Horticulture

1. 2. Soil

2. • Soil is the medium for the growth of plants.
• It provides physical support, moisture and nutrients for growing plants.
• Soil formation, or pedogenesis, essentially involves the formation of loose soil
and its subsequent differentiation into distinct vertical layers or soil profiles.
• This is accomplished by the combined action of several processes, such as
weathering, humification leaching and calcification.
• Weathering is the breakdown and modification of consolidated bed rock
and minerals by physical, chemical and biological processes.
• Physical weathering involves the disintegration of rock by precipitation,
wind action, wave action, freezing and expansion of water within the cracks
and crevices rocks, etc.

3. • Chemical weathering is the disintegration of rock and minerals by
chemical processes which operate at atomic and molecular levels. Biological
weathering takes place by the action of some living organisms, such as lichens,
fungi, etc., which colonize rock surface.
• Humification is the formation of humus from dead organic matter by the action
of saprotrophs which use dead materials as the source of their nutrients.
• Leaching is the removal of materials in solution from the top soil. In this case,
water percolates downward carrying humus and minerals from the surface soil.
• Calcification is the re-deposition of secondary calcium carbonate in a region
from other parts of the soil.

4. Constituents of soil
 Typically, soil has four principal constituents, namely mineral matter
(45%), soil water (25%), soil air (25%), and organic matter (5%). Soil
components can be broadly grouped under three major categories,
physical, chemical and biological.
 They interact with each other and influence the growth and
development of plants.
 A better understanding of the roles of these factors in the soil system
and their effects on plants is important in making appropriate choice
of soil for crop cultivation and also for manipulating the soil for better
crop production.

5. A. Physical components
• The major physical components of soil include soil air and soil water.
• These are very important for the growth and reproduction of soil organisms.
• 1. Soil air
• 2. Soil water or soil solution

6. 1. Soil air
 Soil air, or ‘soil atmosphere', is the air that fills the pore spaces in the soil (pore
spaces or soil spaces are the spaces present in between soil particl
 es). It has the same gases as those of the atmosphere above the ground level, but in
different proportions.
 Soil air has higher CO, content and lower 0, content than the atmospheric air.
 Proper soil aeration is essential for water absorption by plants, growth of roots,
germination of seeds, and also for the activities of soil microbes.
 A poorly aerated soil will have high CO, concentration.
 It is toxic to plants and it seriously affects the absorption of water and mineral
nutrients.
 Typically, soil consists of 25% air. Air is essential for the respiration and growth of
plant roots.
 Water-logged conditions cause the pore spaces to be filled with water and this forces
plants to respire anaerobically.

7. Ø Seeds also require oxygen for germination.
Ø Clayey soils are poorly aerated, whereas sandy soils are well aerated.
Ø Soil oxygen levels of less than 10-12% are stressful to plants.
Ø To improve the aeraiton of clayey soils, crops must be planted in raised beds
which would help to drain the soil so that air can occupy the pore spaces.
Ø Hydrophytes, such as water lilies, are adapted for respiration under water.
Ø Air is also required for the decomposition of organic matter by soil bacteria.
Ø Under water-logged conditions, plant materials do not decompose properly
and so they form materials, such as peat (partially decomposed organic
matter).

8. 2. Soil water or soil solution
• Soil water is a dilute solution of organic and inorganic compounds in water.
• It is held in the soil by the capillary and absorptive forces between soil particles.
• The amount of soil water depends on precipitation and surface evaporation.
• Soil water includes gravitational water, capillary water, hygroscopic water and
combined or bound water.
• Gravitational water is the free water that moves down to the water table due to
gravitational force.
• Capillary water is the water that fills the spaces between non-colloidal soil particles.
• It is held in the soil due to capillary forces.
• Hygroscopic water is the water absorbed by colloidal soil particles.
• Bound water is the water that remains chemically bound to soil materials.
• Of all these, only capillary water and a small portion of gravitational water are available
to plants.

