This document discusses soil formation and the physical properties of soils. It defines soil and lists the major factors that influence soil formation: parent material, climate, biota, relief, and time. The key physical properties discussed are texture, structure, bulk density, porosity, color, consistence, temperature, air, and water. Forest soils tend to have darker color, higher clay content, lower bulk density, greater porosity, better structure, and ability to hold more water and nutrients compared to other soil types due to higher organic matter levels.

1. FUNDAMENTALAND FOREST
SOIL SCIENCE (FCS_ 2072)
Instructor’s Full Name: Habtamu Admas Desta (PhD)
Email: habtamuadmasu35@gmail.com
Telephone: +251913489307

2. CHAPTER I. SOIL FORMATION
1.1. Weathering: physical and chemical
Definition of soil
 Different definitions
Housekeeper-think of soil as a mud
Archaeologists-as records of the past
Geologists-as a skin cover of minerals & rocks
Hydrologists-a sink and reserve of water
Engineers- materials upon which foundation are erected
Urban planers-as a sink for waste disposal
Farmers-a habitat for plants (medium for plant growth)

3. Weathering
 The physical and chemical alteration of rocks and minerals
 Combination of destruction and syntheses
 Two major types of weathering, i.e. mechanical (physical) and
chemical
 Mechanical processes considered as disintegration & chemical
processes as decomposition
 By decomposition, definite chemical changes take place, soluble
materials are released, and new minerals are synthesized

4. 1) Physical or Mechanical weathering (Disintegration) by factors of
a) Temperature – expansion& contraction of minerals, frost, exfoliation of
rocks as well as salt crystallization by evaporation
b) Erosion and deposition - water, ice and wind
c) Plant roots and burrowing animals influences
2, Chemical weathering (Decomposition) by factors of
a) Hydrolysis (chemical breakdown of minerals when combined with water)
b) Hydration (addition of water to minerals e.g. formation of gypsum
CaSO4.2H2O) & attachment of H+ and OH- to molecules
c) Carbonation (formation of CaCO3 through reaction of CO2)
d) Oxidation (reaction of substances with oxygen)
e) Reduction (losses of oxygen)
f) Solution (certain minerals dissolved by acidic solutions)
waterwater

5. Factors Affecting Weathering of Rocks
1. Climatic Conditions (hot & humid climate fastens weathering)
2. Physical Characteristics of rocks (crystals)
3. Chemical and Structural characteristics e.g. Gypsum is easily
weathered because of its solubility.

6. 1.2. Soil forming factors
Five factors of
Parent materials
Climate
Biota
Relief (Topography)
Time

7. a) Nature of parent material (texture, structure, chemical and minerals
composition of rocks)
b) Climate (most important factor particularly temperature and
precipitation)
c) Living organisms/fauna & flora ( especially the native vegetation)
d) Relief (topography) as erosional or eluviation and depositional or
illuviational processes
e) Time that the parent materials passed subjected to soil formation
(soil formation is slow and long process)

8. 1.3. Soil forming processes-formation of forest soil
 Are pedogenic or horizonation processes
a) Additions to the soil/illuviation/deposition
b) Losses from the soil/eluviation/erosion
c) Transfer (translocation) with in the soil
d) Transformations/change with in the soil
 Forest soils are fertile and productive soils with micro-organisms
 Soils in the root zone /rehizosphere
 Forest soils are rich in nutrients, low in bulk density, high in
porosity, high in pH, high in water and air circulation with good
structure

9. 1.4. Soil profile
 Vertical sections of a soil with horizons (horizons are layers of a soil)
 Horizonation is soil development processes (heterogeneous layers)
 Soil formation consists of the evolution of soil horizons
 Haplodization is no horizon creation and no soil development i.e.
homogeneous type of profile
 With six major/master horizons
1. O- horizons (organic)
2. A- horizon (mixture horizon)
3. E (Eluvial)- horizon
4. B (Illuvial)- horizon
5. C- horizon
6. R/D – horizon

10. Soil Horizons
 There are head/master horizons that are designated as O, A, B, C & D
Organic horizons
 above the mineral soil
 as a result of litter derived from dead plants and animals
 occur commonly in forested areas
 absent in grassland regions/haplodized soil
Formation of O- Horizons
 vegetation produced in the shallow waters of lakes and ponds
accumulate as sediments of peat and muck
 Accumulated because of a lack of oxygen in the water for their
decomposition
 Organic soils have 0 horizons; the O refers to soil layers dominated by
organic material

11. 1) A- Horizon (mixture horizon):
 is mineral horizon that lies at/near the surface.
 It is a strong mixture of humified organic matter & mineral soils.
 It is much darker than the underlying E/B horizons.
 Zone of erosional and leaching
 Top soil + Root Zone
2) E (Eluvial)
 horizon of maximum eluviation
 The symbol E is derived from eluviation, meaning, "washed-out."
 (e = ex; luv = washed) of clay,
 Fe, Al (oxides) concentration of resistant minerals such as quartz.
 It is generally lighter in colour than the A horizon
 called bleached horizon.
 It has a lower clay content

