elements, especially from soils with
small reservoir of bases due to the harvest of high yielding crops is responsible for soil acidity.
When soils are worked mechanically, and crops are grown the balance is disturbed and the soils
become more acid. This is the result of base cations being removed with crops and the
simultaneous increase of leaching which takes place when soils are disturbed and worked. Harvest
of high-yielding crops plays the most significant role in increasing soil acidity. During growth,
crops absorb basic elements such as Ca, Mg, and K to satisfy their nutritional requirements. As
crop yields increase, more of these lime-like nutrients are removed from the field. Compared to
the leaf and stem portions of the plant, grain contains minute amounts of these basic nutrients.
Therefore, harvesting high-yielding forages such as Bermuda grass and alfalfa affects soil acidity
more than harvesting grain does.
Changes in land use and management practices often modify most soil physical, chemical and
biological properties to the extent reflected in agricultural productivity. Soil properties deteriorate
due to the conversion of native forest and range land into cultivated land. Such practices result in
an increase in bulk density, decline in soil organic matter content and CEC, which in turn reduce
the fertility status of a certain soil type. In addition, change in land use associated with
deforestation, continuous cultivation, overgrazing, and mineral fertilization can cause significant
variations in soil properties and reduction of output.
5.3. Types of acidity
There are mainly two types of soil acidity: i) Active acidity which occurs because of H+
ion
concentration of the soil solution which is attributable to carbonic acid (H2CO3), water soluble
organic acids and hydrolytically acid salts. This type of acidity can be determined by measuring
the pH value of a water suspension or extract from a soil. It bears directly on the development of
plants and soil micro-organisms; ii) Exchange acidity which refers to those H and Al ions adsorbed
on soil colloids. There exist an equilibrium between the adsorbed and soil solution ions (i.e. active

36
and exchange acidity), permitting the ready movement from one form to another (Figure 6). Such
an equilibrium state is of great practical significance since it provides the basis for the soils
buffering capacity or its resistance to change in pH. Since the adsorbed H and Al ions move into
the soil solution then its acidity is also referred to as adsorbed or potential or reserve acidity.
Reactions of bases (e.g. lime) added to the soil occur first with the active acidity in soil solution.
Subsequently, the pool of reserve acidity gradually releases acidity into the active form.
Figure 6. Equilibrium relationship between exchange (reserve) and solution (active) acidity, and acid or
base inputs.
5.4. Effect of soil acidity on nutrient availability and crop yield
5.4.1. Soil acidity and nutrient availability
The clay mineralogy, pH, presence of oxides and hydroxides of Fe and Al and content of
amorphous materials seem to be the dominant factors affecting P sorption. In the case of highly
weathered soils of Chenca, Nedjo and Endibr, where the dominant minerals are Gibbsite, Goethite,
Kaolinite and desilicated amorphous materials, P sorption is high to very high (Table 18). The
mechanism of phosphate adsorption is considered to be mainly through replacement of hydroxyl
ions on crystal lattices, and hydrated Fe and Al by phosphate ions. Phosphorus sorption capacity
increases with increasing acidity (decreasing pH value). For instance, soils from rift valley of
Ethiopia (e.g. Melkassa) had the lowest P sorption, which are the least weathered (with a pH value
of 7.8). In contrast, the soil from the highlands of Ethiopia (e.g., Chencha) had the highest P
sorption, which has a pH of 4.5, and has higher content of gibbsite, goethite and amorphous
materials than other sites except Endibir for amorphous materials.
Table 18. Amount of P sorbed by some Ethiopian soils at the standard solution P of 0.2 ppm
Soil
origin
Sorbed P pH Fe2O3
(%)
Exch. Al (cmol
(+) kg-1
)
Amorphous
material (%)
Gibbsite and
Goethite (%)
mg kg-1
kg ha-1
Chencha 1200 2400 4.5 11.7 0.40 51 10
Nedjo 950 1900 4.4 16.1 6.16 32 12
Indibir 800 1600 4.8 11.7 1.69 61 0
Melko 600 1200 5.2 15.8 0.37 ND ND
Bako 400 900 6.6 14.4 0.02 41 15
Melkassa 150 300 7.8 0.20 Tr. ND ND
ND: Not determined; Tr: Trace
fast
Reserve
acidity (solids)
H+
Active acidity
Al3+
H+
slow
+ base
+ acid
Al(OH)3