9. • The total amount of water present in the soil is called holard.
• The amount of water the plants can absorb from this is called chresard
(available water which includes capillary water and a small portion of
gravitational water).
• The amount of water which the plants cannot absorb is called echard (non-
available water which includes hygroscopic water, combined water and most of
the gravitational water).
• The amount of water, retained in the soil after the drainage of gravitational
water, is called field capacity of the soil.
• Addition of water, far in excess of the field capacity causes water-logging.
• Soil water is the source of water and inorganic and organic nutrients for plants
Also, it serves as a medium for the dissolution and transport of plant nutrients.
• At normally low or high concentration of soil water adversely affects the life and
growth of plants.

10. • The low concentration of soil water that can cause permanent and irrecoverable wilting
of plants is called permanent wilting point or wilting coefficient.
• Soil water is critical to the growth and development of plants.
• It forms the solvent in which soil nutrients are dissolved before they are absorbed by
plant roots.
• Soil water is the primary source of water for plants.
• It indirectly affects soil air and thereby influences plant growth and development.
• Plants lose large amounts of their water through transpiration.
• Soil water comes from precipitation or irrigation.
• The quantity of water, infiltrated into the soil, depends on the texture and structure of
the soil.
• The downward movement of water depends on the layering or stratification of the soil.
• The goal of irrigation is to retain moisture in the root layer of the soil.
• The unsaturated zone between the ground surface and the water table is called soil
water zone (vadose zone).
• Water passes to the water table through this zone.
• After a heavy rain or irrigation, this zone may become saturated with water.

11. B. Chemical components
• The major chemical components of the soil include mineral matter and organic
matter.
1. Minerals
2.Organic matter

12. 1. Minerals
• Minerals are the major chemical components of the soil.
• They are important in the synthesis of biomolecules in the plant body.
• All about 90 odd soil elements have been identified as constituents of the plant
material.
• A plant body may contain as many as 50 of them.
• These fall under two classes, namely essential elements and non-essential elements.
• Essential elements are indispensable for the growth and development of plants.
• Arnon and Stout (1938) recognized 16 elements to be essential for higher plants (C,
H, O, N, P, K, S, Ca, Mg, B, C1, Cu, Fe, Mn, Mo, Zn).
• Later studies have revealed that the number is still higher.
• For example, Na, V, Co, Se, Si, etc. have also been shown to be essential for some
species of higher plants.

13. • The major criteria for determining the essentiality of an element are the following:
1) The element must be inevitable for the normal growth, development and
reproduction of plants, and in its absence the life cycle of plants may not be
completed.
2) The biological role of the element must be specific and not replaceable by any other
element; in other words, an element is not functionally replaceable by another one.
3) The element must have a direct nutritional role.
4) The element must be an integral part of a structural or a functional molecule of the
plant body (e.g., the presence of N in proteins, and Mg in chlorophyll).
• Of the essential elements, C, H, and O are absorbed from atmospheric air and soil
water.
• So, they are called non-mineral elements.
• The others are absorbed from soil and so they are called mineral elements.
• Nitrogen is obtained from both soil atmosphere.
• Plants absorb mineral elements from soil mainly as inorganic ions, commonly called
mineral nutrients.

14. Macronutrients and micronutrients
• Plants do not require all elements in equal amounts. Based on the level of requirement,
essential elements are classified under two groups, namely macroelements
(macronutrients or major elements) and microelements (micronutrients, minor
elements, or trace elements)
• * Macroelements are the essential elements required by plants in large quantities (e.g..
C, H, O, N, K, Ca, Mg, P, S), whereas microelements include the essential elements
required in trace amounts (e.g., B, CI, Cu, Mn, Mo, Fe, Zn, etc.)
• Macronutrients are further divided into primary nutrients and secondary nutrients.
• Primary nutrients include N, P and K.
• They are required by plants in large amounts and so they are often deficient in the soil.
• Secondary nutrients include Ca, Mg, S, etc.
• They are required in lesser amounts and so they are not usually deficient in the soil.
• Since micronutrients are required only in trace amounts, they are also very rarely
deficient in the soil.
• They are very critical in green house cultivation where artificial growing media are
often used.

15. Macronutrients Micronutrients
Secondary Primary
Iron (Fe)
Manganese (Mn)
Molybdenum (Mo)
Copper (Cu)
Boron (B),
Zinc (Zn),
Chlorine (CI)
Nitrogen (N) Calcium (Ca)
Phosphorus (P) Magnesium (Mg)
Potassium (K) Sulphur (S)

16. a) Primary macronutrients
• Primary macronutrients include
Nitrogen
Phosphorus
Potassium.