12.  Both the A and E horizons are eluvial in a given soil.
 The main feature of the A horizon is the presence of organic matter and a
dark color,
 The E horizon is a light-gray color & having low organic matter content
and a concentration of silt and sand-sized particles of quartz and other
resistant minerals
3) B (Illuvial) horizon
 illuviation from above or below has taken place.
 It is a region of maximum accumulation of Fe & Al oxides and silicate
clays in humid areas
 Mainly Bt horizon & sometimes called subsoil
 In arid areas CaCO3, CaSO4 and other salts may accommodate in the
lower B.
 A and B horizons called solum/true soil

13. 4) C- horizon:
 is the unconsolidated material underlying the solum (A and B).
 It is outside the zones of major biological activities (below 2 m depth)
 is little affected by solum/soil forming processes.
5) R/D – Horizon:
 The consolidated bed rock.
 A soil profile may not show all those horizons.
 There are cases where the surface horizons are eroded and subsurface ones
are exposed.
 Usually B-horizon comes up to the surface.
 In such cases, the profile is called truncated.

15. CHAPTER II. PHYSICAL PROSPERITIES OF SOILS
 texture, structure, particle and bulk densities, pore spaces, soil
colour, soil consistence, soil water, soil air and temperature
3.1. Soil Texture
 relative proportion of particles/separates i.e. sand, silt & clay

16. Rock fragments:
2mm-7.5cm - gravel
7.5-25cm - stone
>25 cm - boulder
Physical nature of the soil separates
1, Sand:
Feels gritty, not sticky unless coated by clay and silt
Has very low degree of plasticity
Has low water holding capacity & rapid drainage due large pores in
between grains
Low organic matter & CEC
Low surface area/large size
High in aeration
2, silt:
 intermediate nature between sand and clay.
 sand and silt separates dominated by quartz

17. 3. Clay
 has a very high surface area/small size to volume ratio
 A given mass of clay has 10000 times as much surface area as the same
mass of medium sized sand.
 affects water, nutrient, gas and the attraction of particles.
 The clay fraction usually has a net negative charge.
 The negative charge adsorbs nutrient cations, including Ca2+, Mg2+, and K+
and retains them in available form for use by roots and microbes.
Mineralogical and chemical composition of soil separates
Coarse Sand
 dominated by Quartz. Gibbsite, hematite and limonite
Clay:
 Kaolinite, illite, vermiculite and montimorillonite or smecitite dominate
the fine clay fraction

18.  Soil textural class names have become standardized to express the
variation of soils in composition of the different size particles (sand,
silt and clay)
Sandy Soil
 contains >70% sand by weight
 Textural classes of such soils are Sand and Loamy Sand
Clay Soil
 contains >35 or 40% clay separate by weight
Loamy Soil
 Exhibits heavy and light properties in about equal proportions
 It is agriculturally important soil
 have the greatest productivity of crops
 A soil with 40% sand, 40% silt and 20% clay is described as a loam
 Textural class name is normally given after the proportion of the
different soil separates is known

19. Soil textural classes

20. Soil Structure/peds
 overall aggregation, or arrangement of the primary soil separates
 influences water movement, heat transfer, aeration, bulk density and
porosity
 classified based on three parameters: by Type/shape, by Class/size
& by grade/strength of the peds
 By types/shapes of soil structure
1. Platy (horizontal & surface clay)
2. Prism-like (vertical & B-horizons of arid) with
 Columnar: when the tops are rounded
 Prismatic: when the tops are level plane and clean cut
3. Block-like
six faced, with the 3 dimensions more or less equal
classified as Blocky & Sub-angular blocky

21. 4. Spheroid
 common in A-horizons that are high in OM
 divided into two types.
 Granular - relatively less porous
 Crumb - very porous
Structure-less soils
1. single grain in sandy soils
2. massive soils in clay soils

23. Importance of Structure
 affects water & air movements and root penetration
 claypan (Bt horizon) difficult soil
 Practical management of soil structure restricted to the topsoil or
plow layer in regard to use of soils for plant growth
 A stable structure at the soil surface promotes more rapid infiltration
 Ped stability can be possible by materials of microbial gum, organic
carbon, iron oxide and clay
Particle and bulk densities of mineral soils
 Density of mineral soils (mass per unit volume)
Particle density:
 mass of a unit volume of soil solids (g/cm3)
 for mineral soil ranged between 2.6 - 2.75 g/cm3
 for organic soils ranged from 0.1 to 0.6 g/cm3.
 The average particle density for mineral soils is usually given
as 2.65 g/cm3

24. Bulk density
 mass per unit volume of oven-dry soil
 In BD, volume for soil solids and pore spaces
 Bulk density of a soil depends on the porosity and OM content of
the soil (inversely related)
Eg. soil has a volume 1cm3 and it weighs 1.33g oven dried. bulk
density 1.33g/1cm3 = 1.33g/cm3. Assume that 50% of the volume of
the soil is occupied by pore spaces.
If soil is compressed and all the pore spaces are removed. What
remains is only a volume of 0.5cm3. The particle density of this soil,
therefore, 1.33g/0.5cm3 = 2.66g/cm3.

