SOIL
is the characteristically unconsolidated mineral & organic materials on the Earth ' s surface often
characterised by horizons or layers , that serves as a natural medium for plant growth and the support of
animal life on land.
Soil developed over a period of time through the interaction of physical, chemical, biological and
morphological properties and characteristics.
PEDOLOGY is the study of soil , it's origin and characteristics.
1st stage in soil formation is the accumulation of characteristically loose, broken and unconsolidated
and heterogeneous parent material called regolith.
Regolith comes from the parent rock
2nd stage is the formation of of top.soil or true soil resulting from the addition of water, gases [ air ] ,
living organisms [ biota ] and decayed matter [ humus ]
The interdependence or interconnectedness among the factors contribute to soil formation.
Jenny ' s formula
Soil = f [ parent material+ climate+ topography+ organisms+ time]
f is a function of
PARENT MATERIAL
~develops from weathering of the underlying parent rock. Minerals are susceptible to different rates
and processes of weathering.
Parent material controls the depth, texture, drainage [ permeability [, and quality of [ nutrients contents ]
and it's colour.
CLIMATE
Climate determines the type of soil. Climate affects weathering of rocks with most rapid decomposition
in humid and hot tropics. Climate affects the humus content eg in the humid tropics ralud
decomposition occurs forming humus.
Exceptionally high rainfalls or excessive rains promote leaching of bases
Evapotranspiration exceeding precipitation promotes capillary action
TOPOGRAPHY
as height of the land progressively increases, amount of rainfall increases, cloud cover, wind increases
while temperature and length of growing season decrease
Aspect- doubt facing slopes in the NH are characteristically warmer and drier than north facing slopes.
Slope angle affects drainage and soil depth. Greater moisture flows and the increased effect of gravity
on steeper slopes accelerate mass movement and soil erosion.
The more gentle the slope the slower rate of movement of water through the soil and the greater
likelihood of waterlogging and pray development.
Soil catena is a variation or sequence of soil types along the slope , so it's related to topography of a
hillside. READ WAUGH p 261
ORGANISMS
Plants, microorganisms such as bacteria and fungi and animals interact in the nutrient cycle
Microorganisms help in the decomposition of matter to produce nutrients
Plants take mineral nutrients from the soil and return them to.it when they die.macroorganisms eg
worms, and termites aerate the soil
TIME
Soils take a long time to develop eg 10mm soil takes upto 400years to form
Under extreme conditions 1mm take 1000 years to develop
Soil horizons develop over time.
SOIL PROPERTIES
1. MINERAL MATTER
soil minerals are obtained from weathering .
Weathering releases nutrients for plant growth.
Primary nutrients are present in the original parent material and which remain unaltered from their
original state. Primary minerals are present throughout the soil forming processes because they are
characteristically insoluble eg quartz
Secondary minerals are produced by weathering reactions and are produced within the soil eg oxides,
hydroxides of primary minerals eg iron resulting from the exposure of air and water.
2. Soil texture is the degree of coarseness and fineness of the mineral matter in the soil.
It is determined by the proportion of sand , silt and clay particles. Soil texture is tested using
sedimentation, drying and feel methods.
Soil texture controls the size and spacing of soil pores. It affects soil water content , water flow and
extent of aeration.
Soil texture controls the availability and retention of nutrients within the soil.
Plant roots penetrate characteristically coarser soils more easily than fine soils.
Texture influences soil structure.
3 SOIL STRUCTURE
is the arrangement or aggregation of soil particles into peds or soil aggregates. Sandy soils have the
weakest structures as they lack the clays, organic content and secretions of organisms needed to cause
the individual particles to aggregate.
A crumb structure provides the optimum balance between air, water and nutrients.
Types of structures are crumb, granular, platy, blocky, prismatic, columnar.
4;ORGANIC MATTER
decomposition of vegetation and animals , or secretions of living organisms produce humus . Bacteria
and fungi break down the organic matter.
L is leaf litter layer - plant remains are still visible
F is fermentation or decomposition layer - decay which biochemically involves yeast is most rapid
H is the humus layer - organic in nature. Decomposition forms characteristically black , slimy organic
material.
A horizon is rich in humus [ mull layer ]
AIR
fills the pore spaces left unoccupied by soil moisture.