17. (1) Nitrogen (N)
• Nitrogen is one of the most widely used elements in plant nutrition.
• Most plants absorb nitrogen from the soil solution primarily as inorganic nitrate
ions (NO,) and, in some cases, as ammonium ions (NH).
• In the plant body, nitrate ions get immobilized (mineral form is changed into
organic forms, such as amino acids, proteins, purines, pyramidines, nucleic
acids, etc.) and become a part of the plant tissue.
• When plants die, their tissues get decomposed and release inorganic nitrogen
ions by the process of mineralization.
Deficiency symptoms
• Stunted plant growth, chlorosis and yellowing of older leaves, premature leaf
fall, delayed or suppressed flowering and fruiting, accumulation of anthocyanin
in stem, leaf petioles and leaf veins, etc. are the major symptoms of nitrogen
deficiency.

18. Phosphorus (P)
• Plants absorb phosphorus from the soil solution primarily as monovalent
orthophosphate anions (H POD) at a soil pH less than 6.8.
• The predominant form of soil phosphorus is HPO3-. But, it is only less readily absorbed
by plants.
• In alkaline soil with pH higher than 7.2, phosphorus exists mostly as trivalent HPO which
is not virtually absorbed by plants.
• In plants, phosphorus is found in proteins, nucleic acids (DNA and RNA), ATP and ADP.
• Phosphorus induces root proliferation and early crop maturing.
Deficiency symptoms
• Stunted plant growth, intense green colouration, distortion and malformation of leaves,
rapid senescence and premature fall of older leaves, short and slender stem and leaf
stalk, delayed flowering and fruit maturation, etc. are the major symptoms of
phosphorus deficiency.

19. Potassium (K)
• Most plants require potassium in large amounts.
• Potassium has significant roles in protein synthesis, respiration, phloem
translocation, osmoregulation, opening and closing of stomatal pore, sleep
movements, daily changes in the orientation of leaves, storage of starch, growth
of meristematic tissue, etc.
• Potassium ions activate a number of enzymes, especially those involved in
photosynthesis and respiration.
• Plants absorb potassium from soil in its monovalent cationic form (KTM).
Deficiency symptom
• Potassium is frequently deficient in sandy soils because of its high water-
solubility and the ease with which it can readily leach out of sandy soil. The
common symptoms of its deficiency include short and weak stem, stunted
growth, dieback of shoot, chlorosis and mottling in older leaves, small necrotic
patches at leaf tips, in between leaf veins, and along leaf margins, suppression of
flowering, etc.

20. b) Secondary macronutrients
• Secondary macronutrients include
Calcium
Magnesium
Sulphur.

21. Calcium (Ca)
• Calcium is not only an essential plant nutrient but also is very important in regulating
soil acidity.
• It makes soil nutrients readily available to plants in appropriate amounts.
• It is absorbed by plants as divalent cations (Ca-*).
• Calcium is important in the growth and division of cells, formation of cell wall and
mitotic spindle, nitrogen accumulation, etc.
• Also, it maintains the physical integrity and the normal functional state of cell
membranes, and acts as a second messenger in hormonal and environmental
responses.
• Calcium is abundant in most types of soil and so it is very seldom deficient in plants
under normal natural conditions.
• When it is deficient, plant tissue formation becomes incomplete, terminal buds may
cease to grow, leaving a blunt end, and roots grow poorly and become short and thick.
• The common deficiency symptoms include death of apical meristems, stunted stem
growth, chlorosis and necrosis of the edges of young leaves, distorted and yellow-edged
leaves, poorly developed and gelatinous roots, etc.

22. Magnesium (Mg)
• Magnesium is absorbed as Mg2+ ions.
• It forms the central atom of chlorophyll molecules, stabilizes ribosome structure
and activates several enzymes.
• It is also essential in the formation of fats and sugars.
• Magnesium is mobile in plants, and thus its deficiency appears first in older
leaves.
• Its deficiency usually occurs in strongly acidic soils.
• The common deficiency symptoms are chlorosis and necrotic spots in leaves,
premature leaf fall, etc.