25. Total Porosity
 Pore space of mineral soils
 occupied by air and water
 % Pore space = 100 - % solid space = (1 – BD/PD)100
 clay soil has the highest total porosity
 higher the OM contents, the higher porosity
 As soil depth increases, porosity decreases (low OM & over weight
of the above)
Size of pores: macro & micro – pores
E.g.. bulk density 1.4g/cm3 and particle density 2.65 g/cm3. Calculate
its % pore space and % solid space
Soil colour
 result of OM and Fe contents of the soil
 Important as indirect measure of important characteristics water
drainage, aeration, & OM

26. Judgements from soil colour
 Brown to black colour: results from OM
 White to light grey: OM leached down, sandy soils and E-horizons
 Yellow to Red: due to iron oxides & in warm areas
 Bluish grey: un-oxidized iron, lack of oxygen
 Mottling: Alternating water saturation and drying of the subsoil
 Greenish, bluish, and grey colours in the soil indicate wetness while
bright colours (reds and yellows), indicate well-drained soils
 The light & grayish colors of E horizons by illuviation of iron
oxides & low OM content
 matching the color of a soil sample with color chips in a Munsell
soil-color book
 having color chips arranged systematically according to their hue
(dominant wavelength), value (quantity of light), and chroma (purity
of the dominant wavelength of the light)
Eg. 10YR 6/4, 10YR is the hue, 6 is the value, and 4 is the chroma

27. Soil consistence
 behaviour of soil towards mechanical stresses or manipulations
 determined by cohesive & adhesive properties of the entire soil mass
 strength of forces b/n sand, silt, and clay particles
 Consistence is important for tillage and traffic considerations
 described at three moisture levels: wet, moist & dry
 Plasticity of a soil is the capability of soil being molded
Soil Air
 Soil air differs from atmospheric air in many aspects
 atmosphere contains by volume nearly 79% N, 21% oxygen and
0.03% carbon dioxide
 Soil air contains high relative humidity & CO2
 Respiration of roots & organisms, consumes oxygen and produces
carbon dioxide
 soil air contains 10 to 100 times more carbon dioxide and slightly
less oxygen than does the atmosphere
 carbon dioxide diffuse out of the soil and oxygen diffuse into the
soil

28. Soil Temperature
 Many seeds need a certain minimum temperature for germination
 Below freezing, there is extremely limited biological activity
 A soil horizon as cold as 5°C acts as a determinant to the elongation of
roots
 alternate freezing and thawing of soils results in the alternate expansion
and contraction of soils
 Soil has high temperature than the atmospheric air because of high bulk
density and respiration effects
 In the absence of soil temp data, it can be estimated by adding 2.5oc to
mean annual air temperature
Factors influencing soil temperature
i. Local climate: Soil temperature is highly correlated to air temperature
ii. Slope steepness and aspect
iii. Topography

29. i. Cover: Plants shade the soil, reducing the temperature
ii. Soil colour: Dark-coloured soils absorb heat more
iii. Mulching: reduces heat by reducing insolation
Soil temperature influences on soil properties
i. Biological activity
ii. Organic matter accumulation: Lower temperature = higher organic
matter accumulation
iii. Weathering of parent materials: Fluctuating temperatures help the
physical breaking down of rock and mineral grains
iv. Nutrient availability: Many nutrients are unavailable or poorly
available at low temperatures (low biological/MOs activities)

30. Physical prosperities of forest soils
Forest soils have physical properties of
 Black or brown colour by OM
 More of clay texture due to low erosion & leaching
 Low bulk density by OM
 High porosity by OM
 High water holding capacity & good aeration by OM
 High infiltration rate by OM
 Good soil structure by OM
 Better root penetration
 Low compaction/sealing and good consistency by OM
 High in CEC, pH and nutrients (C, N, S, Ca, Mg, etc) by OM
 High in carbon contents (carbon sequestration)
 Rich in microorganisms
 Colloidal sites

31. CHAPTER III. SOIL WATER
 Soil-water is the part of the hydrosphere where water is held in
the soil, either by adhesive forces existing between water &
soil material or by capillary force caused by the soil pores &
the surface tension of the water.
 Water containing a variety of mineral substances in solution,
dissolved oxygen & carbon dioxide
 Water is essential for plant growth.
 Without enough water, normal plant functions are disturbed, and the
plant gradually wilts, stops growing and dies.
 Soil water is also called rhizic water.
 There are three main types of soil water
 Gravitational water (water moving through soil by the force of
gravity) macro pores water, unavailable

32.  Capillary water (water held in the micro pores of the soil & water that
composes the soil solution), available to plants as it is trapped in the soil
solution right next to the roots if the plant
 hygroscopic water (very thin film surrounding soil particles), water is
found on the soil particles and not in the pores, generally not available to
the plant
Importance of Soil Water
 Medium through which nutrients reach & absorbed by the roots
 When the soil solution is deficient in one or more of the nutrient elements
needed for plant growth, the soil is infertile
 The upper and lower limits of water availability for a particular soil
depend on its field capacity and wilting percentage respectively
3.1. Classification of soil water
 retention refers to moisture holding by soil particles
1. Maximum retention capacity- all the pores in the soil are filled with
water, saturated