Air in the soil contains more carbon dioxide released by plants and biota , and more water vapour and
less oxygen as this is consumed by bacteria.
Biota need more oxygen and give off carbon dioxide by respiration.
June 2019
7a.
5. SOIL MOISTURE
is essentially important as it helps in the development of of horizons.
Essentially supplies water for soil living organisms
Essentially supplies water for plants
Essentially facilitates soil forming processes such as weathering, humification, chelation, cheluviation,
leaching, podzolisation, calcification, ferratilisation
Essentially dissolves soil nutrients so that to they can easily be used by plants
Essentially facilitates processes of translocation [ see point no 4 ]
Essentially cools or regulate soil temperatures
The amount of moisture@ a given time is expressed as
W~R - { E+ T + D ]
[Input ]. [ Outputs ]
W = Water in the soil
~ proportional to
R = Rainfall/ precipitation
T= Transpiration
E = Evaporation
D = Drainage
GRAVITATIONAL WATER OR FREE WATER [ Gravitational water ]
When soil pore spaces are saturated, cessation of infiltration occurs , and water with a low surface
tension drains away rapidly under gravity
Gravitational water is available to plants when the soil is wet but unavailable when water has drained
away.
FIELD CAPACITY is the remaining moisture that is heldby the soil after excess water has drained away.
Moisture @ field capacity is held as hygroscopic water or capillary water.
HYGROSCOPIC WATER is always present, unless the soil becomes completely dry
CAPILLARY WATER is attracted to and forms a film around the hygroscopic water but has a lower
cohesive strength
Capillary water is freely available for plants roots. Capillary water can be lost through evapotranspiration
6. AIR
Essentially promotes air for plants and animals for processes of respiration
Essentially aerates soils providing a cooling effect in the soil
Essentially provide breathing air for soil living organisms
Air fills pore spaces left unoccupied by moisture.
7. SOIL ORGANISMS ( biota )
include bacteria , fungi and earthworm. Soil organisms are active in warmer well drained and aerated
soil than in cold air.
Decomposition is by detrivores such as termites, mites, worms ,woodlice and slugs
Bacteria and fungi secrete enzymes which break down compounds in the detritus.
Bacteria help in nitrogen fixation in the nitrogen biogeochemical cycle .
Fungi help to bind individual soil particles together to give a crumb structure, while burrowing animals
create passageways that help in air and water circulation and facilitate root penetration.
8 SOIL NUTRIENTS
are chemical elements found in the soil which are essential for plant growth and the maintenance of soil
fertility .
Sources of nutrients
Weathering of minerals
Decomposition release nutrients
Rainwater
application of fertilisers
Positively charged nutrients are called cations eg Ca+, Mg+, K+ , Na
Negatively charged nutrients are called anions
Cation exchange allows cations to be moved between clay particles and soil solution
Cation exchange capacity [ CEC ] is a measure of the ability of a soil of retain cations for plant use.
Sand soil has a low cation exchange capacity so are less able to keep essential plant nutrients
SOIL pH is a measure of the degree of acidity or alkalinity if the soil
It is a measure of the concentration of charged hydrogen ions.
9 ACIDITY [ PH ]
Soil contains positively charged hydrogen cations. Acidity or alkalinity is a measure of of the degree of
concentrations of hygrogen cations.
PH scale measures the concentrations of cations
6 & below is acidic
7 is neutral
8& above is alkaline
Acid soils need constant liming. Too acidic soils releases iron and aluminium which in excess can be
poisonous to plants.
10. SOIL TEMPERATURE
incoming radiation is absorbed, reflected or scattered by the Earth surface
Topsoil if uncovered vegetatitively , heats up more rapidly than subsoil during the day & loses heat more
rapidly@ night.
A characteristically warm soil have higher biota activity giving rapid break down of organic matter. Seeds
germinate more rapidly in a warm soil than in a cold soil.
SOIL PROFILE is a vertical section through the soil showing its different horizons.
Horizons are layers reflecting the balance between soil processes and the time that has been available
for their development.
UPPER LAYER OR A horizon is where biological activity humus content are @ their maximum. It's a zone
most affected by the leaching of of characteristically soluble materials and by the downward movement
or [ ELUVIATION ] of clay particles.