23. Sulphur (S)
• Sulphur is obtained primarily from the decomposition of metal sulphides in
igneous rocks.
• It exists in soil as well as in humus as sulphates and sulphides.
• It is absorbed by plants as sulphate anions (SO2).
• The unique flavour of onion, cabbage and other cruciferous plants is due to the
presence of certain sulphur compounds.
• Sulphur is an ingredient of some vitamins and amino acids.
• It is important in the formation of disulfide bonds in the tertiary structure of
proteins.
• The commonest symptoms of sulphur deficiency are chlorotic foliage, premature
leaf fall, inhibition of apical growth, suppressed fruit formation, etc.
• Sulphur is not usually added as a fertilizer element, but is added indirectly when
sulphate forms of other elements are applied.

24. c) Micronutrients
Boron (B)
Iron (Fe)
Molybdenum (Mo)
Manganese (Mn)
Zinc (Zn)
Copper (Cu)
Chlorine (CI)
Organic matter

25. Boron (B)
• Plants absorb Boron as borate anions (BO2-). It is mobile in the plant system,
and it affects flowering, fruiting, cell division, water relations, etc.
• Deficiency results in the death of terminal buds and a kind of growth pattern at
the top of the plant, called witche's broom.
• Young leaves become thick, leathery and chlorotic and flowering becomes
scanty.
• Other deficiency symptoms include shortened internodes, bushy appearance of
plants, stem crack, heart rot in storage roots, etc.

26. Iron (Fe)
• Iron is more abundant in most soils than other trace elements.
• Its deficiency occurs in alkaline or acidic soils. It can be absorbed through leaves
or roots as Feat ions.
• Iron is a component of cytochromes, ferrodoxin, flavoproteins, leghaemoglobin,
etc.
• It acts as a catalyst in the synthesis of chlorophyll, and as an enzyme activator in
many biochemical reactions.
• The symptoms of iron deficiency include chlorosis of young leaves, inhibition of
chloroplast formation and impairment of aerobic respiration.
• In severe cases, leaves become whitish.

27. Molybdenum (Mo)
• This element is unavailable to plants grown under very low pH conditions.
• Molybdenum is essential for the synthesis of proteins and some enzymes that
reduce nitrogen (e.g., nitrate reducatase).
• Molybdenum deficiency is highly species-specific.
• It is particularly widespread among legumes, maize and the members of the
family Brassicaceae.
• It is severe in acidic soils with high iron precipitates, which strongly absorb
molybdenum ions.
• Vegetables, cereals, and forage grasses show very visible symptoms of
molybdenum deficiency.
• The common symptoms of molybdenum deficiency include chlorosis, necrosis,
mottling, folding and wilting of leaves, low flowering, premature flower fall, etc.
• Liming of soil is a corrective measure to reduce the hazards of Mo deficiency.

28. Manganese (Mn)
• Manganese serves as the cofactor of several enzymes, especially of carboxylases
and dehydrogenases.
• It is crucial in photosynthesis because of its role in chlorophyll synthesis.
• Plants absorb Mn as Mn2+.
• Its deficiency symptoms include chlorosis and necrotic gray spots in younger
leaves, as in iron deficiency.
• Disintegration of chloroplasts is also common.
• Mn deficiency is often aggravated by low soil pH (<6) and high organic content.

29. Zinc (Zn)
• Zn is absorbed as Zn2+ ions by plant roots. It is an enzyme activator.
• When it is deficient, leaves become drastically reduced in size and stem
internodes get shortened, giving a rosette appearance.
• Necrosis and chlorosis in leaves, distorted leaf shape, reduced fruit size,
suppressed seed formation, etc. are other symptoms.

30. Copper (Cu)
• Soils that are rich in organic matter are more prone to copper deficiency. Plants
absorb Cu as divalent cupric ion (Cu2+).
• Copper primarily serves as a cofactor of many oxidative enzymes.
• Also, it is important in chlorophyll synthesis and acts as a catalyst in
carbohydrate and protein metabolism.
• The common symptoms of Cu deficiency include stunted growth, distortion of
young leaves, chlorosis and necrosis at the tips and margins of leaves, etc.