33. 2. Field capacity (FC)- macro pores filled by air while the micro
pores still contain water. Plants take up the water actively
- amount of water remaining in a soil two or three days after it has
been thoroughly wetted
3. Wilting Coefficient (critical moisture, WP)- due to
evapotranspiration at FC. plants start to wilt. If the condition persists,
plants remain wilted i.e. they exist in a permanently wilted condition
- Water is lost from the soil by downward percolation &
evapotranspiration
4. Hygroscopic coefficient- retention of water molecules held around
soil particles or colloids, as adsorbed moisture by more drying
5. Air dry: the moisture content of an air-dry soil is at equilibrium with the
atmosphere
6. Oven dry: the moisture content remaining in the soil after the soil has
been dried at 105 - 1100c until no more water is lost

34. Conventional Soil Moisture Classification Schemes
 two types of soil water classification
1. Physical classification
 gravitational water: Water in excess of the field capacity
(saturated)
 Capillary water: water held in micro pores (includes most water
taken up by growing plants)
 Hygroscopic water: water bound tightly by the soil colloids
2. Biological Classification:
 available water: Moisture retained in the soil between the
field capacity and permanent wilting coefficient
 Readily available water (RAW) is that portion of available water
that the crop can use without affecting its evapotranspiration and
growth
 unavailable water: Water held at Wilting Coefficient.
Gravitational water is also unavailable water

36. Relationship of soil texture to available water-holding capacity of
soils. The d/ce b/n the water content at FC & water content at PWP is
the available water content

37.  Most of the water that enters the plant roots
does not stay in the plant.
 Less than 1% of the water withdrawn by the
plant actually is used in photosynthesis (
assimilated by the plant)
 The rest of the water moves to the leaf
surfaces, where it transpires (evaporates) to the
atmosphere.
 The rate at which a plant takes up water is
controlled by its physical characteristics, the
atmosphere and soil environment
 Plants can extract only the soil water that is in
contact with their roots
 During the course of growing season, plants
extract more water from the upper part of their
root zone than from the lower part.

38. Exercise: A soil having the following weights will serve as an example
for some simple soil moisture calculation
• Weight of soil at FC = 190g
• Weight of air dry soil = 140g
• Weight of soil at WP = 160g
• Weight of oven dry soil = 130g
Calculate A. percentage of H2O at FC
B. percentage of H2O at PWP
C. percentage of available H2O
Solution
130g (oven dry soil) = 100% soil (0% water)
190g FC Water = ?
(190g*100)/130g = 19000/130 = 146.15 (100% soil + water)
146.15 – 100% soil = 46.15% water at FC

39. Factors affecting the amount and use of available soil moisture
1. Crop type
2. Climate
3. Soil (texture, structure, moisture content, salt content, etc)
4. Organic matter
5. Crop growth stage
 Initial stage
 Crop development stage
 Mid-season stage
 Late season stage
6. Topography
7. Soil depth and layering
8. Depth of ground water table
9. Compaction effects
10. Osmotic effects

40. Movement of soil water
 Amount of water varies with time & depth b/c of
 supply (rain fall, irrigation, infiltration, flood) &
 demand (evapotranspiration, uptake, percolation) by its environment
i.e. drainage & wicking
Three types of water movement
 Saturated flow/steady state
 Unsaturated flow macro pores are filled with air & micro pores with
water
 Vapour movement (capillary/wicking)

41. Loss of soil water and hydrological cycle
 Hydrological Cycle is a series of movements of water above, on, and
below the surface of the earth
 continues cycle of water b/n earth & atmosphere
 movement of water occur in solid, liquid &vapour forms
 water cycle consists of four distinct stages (storage, evaporation,
precipitation and runoff)
 Evaporation is the process by which liquid water changes to water
vapour
 Evaporation of ice is called sublimation (ice to vapour)
 Evaporation from the leaf pores, or stomata of plants is called
transpiration

42.  The amount of water evaporates from the ocean, land, plants,
and ice caps are equal to precipitation falls back on the earth
 Loss of water from the soil by
 Percolation – down ward movement of free water below the root zone
 Runoff- loss of excess water from the soil surface
SOIL WATER BALANCE
 Balance b/n input and output
 Rainfall, irrigation and capillary rise of groundwater towards the
root zone add water to the root zone and decrease the root zone
depletion.
 Soil evaporation, crop transpiration and percolation losses remove
water from the root zone and increase the depletion.

43. CHAPTER IV. SOILAIR AND TEMPERATURE
 Soil air is air occupied by soil pores (macro and micro pores)
 Soil aeration exchange of CO2 & O2 gases b/n soil pore space &
the atmosphere
 Well aerated soil
 Enabling growing aerobic organisms in adequate amounts
 Encouraging optimum rates essential metabolic processes
 Accelerating root growth and plant development
 The content of CO2 in soil air may vary from 10 -10,000 times
 Soil air contains a much more CO2 & less O2 than atmospheric air
 Poor aeration causes abnormal development of roots
Gas Soil air volume (%) Atmosphere air volume (%)
Oxygen 20.0 21.0
Nitrogen 78.6 78.03
Carbon die oxide 0.5 0.03
Argon 0.9 0.94

44. Availability of air (oxygen)
 Gaseous diffusion in soil dependent on pore space continuity
 Oxygen availability in field is regulated by three factors of soil macro
porosity, soil water content and consumption by roots & micro-organisms
 When oxygen is diffusing through a macro-pore and encounters a micro-
pore (filled with water), the water-filled micro-pore acts as a barrier to
further gas movement and make oxygen unavailable
 Oxygen deficiencies are created when soils become water saturated
 Clayey soils are susceptible to poor soil aeration when wet because most
of the pore space consists of micro-pores filled with water
 A desirable soil for plant growth has a total porosity of 50%, which is one
half macro-pore porosity and one half micro-pore porosity
 Such a soil has a good balance between the retention of water for plant use and an
oxygen supply for root respiration
 Occurred at field capacity