ELUVIATION is the washing out of organic and mineral matter from the A horizon.
B HORIZON is the zone of accumulation or ILLUVIATION ,where materials removed from A horizon are
redeposited. Illuviation is the process of inwashing and the redeposition of organic and mineral matter
from A horizon.
~ A horizon and B horizon make up the true soil
C HORIZON consists of recently weathered parent material or regolith resting on the bedrock.
Note humus is incorporated into the soil to give a characteristically crumbly , black nutrient rich layer
called MULL
Slow decomposition of humus in cold areas produces MOR
SOIL FORMING PROCESSES
WEATHERING is the disintegration and decomposition of the rocks in situ to form largely regolith.
Weathering nleaves primary minerals as residues and produces secondary minerals. Minerals are
released as inputs in soil system.
HUMIFICATION AND CHELUVIATION
HUMIFICATION is the process by which organic matter is decomposed to form humus
CHELUVIATION involves organic matter decomposition to form organic acids which attack clay and
combine with cations eg iron and aluminium to form chelates ( characteristically soluble ) which move
downwards through the soil profile. Fe and Al are redeposited.
Cheluviation is the process in which minerals are dissolved and transported downwards under the
influence of chelating agents.
Chelating agents are the organic acids produced after the decomposition of organic matter.
TRANSLOCATION
is the movement of soil components in solution or suspension or direction ( upwards or downwards)
i. ELUVIATION AND ILLUVIATION
ELUVIATION is the outwashing or shedding or degrading of the A horizon of it's nutrients , water, soil ,
organic matter, mineral matter.
ILLUVIATION is the accumulation of water , soil , organic matter, mineral in B horizon
ii. LEACHING is a translocational process involving the removal of soluble material in solution.
Leaching occurs when precipitation exceeds evapotranspiration✓and soil drainage is good✓. Organic
acids causes breakdown of clays and Ca and Mg are eluviated from A horizon , making the soil
increasingly acidic✓
Redeposition of clays occur in the illuviation zone.
iii. Podsolisation is a process caused by excessive leaching where precipitation exceeds
evapotranspiration where soils are well drained.
The removal of iron (Fe) and aluminium ( Al) oxides.
Organic acids leads to rapid breakdown of clays leaching bases ca , Mg , Na, K.
Fe and Al are released leaving the soil characteristically white or grey in colour.
A horizon is bleached and B horizon is reddish brown due to illuviation.
Iron deposits firm an iron pan which is a podsol.
iv. GLEYING
is a process occurring in depressions or valleys with impermeable rock which leads to poor drainage
giving anaerobic or waterlogged conditions. Depression is filled with stagnant water.
v. SALINISATION occurs when evapotranspiration exceeds precipitation where the water table is near
the surface. Moisture evaporates from the surface, salts are drawn upwards in solution by capillary
action.
FERRATILLITISATION
is a process involving intense chemical weathering and leaching of bases eg silica leaving behind
sesquioxides [ oxides made up of 3 oxygen atoms and 2 atoms of another element ]
Eg Aluminium oxides Al2 O3
Iron Oxides Fe2 O3
Soils n Tropical rainforest are called LATOSOILS / FERRALLITIC SOILS formed due to ferrallitisation.
FERRALITISATION
is the process in which rock is changed into a soil consisting of clay [ kaolinite ] and sesquioxides, in the
form of hydrated oxides of iron and aluminium.
~ in humid tropics, with consistently high temperatures and rainfall throughout the year, chemical
weathering rapidly breaks down the rock .
~ at first produces clays which also break down to form silica.
~ leaching removes silica and sesquioxides of FE & AL remain giving the soil a characteristically red
colour.
~ FERRALITISATION is the REVERSE OF PODZSOLISATION ,where silica remains and Fe and Al are
removed.
~ in Tropical rainforest , ferrallitic✓✓soils are formed
~ In savanna areas , with alternating dry and wet climate , ferruginous✓✓soils are formed
Halomorphic soils support halophytes such as bushsalt.
Chernozorms or black earth are organic in nature resulting from decomposition of grass cover to form
mull humus which forms a characteristically black crumbly top soil or structure
Rapid decomposition of grass in the A horizon forms black earth.