31. Chlorine (CI)
• Chlorine is absorbed as chloride ions (CI-).
• Deficiency is rare.
• Deficiency symptoms include reduced plant growth, chlorosis, necrosis and
bronzing of leaves, wilting of leaf tips, etc.

32. 2.Organic matter
• The organic matter of the soil is the product of the decomposition of the dead
remains of animals and plants, animal wastes and green manure.
• Some bacteria and fungi, inhabiting the soil, serve as the decomposers of these
materials.
• They enrich the soil with organic matter through decomposition, nutrient cycling,
etc.
• The organic matter of the soil mainly includes humus and litter or detritus.
• Humus is the amorphous, dark-coloured and fully decomposed organic matter.
• On the other hand, litter is the partially decomposed organic matter of fallen
leaves, twigs, bark, green manure, etc.

33. Significance of organic matter
• Organic matter is very significant in all types of soil. Its major significances are the
following
Increases the aeration, hydration, water-holding capacity and the drainage
potential of the soil.
Provides nutrients and thereby enhances the fertility and productivity of the
soil.
Improves the soil texture by facilitating the binding together of mineral particles
to form aggregates.
Serves as a buffer against the rapid and abnormal changes in soil pH.
Forms a potentially rich reserve of nutrients for plants.

34. Humus stores large amounts of cations, such as Mg2+ ,Na +and H+.
Coarse organic matter on the soil surface reduces the impact of rainfall and
permits clear water to seep gently into the soil.
Organic matter prevents surface runoff and soil erosion so that more water
would be available for plant growth.
Fresh organic matter provides food for soil organisms, such as earthworms,
ants, and rodents. These animals burrow in the soil and thereby make the soil
more porous. This, in turn, permits plant roots to obtain oxygen and to release
carbondioxide as they grow.
Organic matter forms a soft mulch over the soil surface and thus lowers soil
temperature in summer and keeps the soil warmer in winter.
Organic mulch reduces evaporation.

35. C. Biological components of soil
[Soil organisms or soil biota]
• Soil is an ideal habitat for a great variety of organisms, called soil biota.
• They vary with soil types. So, each type of soil will be inhabited by specific types
of microbes, flora and fauna.
• Based on size, soil organisms are grouped under three categories, namely
microbiota, mesobiota and macrobiota.
• Microbiota includes minute organisms, such as bacteria, cyanobacteria, algae,
fungi, protists, nematodes, etc.
• Mesobiota includes medium-sized organisms, such as rotifers, nematodes,
annelid worms, mites, land snails, some insects, etc.
• Macrobiota includes large-sized organisms, such as rooted plants, earth-worms,
centipedes, millipedes, some insects, burrowing vertebrates, etc.

36. • The spaces around the roots of higher plants (rhizosphere) provide a special
environment for the rapid proliferation of microorganisms due to exudation of
energyrich compounds.
• Soil bacteria mostly colonize in the rhizosphere. About 7 billion bacteria are
present in a gram of rhizosphere soil, compared to the 25 million in non
rhizosphere soil.
• The most abundant genera of soil bacteria are Pseudomonas, Azotobacter,
Bacillus and Agrobacterium.
• The important genera of bacteria from plant nutrition point of view are
Rhizobium, Clostridium, Nitrobacter and Nitrosomonas.
• Fungi dominate in acidic soils, whereas bacteria and actinomycetes are inactive
in a pH range of 4.5 - 6.5.
• Important genera of fungi are Aspergillus, Fusarium, Trichoderma, Mucor,
Penicillium etc.

37. • They perform two important functions:
1. act as scavengers and decompose various biodegradables (fungi can
decompose lignin while bacteria cannot)
2. certain fungi feed on protists and nematodes and thus maintain microbial
equilibrium in the soil.
• Soil flora consists of algae belonging to the class Cyanophyta, Chlorophyta
Xanthophyta and Bacillariophyta.
• The most abundant genera are Nostoc and Anabaena.
• They fix atmospheric nitrogen and are used as biofertilizers.