45. Factors affecting soil aeration
1. Amount of air space/pores
 The top soil contains much more pore spaces than the sub-soil
& gaseous exchange is more in the top soil than in sub-soil
2. Soil organic matter (the more SOM, the more aeration)
3. Soil moisture (the more water content of the soil, the less aeration)
4. Cultivation
5. Compaction

46. Redox Potential
 Redox potential (Eh) is the measurement of the tendency of an environment
to oxidize or reduce substrates
 In well-oxidized environment, the redox potential will be highly positive
 In reduced environments (saturated soil) the redox potential will be low
 Reduced soils (saturated/flooded soils) affected by Fe & Mn toxicity as
well as deficiency of available Sulphur (sulphate) and nitrate
 Anaerobic soils retard the development of root hairs
 Oxygen is needed for aerobic respiration in soil
C6H12O6 + 6O2 ↔6CO2 + 6H2O

47. Aeration in relation to soil & root ability (crop management)
 For crops in the field, aeration enhanced by maintaining soil
aggregation, tilage & drainage
 Clay soils have low aeration for root activities and plant growth
 Compacted soils and soils with high bulk density is poor in aeration
and root penetration
 Flooded areas and saturated fields / waterlogged regions are poor in
aeration and root penetration
 Drainage of flooded fields and tillage of compacted soils improve
soil aeration as crop management
 Sodic soils with structure disturbance have low aeration and root
penetration which needs gypsum and then leaching for its
management and reclamation

48. Soil Temperature
 Soil temperature influences physical, chemical & microbiological
processes that take place in soil
 Soil temperature is required for calculating most belowground
ecosystem processes of root growth & respiration, decomposition
and nitrogen mineralization
 Soil temperature affects water and nutrient uptakes, microbial
activities, nutrient cycling, root growth, and many other processes
 The temperature of soil is a significant parameter in agriculture for
efficient plant growth & seed germination
 Most soil organisms function best at an optimum soil temperature

49.  Soil temperature impacts the rate of nitrification
 It also influences soil moisture content, aeration and availability of
plant nutrients
 Soil temperature at 5 cm depth followed a similar pattern to the air
temperature
 The factors that affect the amount of heat supplied at the soil
surface & temperature include:
 Soil colour, Soil mulch, Slope of the land surface,
 Vegetative cover, Organic matter content,
 Evaporation, compaction, moisture & Solar radiation

50. Processes affected by soil temperature
 Soil temperature affects the physical, chemical & biological
processes
 Plants are more sensitive to soil temperature than air temperature as
it affects shoot growth & photosynthesis than root growth
 Soil temperature greatly affects seed germination
 Soil temperature affects plant growth indirectly by affecting water
and nutrient uptake as well as root growth
 At a constant moisture content, a decrease in temperature results in
a decrease in water and nutrient uptake
 At low temperatures, transport from the root to the shoot and vice
versa is reduced

51.  Soil temperature is one of the most important transient soil physical
ecosystem processes, including root growth and respiration,
decomposition
 Soil temperature alters the rate of organic matter decomposition
and mineralization of different organic materials
 Microbial processes are influenced by soil temperature changes
 Most soil organisms function best at an optimum soil temperature
 Soil temperature impacts the rate of nitrification, influences soil
moisture content, aeration and availability of plant nutrients

52. Thermal properties of soils
 Dry soil is more easily heated than wet soil
 Thermal properties dictate the storage and movement of heat in soils
 Heat flux in soils is a function of time and depth
 Evaporation has the potential of cooling the soil
 The ability to monitor soil heat capacity is an important tool in
managing the soil temperature regime to affect seed germination and
crop growth

53. Soil temperature control
 Soil temperature can be managed/controlled by soil mulching /
covering & reducing excess soil moisture
 Organic mulching & plant-residue management influence soil
temperature
 Poorly drained soils have temperature 3 – 6 degree Celsius lower
than well drained soils

54. CHAPTER V. SOIL COLLOIDS: THEIR NATURE AND
PRACTICAL SIGNIFICANCE
 Colloidal particles referred to as micelles/ micro cells/exchange sites
 Are seat for soil chemical and physical activities
 Colloids are occupied by negatively charged very small sized clay
and humus
 particles with diameter less than 0.001 mm
 All clays (diameter < 0.002 mm) are not colloids
 sites/store house within the soil where ions of essential mineral
nutrients
 essential ions can be withdrawn from the colloidal bank sites and
taken up by plant roots

55.  Important from the point of view of nutrient availability to plants
 On an oxide basis, silicon and aluminum are first and second in
abundance next to oxygen
Mineral contents of the crust

56. General properties and types of soil colloids
 Extremely small size
 Having large surface area (a unit mass of colloidal clay is at least
1000 times greater than that of 1g of coarse sand)
 Dominated by electronegative charges except in a very acid soils
 Adsorbing cations and anions (colloids attract ions of opposite
charges)
 Adsorption of water- the larger surface area of colloids, the greater
water amount that it holds