Chernozorms are regarded as the optimum soil for agriculture as they are characteristically deep, rich in
organic matter, retain nutrients, have ideal crumb structure with well formed soil peds.
SOIL FERTILITY is the ability of the soil to provide unconstrained or optimum growth for plants.
Measures to improve soil fertility
~ application of nitrogen fertilisers to provide nitrates into the soil [ see the Nitrogen cycle ]
~ practising crop rotation with leguminous plants to fix nitrogen
~ practising strip cropping
~ addition of compost manure to provide organic matter to the soil
~ addition of lime to reduce soil acidity
~ addition of green manure into the soil to increase fertility
SOIL EROSION
Measures
Intercropping involving growing different crops in the same field.
~ zero tillage eg Potholing system [ Pfumvudza ] [ conservation agriculture ]
~® practising terracing to reduce speed of runoff
~ afforestation and reforestation programmes. Vegetation cover reduce raindrop impact through
interception hence reducing soil dislodgement
Planting trees as a shelter - forest network against shifting soil.✓
~ practising zero grazing [ keeping livestock indoors]
~ soil bank policy - Environmentally sensitive areas are left uncultivated for along time and thefarmer is
paid for leaving the land unused.
ZONAL SOILS
are characteristically mature soils resulting from the maximum effects of climate and living matter
[ vegetation ] upon parent rock in areas where there are no extremes of weathering, relief or drainage
and where the climate has been stable for a long time.
Azonals had time to develop eg chernozems
Thick grass cover and the importance of roots as a source of organic matter together provide a plentiful
supply of mill humus which forms a characteristically black crumbly topsoils. Presence of biota for
decomposition to form humus throughout the A horizon.
Chernozems are regarded as the optimum soil for agriculture as they are characteristically deep, rich in
organic matter
AZORNAL SOILS
~ are characteristically immobile and are of recent origin and have had insufficient time to operate fully.
~ soils show the characteristics of their origin [ parent material ]
~ soils have no well defined horizons
INTRAZONAL SOILS
~ exhibit the predominance of a single factor such as parent rock or extreme of drainage
1 CALCIMORPHIC SOILS
~ soils devop on a limestone rock eh terra rossa and rendzina
2HYDROMORPHIC SOILS
~ are those having a constantly high water content [ gleyed soils and peat ]
3. HALOMORPHIC SOILS
~ have high levels of soluble salts which render them saline or salt
CALCIMORPHIC
i. RENDZINA ~develops on limestone where grasses produce leaf litter rich in bases.decomposition
occurs to form humus. Release of calcium from the parent rock and a lack of hydrogen cations
SOIL FERTILITY is the ability of the soil to provide for the unconstrained or optimum growth of plants.
The capacity of a soil to produce characteristically high or low yields depends on the nutrients content
structure, texture, drainage acidity and orgacnic content of a particular soil.
Ensure soil fertility through
~ addition of compost manure
~ application of nitrogenous fertilisers
~ burying plant remains
~ implementation of crop rotation involving leguminous crops to add nutrients in the soil.
SOIL DEGRADATION is the deterioration of the soil in it's qualitative flaire until it can no longer provide
unconstrained conditions for plant growth
Physical erosion is when soil erosion results from the action of the wind or water. It's a natural process
accelerated by human activity.
Chemical carried by water can cause diffuse pollution , while biological degradation is when organic
matter in the form of plant remains or organic manure is washed out of the soil.
~ massive deforestation has led to bare ground that is susceptible to soil erosion eg Burkina Faso,
Ethiopia
~ splash erosion didlodges soil particles and make them prone to erosion
~ monoculture has led to soil impoverishment as there is little replacement of nutrients
~ overgrazing has exacerbated Environmental degradation
MITIGATION STRATEGIES FOR SOIL DEGRADATION
~ implementation of afforestation programme as tree .leaves intercept precipitation and roots bind the
soil together
~ Marram grass anchors the soil, and brushwood is planted in gullies
~ leguminous crops eg groundnuts, beans are planted to fix nitrogen in the soil thus improving it's
quality
~ tres are planted as windbreaks and shelterbelts to reduce the effectiveness of wind erosion
~ strip cropping in strips or the intercropping of different crops in the same field to protect the soil
~ addition of organic manure helps to bind the characteristically loose soil and reduces it's vulnerability
to erosion