38. Physical properties of soil
• The physical properties of soil include soil texture and soil structure.

39. (i) Soil texture
• Soil texture may be defined as the proportions (percentages) of sand, silt, and clay
particles in a soil.
• Based on size, soil particles may be physically separated into three classes, called soil
separates.
• They are sand, silt, and clay.
• An agricultural soil normally contains all the three soil separates in varying proportions.
• The soil which contains mostly sand is called sandy soil, and the soil which contains
mostly clay is called clayey soil.
• When the three soil separates occur in almost equal proportions, the soil is called
loam.
• A perfect loamy soil does not occur in nature.
• Instead, one or two separates often predominate in the loam.
• Loamy soils are accordingly classified into sandy loam, clayey loam, silt loam and silt-
clay loam.

40. Major soil types
1. Sandy soil - Mostly sand
2. Clayey soil - Mostly clay
3. Loamy soil - Sand, silt and clay in almost equal proportion
(a) Sandy loam - Sand is predominant
(b) Clayey loam - Clay is predominant
(C) Silt loam - Silt is predominant
(d) Silt-clay loam - Silt and clay predominate.

41. Soil type can be determined by the procedure
explained by the concept diagram .

43. • Soil texture, in practice, cannot be changed in the field.
• However, the water retention capacity of sandy soils can be improved by
adding organic matter.
• Soil texture has implications in soil fertility.
• Clayey soils have high cation exchange capacity (CEC), i.e., the ability to attract
and hold cations.
• Sandy soils have low cation exchange capacity, which indicates low soil
nutritional status.
• Sandy soils have large pore spaces and hence they dry faster than clayey soils.

44. (ii) Soil structure
• Soil structure is the pattern of arrangement or grouping of individual soil
particles.
• In the soil, individual primary particles are arranged into soil aggregates or
secondary units of different shape and size, called peds.
• Soil structure affects pore size, water-holding capacity, infiltration rate, and the
permeability of water and air.
• Peds are easily friable and hence the soil structure may change, when the soil is
disturbed.
• Factors that change the soil structure include raindrops, tillage, traffic, etc.
• The vehicular transport and walking over soil cause soil compaction, which
retards drainage and rooting.

45. Chemical properties of soil
• Horticulturally important chemical properties of soil are
Soil fertility,
Mineral nutrition of plants and
Soil reaction (soil pH).

46. (i) Soil fertility
• Soil fertility refers to the ability of a soil to supply all the essential nutrients in
optimal amount in a readily available form.
• Cation exchange capacity (CEC) is an index of soil fertility.
• Many essential plant nutrients are positively charged ions, called cations (e.g.,
Ca2+, Na+, K+, and Mg2+).
• A fertile soil has the capacity to attract and hold these nutrients.
• The most efficient soil materials in cation adsorption are those with large
surface areas.
• Soil components, such as clay particles as well as soil organic matter, are
colloidal in nature.
• They possess large surface area and negative charge.
• Hence, they have greater attraction for cations and very high capacity for cation
exchange.

47. (ii) Mineral nutrition
• Mineral nutrition is the overall process by which plants absorb mineral nutrients
from soil, transport them to the different parts of their body, and finally utilize
them for the synthesis of biomolecules.
• Thus, mineral nutrition is completed in three stages, namely absorption,
conduction and assimilation.
• Plants absorb mineral elements from the soil through their root system mostly
as inorganic ions.

48. General roles of mineral nutrients
• Mineral elements, in their ionic and molecular states, serve several functions in the plant body.
Some of such functions are given below:
(i) Form the structural framework of protoplast and cell wall (e.g. C, H, O, N, P, S).
(ii) Enter into the synthesis of several biologically important molecules.
• [Nitrogen is a major constituent of proteins, enzymes, nucleic acids, vitamins, hormones,
pigments, cytochromes, etc.
• Phosphorus is the structural constituent of nucleic acids, nucleotides, nucleotide phosphates
(ATP, GTP, CTP), etc. Magnesium is the metallic part of chlorophyll.
• Calcium is a major component of the calcium pectate of the middle lamella of cell wall.
• Iron is the metallic part of cytochromes. Sulphur is a constituent of some essential amino
acids, such as methionine and cysteine].
(iii) Regulate the pH of the cytoplasm and vacuole sap.
(iv) Control the permeability of cell membranes (monovalent cations increase the permeability,
whereas divalent and trivalent cations decrease permeability).
(v) Mineral ions of the vacuole sap influence the osmotic potential of the cell.
(vi) Some mineral elements serve as activators or co-factors of enzymes (e.g. N, K, Ca, Mg, Mn).
(vii) Potassium ions play an active role in the opening and closing of stomata.