57. Types of soil colloids
four major types of soil colloids
1) Layer silicates clays
2) Iron and aluminium oxide clays,
3) Allophane and associated clays
4) Humus
two broad categories as inorganic (clay minerals)
and organic (humus)

58. Layer Silicate Clays
 are overwhelmingly negatively charged with high cation exchange
capacity (CEC) which are dominant in soils
 cation exchange capacity originates mainly from isomorphic
substitution
 Silicate clay minerals are the dominant inorganic colloids
 properties with layer like crystalline structures & build by Silicon-
tetrahedral and Aluminium octahedral sheet
 layers are comprised of planes of closely packed oxygen atoms
held together by silicon, aluminium
 Six oxygen atoms coordinating with
a central Al or Mg atom form the
shape of an eight-sided geometric
solid, or octahedron

59. Allophane and associated clays
 Non crystalline (amorphous) minerals exist by glassy ash and
cinders
 Volcanic origin
 lack ordered three-dimensional crystalline structures
Iron and aluminium oxides
 occur in the highly weathered acidic soils of tropics and
semi-tropics having red colours
 not sticky and plastic
 Examples of Iron and Aluminium oxides are goethite
(FeOOH), haematite (Fe2O3) and gibbsite AL (OH) 3.
 In very acid soils (pH < 4), they carry a net positive charge
(H+) and attract negatively charged ions

60. Organic colloids (humus)
 Humus is composed mainly of C, H and O
 It has very high CEC (net negative charge)
 A swarm of cations surrounds a highly charged micelle
 Humus is not crystalline, having variable sizes, but smaller than
montimorillonite clay

61. Genesis of layered silicate clays
 Silicate clays developed by two processes
1, physical and chemical alteration of minerals
2, decomposition of primary minerals & recrystallization in to silicate clays
 Alteration – result of weathering where the mineral is broken
down in size to colloidal range & become less rigid crystal
structure
 Recrystallization – complete break down of crystalline structure &
recrystallization of clay minerals from products of this breakdown i.e. result
of much more intense weathering than alteration process
eg formation of kaolinite from a 2:1 mineral solutions
 Fine grained micas, chlorite & vermiculite are formed through mild
weathering of primary aluminosilicate minerals whereas kaolinite &
oxides of iron & aluminium are high degree of weathering

62. Types and characteristics of crystalline silicate clays
 Based on the number & arrangement of layers, silicate clays are
classified into four groups
1. 1:1 Type: (E.g. kaolinte, halloysite, nacrite, dickite)
 held together by rigid hydrogen bonding
 Layer consisting of one tetrahedral silica sheet & one octahedral
alumina sheet
 Each layer contains one Si tetrahedral and one Al octahedral
sheet
 Kaolinite is the most dominant in the soil
 pH dependent
 low CEC
 large in size/coarse clay
 resistant to weathering/highly leached
 found in older/ intensively weathered soils
 very low plasticity, cohesion, shrinkage and swelling

63. 2:1 type expanding minerals (montimorillonite/ smectite
and vermiculite)
 One octahedral sheet sandwiched b/n two tetrahedral sheets
 Weak oxygen-to-oxygen linkages
 One octahedral sheet sandwiched b/n two octahedral sheets
 have expansion of crystals
 high CEC/high negative charges
 small in size/fine clay
 high plasticity, cohesion, swelling-shrinkage
2:1 type Non-expanding minerals (illite, chlorites)
 similar to 2:1 expansion but K ion fits between the crystal lattice
 no expansion
 properties lie between that of kaolinite and montimorillonite

64. 2:2 type minerals (silicates of Mg with some Fe and Al)
 two silica and two Mg make up the unit
 Have properties similar to kaelonite

65. Sources and types of charges in colloids
Two major sources of charges
 Hydroxyls & other functional groups releasing H+
 Isomorphs substitution in some clay crystals of one cation by
another similar size but differing in charge
All colloids associated with OH- groups are largely pH dependent
1. Negative charges
 originated from two sources
i. Exposed crystal edges (pH dependent):
 Unsatisfied valences at the broken edges of the silicon and
aluminium sheets
 minerals such as kaolinite have some exposed oxygen and
hydroxyl groups, which act as negatively charged sites
 At high pH, the H+ dissociates leaving a negative charge carried by
the oxygen
 this type of charge is pH dependent charge/depend on soil pH
 Charges associated with humus, 1:1 type clays, oxides of iron &
aluminium, allophane are pH deependent

66. ii. Ionic (isomorphic) substitution
 Are constant/permanent charges
 Silicon in tetrahedral & Al in the octahedral are subjected to
substitution by atoms of similar size
 When there is no substitution, positive & negative charges are
balanced
 Negative charge resulted when a lower charged ion substitutes a
higher charged ion
 if Al3+ atom replaces Si4+ in tetrahedron, one extra negative
(deficiency) charge will be created
 Similar result will happen when Mg replaces the Al in the
octahedron unit
 charge formed by this process does not change with the soil’s pH
(permanent charge)