37
The solubility and availability of important nutrients to plants is closely related to the pH of the
soil. Soil pH affects the availability of plant nutrients. Soil acidity converts available soil nutrients
into unavailable forms and soils affected by soil acidity are poor in their basic cations are essential
to crop growth and development. For example, effects of high acidity in a soil are shortage of
available Ca, K, Mg, P and Mo on the one hand, and excess of soluble Al, Mn and other metallic
ions on the other. Acid soil limits the availability of crucial nutrients such as P, K, Ca and Mg, and
affects the movement of soil organisms that are important for plants health. If a particular soil is
too acidic for plants to grow healthy, it is necessary to raise the pH by applying an alkaline
substance.
Soil acidity and associated low nutrient availability is one of the constraints to crop production on
acid soils. If a pH of a soil is less than 5.5, phosphate can readily be rendered unavailable to plants
as it is the most immobile of the major plant nutrients and yields of crops grown in such soils are
very low. Soils with pH ranging between 5.5 and 7 allow the readiest availability of plant nutrients
in which P fixation is lower and its availability to plants is higher. At this pH range, toxicity and
deficiency of Fe and Mn may be avoided. The quantity of P in soil solution needed for optimum
growth of crops ranges between 0.13 to 1.31 kg P ha-1
as growing crops absorb about 0.44 kg P ha-1
per day. The labile fraction in the topsoil layer is in the range of 65 to 218 kg P ha-1
, which could
replenish soil solution P.
Phosphate sorption (the loss of orthophosphate from soil solution to solid phases) takes place by
specific adsorption and precipitation reactions. Specific adsorption occurs when P anions replace
the hydroxyl groups on the surface of Al and Fe oxides and hydrous oxides, while precipitation
reaction occurs when insoluble P compounds form and precipitate. At very low soil pH (≤4.5–5.0),
addition of P to soils can result in precipitation of Al and Fe phosphates, whereas at high pH (>6.5)
insoluble calcium phosphates can be formed. In many situations, however, specific adsorption
reactions are the main regulators of soil solution P concentrations. Specific adsorption of P is
affected by many factors including pH, ionic strength of the background electrolyte and anion
competition.
5.4.2. Soil acidity and crop performance
As crops differ in their susceptibility to soil acidity, the correct pH depends on the type of crop
grown. For example, food and forage legumes, such as beans, peas, and desmodium forage,
possess nodules on their roots where bacteria can take N from the air and change it to a form usable
by the plant. However, some strains of the bacteria do not thrive at pH values below 6, thus pH 6
or above is best for the legumes that require those particular strains of the bacteria. In contrast,
potato scab disease is more prevalent when soil pH is above 5.5; thus, the recommended soil pH
for optimum growth of potato is 5.0 to 5.5, although potato plants can grow well at higher pH.
Whereas plants such as azalea and camelia grow well only at pH values below 5.5 and suffer from
iron (Fe) and Mn deficiencies at higher pH. The pH of soils for best nutrient availability and crop

38
yields is considered to be between 6.0 and 7.0, which is the most preferred range by common field
crops. A summary of crop relation to soil reaction is given in Table 19.
Crops like cotton, alfalfa, oats and cabbage do not tolerate acid soils and are considered suitable
to neutral soils with a pH range of 7-8. Wheat, barley, maize, clover and beans grow well on neutral
to mildly acid soils (pH 6-7). Grasses tend to tolerate acidic soils better than legumes, so liming to
pH 5.5 may control acidity without limiting production. Legumes, however, need more Ca and
perform best between pH 6.5 and 7.5. Among crops tolerant to acid soils are millet, sorghum,
sweet potato, potato, tomato, flax, tea, rye, carrot, and lupine. Poor plant vigor, uneven crop
growth, poor nodulation of legumes, stunted root growth, persistence of acid-tolerant weeds,
increased incidence of diseases and abnormal leaf colors are major symptoms of increased soil
acidity which may lead to reduced yields. Increased acidity is likely to lead to poor water use
efficiency because of nutrient deficiencies and imbalance, and or induced Al and Mn toxicity. High
concentration of Al also affects uptake and translocation of nutrients, especially immobilization of
P in the roots, cell division, respiration, nitrogen mobilization and glucose phosphorylation of
plants.
Table 20. Crop relation to soil reaction
Crops Optimum pH for best
growth
Crops Optimum pH for
best growth
Alfalfa, cotton, oats,
chickpea
7.0-8.0 Tomato 5.5-7.5
Cabbage 6.0-6.5 Sugar beet 5.8-7.0
Wheat, barley 6.0-7.0 Onion 5.8-6.5
Maize 6.0-7.2 Carrot 5.5-7.0
Faba bean 6.0-8.0 Potato, sweet potato,
lupine
4.5-6.5
Field pea, clover 6.0-7.0 Mango 5.0-6.0
Lentil 6.5-8.0 Papaya 6.0-6.5
Soybean 6.2-7.0 Avocado 5.0-8.0
Sorghum, millets 5.5-7.5 Pineapple 4.5-6.5
Flax 5.0-7.0 Deciduous fruits 6.5-7.5
Beans 5.5-8.0 Sugarcane 5.0-8.5
Rye 5.0-7.5 Tea 4.0-6.0
Soil acidity, at pH 5.5 or lower, can inhibit the growth of sensitive plant species, though it has little
effect on insensitive species even at pH lower than 4. This pH effect is compounded and often
surpassed by Al and Mn toxicity, Ca and Mo deficiency (Somani 1996; Fox 1979; Baquy et al.
2017). Roots are commonly the first organs to show injury owing to acid due to Al toxicity; they

Soilfertilitymgt_Guidebook.pdf

Soilfertilitymgt_Guidebook.pdf

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