49. Specific biological roles of essential elements

52. Symptoms of mineral deficiency
• The visible symptoms of mineral deficiency in plants can be used to detect which
minerals are deficient in the soil so that appropriate remedial measures can be
taken.
• The commonest mineral deficiency symptoms include stunted growth, die back
of shoots (death of shoot meristems), chlorosis (loss of chlorophyll and the
yellowing and mottling of leaves), necrosis (death of tissues), premature leaf fall,
delayed or suppressed flowering and fruiting, distorted leaves, malformed parts,
poor reproductive development, etc.
• The plants, which show specific deficiency symptoms for a particular element,
can be used as indicator plants for assessing the nutrient level and quality of soil.
• The major roles and deficiency symptoms of the different elements are given in
the table.

53. (iii) Soil reaction or soil pH
• Soil reaction, or soil pH, is a measure of the hydrogen ion concentration of the
soil solution.
• It is an indication of the soil's degree of acidity or alkalinity.
• A pH of 7 is neutral.
• Values above 7 are considered alkaline, and values below 7 are acidic.
• Soil pH varies greatly with different types of soil.
• Accordingly, soil can be acidic, alkaline, or neutral.
• The normal pH of most soils usually ranges between 4.5 and 8.5.
• In rain-fed tropical areas, soil is usually acidic.
• But, in dry areas, it may be alkaline.

54. • In general, the upper layers of the soil are more acidic than the lower layers in
humid climate.
• But, in dry climate, the upper layers are less acidic than the lower ones.
• Plants and other soil organisms have different degrees of tolerance to soil pH.
• Thus, soil pH serves as a limiting factor that controls the growth and distribution
of plants and other soil organisms.
• Soil pH may rise in soils that are exposed to low rainfall, or are poorly drained.
• Salts tend to accumulate under these conditions.
• Soils formed from calcareous parent material have high alkalinity.
• Acidic soils (low pH) occur when soils are exposed to heavy rainfall and good
drainage because the bases leach to lower depths, or are washed away in the
runoff.

55. • Adequate amounts of soil nutrients ensure the availability of sufficient nutrients
to plants.
• Still then, soil moisture, soil temperature and soil pH interfere with the
availability of nutrients.
• Plant processes are generally slowed down by low temperatures
• Most horticultural crops tolerate a soil pH within the range of 4 to 8 (see the
table showing the relation between soil pH and the availability of nutrients). Soil
pH regulates the availability of nutrients.
• A pH of 7+ 1 appears to be a safe range for most nutrient elements in the soil.
• Decomposition of organic matter is solwest at pH below 6 and fastest between
6-8.

56. Soil types and treatment
a) Acidic soils
• Soil becomes acidic due to many reasons.
• For example, soils formed from acidic parent materials are naturally acidic.
• Sometimes, rain may cause the downward leaching of the lime in the soil,
making the top soil acidic.
• Often, the hydrogen ions secreted by the plant roots make the soil acidic.
• Most nitrogen carriers are acids and so their addition may acidify the soil.
• Low soil pH may be corrected in practice by liming.
• Liming is the addition of either calcium alone or calcium and magnesium
together to the soil.
• In the strict sense, the term lime refers only to calcium oxide (Cao).
• However, it is often loosely applied to include calcium carbonate, calcium
hydroxide and calcium-magnesium carbonate also.

57. Effects of liming on soil
1. Lime neutralizes soil acidity.
2. Lime promotes the growth of beneficial bacteria, which decompose organic matter and
promote nitrification.
3. Liming increases the availability of phosphorus and potassium to plants.
4. Liming improves the physical condition of the soil by increasing the infiltration capacity.
• The liming materials used in agriculture include powdered lime stone (CaCO3), burnt lime
(Cao) and hydrated lime (Ca (OH)2)
• Lime stone refers to rocks consisting chiefly of calcium carbonate or calcium and magnesium
carbonate (CaMg (CO)2.
• When magnesium carbonate is present, the material is called dolomite.
• Lime should be applied before ploughing, or it should be mixed with the soil after ploughing
the land.
• When large quantities of lime are to be applied, it is better to apply small quantities each in
two or three instalments, rather than administering a heavy single dose.