67. 2. Positive charges
 Isomorphic substitution can be source of positive charges if
 the substituting cation has a higher charge than the ion, which
substitutes it
 e.g. if Al3+ ion substituted by Mg2+ ion, a positive charge remain
Physical and chemical properties of mineral colloids (CEC, AEC,
nutrient availability)
 Charges associated with soil particles attract simple and complex ions
of opposite charges
 In temperate regions, negative charge predominates with high CEC
 In tropics, positive charges predominate and anion exchange is
relatively more prominent
 CEC is positively correlated with pH while AEC correlated inversely
with pH of the soil

68.  Refers to the maximum number of positive charges that a given
amount of soil can adsorb
 The maximum number of negative charges that a given amount of
soil has
 CEC is defined as the sum of positive (+) charges of the adsorbed
cations
 The sum of the CEC of humus, silicate clays, hydrous oxides and
others
 The interchange between a cation in soil solution and another cation
on the surface of any negatively charged material (micelles)
 Cations are adsorbed and exchanged on a chemically equivalent
basis
 i.e. one K+ replaces one Na+ & two K+ are required to replace for one Ca++
Cation exchange capacity (CEC)

69.  CEC dependent on a number of factors
 pH (charge increases with pH)
 Type & amount of clays
 OM
 CEC is expressed in equivalents or milliequivalents or
centimoles per kg
 (1 milliequivalent/100 g = 1 cmolc/kg)
 “Equivalent weight is 1gm of H+ (atomic weight of an ion that
replaces 1gm of H+)”
Types of colloids and their CEC
Colloid Type CEC (meq/100g of soil
Humus 100 – 400
Vermiculite 150
Montimorillonite 100
Chlorite, illite 30
Kaolinite 8
Hydrous oxides 4

70. Anion exchange and adsorption (AEC)
 Anion exchange sites arise from the protonation of hydroxyls layer
clays
 The positive charges of kaolinite, Fe/Al oxides and allophane attract
and adsorbe anions
 AEC inversely related to soil pH and importance in acid soils
dominated by oxidic clays
 The availability to plants of the anions nitrate, phosphate, and
sulfate is related to mineralization from OM as well as anion
exchange
 Kaolinite & hydrous oxides of Fe & Al have positive charges on
their crystal surfaces
 H+ ion concentration in the soil solution increases soil acidity
 The protonation or adding of H+ to the OH-
E.g. Al-O---H Al-OH2
+
at low pH

71. Nutrient availability and uptake
 Nutrients are up taken by plants in their available forms, soluble form and
must be located at the root surface
 Uptake of anions by roots is accompanied by the excretion of OH- or HC03
-
& cations by exudation of H+ (acidity in rehyzosphere)
 An-equilibrium tends to be established between the number of cations
adsorbed in colloids and the number of cations in solution
 A direct exchange may take place between nutrient ions adsorbed on the
surface of soil colloids & H+ ions from the surface of root cell walls
 Number of cations in soil solution is much smaller (1%) than the number
adsorbed in colloids
 Roots absorb cations from the soil solution and upset the equilibrium

72. Three basic mechanisms by which the concentration of nutrient ions
at the root surface is maintained
1. root interception comes into play as roots continually grow into
new, un depleted soil
2. mass flow, as when dissolved nutrients are carried along with the
flowing soil water toward a root
3. diffusion from areas of greater concentration toward the nutrient-
depleted areas of lower concentration around the root surface
 Soil compaction, cold temperatures & low soil moisture content,
reduce root interception, mass flow or diffusion & result in poor
nutrient uptake by plants even in soils with adequate supplies of
soluble nutrient
 Availability of nutrients for uptake can also be negatively or
positively influenced by the activities of microorganisms
 Nutrients are up taken in their available forms

73. Common Forms of the Essential Elements Available & Absorbed by
Plant Roots from Soils

74. CHAPTER VI. SOIL-PLANT RELATIONSHIP AND
NUTRIENT AVAILABILITY
 Agricultural production and productivity are directly linked with
nutrient availability
 Soil pH affects nutrients available for plant growth
 In highly acidic soil, Al & Mn can become more available and more
toxic to plant while Ca, P, and Mg are less available to the plant
 In highly alkaline soil, P and most micronutrients (Fe, Mn, Cu, Zn)
become less available
 Plant growth and development largely depend on the combination
and concentration of mineral nutrients available in the soil
 The nutrients may not be available in certain soils, or may be present
in forms that the plants cannot use
 Soil properties like water content & compaction may exacerbate
these problems

76. Inherent fertility factors; factors relating to nutrient availability
and root ability (root exudation and rhizosphere; root morphology)
 Parental rock material, texture, humus and water content, pH,
aeration, temperature, root surface area, the rhizoflora, and soil
microorganism population & mycorrhizal development are some
factors for nutrient availability
 The rhizosphere is a densely populated area in which the roots must
compete with the invading root systems of neighbouring plant species
for
 space, water, and mineral nutrients, and
 with soil-borne microorganisms, including bacteria, fungi, and insects feeding
on an abundant source of organic material
 Thus, root-root, root-microbe, and root-insect communications are
likely continuous occurrences in this biologically active soil zone
 Root exudates refer to a suite of substances in the rhizosphere that are
secreted by the roots of living plants and microbially modified
products of these substances
 Root exudates are a pathway for plant–microbial communication

77.  The rhizosphere is the volume of soil adjacent to & influenced by the
plant root, is regarded as a “hot spot” for microbial colonization and
activity
 Microorganisms in the rhizosphere of plants dominate the cycling of
nutrients in soil-plant systems
 Rhizosphere microorganisms increase the ability of plants to acquire
nutrients from soil by either increasing the extent of the root system
(e.g. through fungal hyphae) or solubilizing macronutrients like
phosphorus or sulphur
 The majority of root exudates including primary metabolites (sugars,
amino acids, and organic acids) are believed to be passively lost from
the root and used by rhizosphere-dwelling microbes
 Root Exudates as Determinant of Rhizospheric Microbial Biodiversity

78.  Root morphology affects nutrient uptake of plants
 Root length and radius can influence nutrient uptake by plant roots
growing in soil
 Total nutrient uptake depends on root surface area, and the rate of
increase and uptake per unit of root surface

79. Characteristics of nutrient uptake process, Intercept and contact
exchange
 Nutrients are taken up by roots mainly as inorganic ions from soil
solution
 The rate of uptake depends primarily on the concentration in the soil
solution immediately adjacent to the root
 The rate of nutrient uptake is independent of the rate of water uptake,
but the concentrations of nutrients at root surfaces depend strongly on
soil water content
 Soil water content is important because it affects root growth and
nutrient transport to the root surface in both the water flux created by
transpiration (called mass flow), and the diffusive flux towards or
away from the root
 The forms of ions taken up by roots differ somewhat with
 plant species and growing conditions, and they are regulated by a combination
of soil processes, the importance of which depends on the nutrient in question

80. Transport of nutrients from soils to the root system (mass
flow & diffusion; active and passive transport
 Nutrients move to roots in different mechanisms
1. Mass flow
 is the movement of dissolved nutrients into a plant as the plant absorbs
water for transpiration
 The process is responsible for most transport of nitrate, sulfate, calcium and
magnesium.
2. Diffusion
 is the movement of nutrients to the root surface in response to a
concentration gradient
 Passive uptake is driven by diffusion, and the uptake is dependent on
transpiration
 When nutrients are found in higher concentrations in one area than another,
there is a net movement to the low-concentration area so that equilibrium is
reached
 Thus, a high concentration in the soil solution and a low concentration at
the root cause the nutrients to move to the root surface, where they can be
taken up eg. P & K transport

81. 3. Root interception
 occurs when growth of a root causes contact with soil colloids
which contain nutrients & the root then absorbs the nutrients
 It is an important mode of transport for calcium and magnesium,
but in general is a minor pathway for nutrient transfer
 The actual pathway of nutrients into the root itself may be
 passive (no energy required; the nutrient enters with water) or
 active (energy required; the nutrient is moved into the root by a
"carrier" molecule or ion)
Uptake of water and nutrients by roots
 Root hairs, along with the rest of the root surface, are the major sites
of water and nutrient uptake.
 Water moves into the root through osmosis and capillary action

82.  Osmosis is the movement of soil water from areas of low solute
concentration to areas of high solute concentration
 Soil water contains dissolved particles/solutes, such as plant nutrients.
 Capillary action results from water’s adhesive (attraction to solid
surfaces) and cohesion (attraction to other water molecules)
 Capillary action enables water to move upwards, against the force of
gravity, into the plant water from the surrounding soil
 Nutrient ions move into the plant root by diffusion and cation
exchange

83.  Cation ion exchange occurs when nutrient cations are attracted to
charged surface of cells within the root, called cortex cells
 When cation exchange occurs, the plant root releases a hydrogen ion
 Thus, cation exchange in the root causes the pH of the immediately
surrounding soil to decrease/acidic

84. Long distance transport within the plant (xylem & phloem transport
 As a generalization, mineral nutrients and water are taken up from
the soil and transported upward, whereas products of photosynthesis
are produced in green leaves and transported downward
 Once water and nutrient ions enter the plant root, they move though
spaces that exist within the root tissue between neighbouring cells
 Water and nutrients are then transported into the xylem, which
conducts water and nutrients to all parts of the plant
 Xylem transports and stores water and water-soluble nutrients in
vascular plants
 Phloem is responsible for transporting sugars, proteins, and other
organic molecules in plants

85. CHAPTER VII. CHEMICAL PROPERTIES OF MINERAL SOILS
1. Soil acidity

86. Group Assignment
Group 1. Write an essay about
Soil organic matter (source, decomposition and
constituents of organic matter),
Influence of soil organic matter on soil properties and plant
growth
Factors and practices influencing soil organic matter
content
Soils and greenhouse effects
Sources
1. Brady, N.C. and Weil, R. R. The nature and properties of
soils: 3rd or 5th Edition. Macmillan publishing
2. HENRY D. FOTH. FUNDAMENTALS OF SOIL SCIENCE.
8TH EDITION
3. Refer Others

87. Group 2. Write an essay about
Essential plant nutrients
Macro-nutrients : available forms, functions
Micronutrients: Available forms, and functions in
plants
Nutrient levels in plants; deficiency and toxicity
symtoms
Nitrogen fixation (types and involved organisms)
Sources
1. Brady, N.C. and Weil, R. R. The nature and properties of
soils: 3rd or 5th Edition. Macmillan publishing
2. HENRY D. FOTH. FUNDAMENTALS OF SOIL SCIENCE.
8TH EDITION
3. Refer Others