59. b) Saline soils
• These are the soils which contain enough soluble salts, but never an excess of
exchangeable sodium.
• Excess of soluble salts, in all probability, impairs the productivity of plants.
• Generally, the pH of such soils is less than 8.5. Saline soils are often recognized
by the presence of white grains of salts on the surface due to the presence of
an excess of the chlorides, sulphates and sometimes nitrates of Ca and Mg.
• Therefore, saline soil is often referred to as white alkali soil.

60. c) Saline alkali soils
• These are the soils containing soluble salts in sufficient quantities
which interfere with the growth of most crop plants.
• They contain sufficient exchangeable sodium which adversely affects
the soil properties and plant growth.
• The pH of saline alkali soil is above 8.5.

61. d) Non-saline alkali soils
• These are the soils containing sufficient quantity of exchangeable sodium, but
not appreciable quantities of soluble salts.
• The pH of such soils usually ranges between 8.5 and 10.
• Exchangeable sodium may interfere with the growth of most plants.
• The treatment of saline and alkali soils gypsum, sulphur, iron sulphate and lime
stone. Gypsum is a natural sulphate of calcium.
• It reacts with exchangeable sodium and converts the latter to sodium sulphate.
• Sodium sulphate leaches from the top soil to reduce the soil pH.
• Gypsum must be applied on the soil surface and then mixed with soil two to four
weeks before sowing a crop.

62. Soil analysis / soil testing
• One of the major requirements for a good crop yield is the presence of essential
plant nutrients in the soil in adequate quantities and readily utilizable forms.
• For maximum production and rational soil management, a sound knowledge of
the fertility status and physical properties of the soil is essential.
• Soil testing is the best method of determining the fertility status of the soils.
• Based on the recommendations of a soil expert, soil amendments can be made.
• In fact, soil testing is an essential part of any scheme of agricultural
development.
• Soil testing laboratories have been established in almost all the states.
• Soils are analysed and recommendations are made in respect of the fertilizer
requirement for different crops.

63. Soil sampling
• Soil tests and the interpretations of their results are based on the samples analysed.
• It is, therefore, important that soil samples should be properly collected and they
should represent particular areas.
• For routine soil testing, the field is divided into portions according to variations.
• Separate samples are collected from the different portions.
• These samples are mixed thoroughly and spread on a clean sheet of paper, or on a
piece of cloth, and divided into four equal parts.
• Two opposite quarters are rejected and samples from the other two are mixed and the
procedure is repeated till the desired quantity of the sample is obtained (1-2Kg).
• The final sample is collected in a plastic bag and placed in a box or cloth bag. Moist
samples must be dried before collecting.
• Care has to be taken to avoid contamination.
• The bag or the box must be properly labelled and sent to the nearest soil testing
laboratory along with the information sheet.

64. Principles of soil sampling
When the different areas within a field differ distinctly from each other in crop
growth, soil type, elevation, or cropping history, divide the field suitably and sample
each area separately.
Avoid collecting samples from old bunds, marshy spots, hedges, etc, which do not
represent the field.
Do not sample a field within three months after the application of lime or fertilizers.
Depth of sampling should be according to the farming situations:
(a) Take samples from 15 cm depth in ordinary farming situation.
(b) If perennial crops are to be grown, collect soil sample from 25 cm depth.
(c) For pastures and lawns, take samples from 5 cm depth.
 Samples must be collected from areas between the lines, where crops have
been planted in lines.
 Use proper sampling tools. A soil tube, an auger, or a spade can be used according
to the situations. In wet lands, soil auger is the best tool, while in dry lands both the
soil tube and spade can be used conveniently. The soil parameters, analyzed in the
laboratory, must include soil pH, total soluble salts, available nitrogen, available
phosphorus and available potassium.

66. Soil test interpretation and fertilizer
recommendations
• Based on the result of the analysis of soil samples and the results of
field experiments, fertilizers are recommended.
• Additional information regarding the time and the method of
application of fertilizers, precautions to be taken, etc. may also be
obtained, if necessary.