SOIL PROFIILE, SOIL STRUCTURE AND SOIL TEXTURE
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SOIL PROFILE
soil profile is the vertical arrangement of the different soil layers from top to bottom.
As soil-forming processes continue, soil is found to be arranged in a definite sequence of soil horizons or layers. This vertical arrangement of various soil horizons is called soil profile. The thickness of the layers varies from place to place. In places where the soil forming processes have been in action for too long and if erosion has not taken place, the layers are thicker making the profile deeper. In some places where soil where erosion has occurred, the top layers of the profile may be missing. Such profiles are called truncated soil profiles.
Horizon 0 (Organic horizon)
•This horizon contains organic materials that are at different stages of decomposition on the surface of the soil.
•It is commonly found in highly vegetated areas, such as the forests.
• It is not soil as yet.
Horizon A (Topsoil)
• In this horizon, there is abundance of living organisms that are actively breaking down organic materials to organic matter and humus.
•It has plenty of organic matter which makes it look dark or black in colour.
•Also, because of the presence of organic matter, it is rich in plant nutrients.
•The soil particles are loosely packed, which enables water and air to move freely through it.
•Crumb and granular structures are commonly found in this horizon.
•As rainwater passes through this horizon, it carries away plant nutrients, compounds of iron and aluminium and clay particles to the underlying horizons. This is called Eluviation. It is a layer that is highly leached.
Horizon B (Subsoil)
•This is the horizon where most of the materials leached from horizon A are deposited and is referred to as the or
•Fewer living organisms exist in this horizon.
•Organic matter content is low or, is lacking in some places which makes the soil brown or red in colour.
•Soil particles are tightly packed which reduces the amount of air present.
• The compactness and the high clay content of soil in this horizon enables it to hold a lot of water and become water logged.
• Block and prismatic structures are commonly found in this horizon.
Horizon C (Partly weathered rock or parent material)
• This horizon is made up of coarse, partly weathered parent material that originates from the parent rock as a result of weathering.
• Soil particles are very tightly packed, and therefore, poorly aerated. This horizon lacks organic matter.
•Apart from the tree roots, there are no living organisms in this horizon. It is a layer outside the biological activities.
Horizon D (Bedrock)
•This is the original parent material which is still intact. It has not experienced weathering forces due to protection by the overlying horizons.
•Since it is a solid mass of rock, it lacks living organisms, organic matter and air.
It is important to note that in between any two horizons, there is a transition zone, whereby one horizon gradually merges into the next. The characteristics of the transition zone reflect the properties of the two adjacent horizons.
Importance of soil profile to crop production
It can be used to determine the suitability of a soil for agriculture. The deeper the profile, especially the topsoil layer, the better the soil is for crop production. The deeper soil profile provides the roots of crops with a bigger volume of soil to exploit.
It helps to determine the type of crop to grow. Shallow rooted crops are grown in shallow profiles and vice versa.
SOIL STRUCTURE
This refers to the arrangement of soil particles or the physical appearance of the diiferent soil particles.
THE DIFFERENT TYPES OF SOIL STRUCTURE
1. Blocky/cuboidal structure. In this arrangement, soil particles are arranged like cubes or blocks. They are hexagonal in appearance and have sharp edges. This type of structure is found in subsoils and anthill soils.
2. Columnar structure. Aggregates form column-like structures which may be 12 cm or more in diameter, and their tops are round. This structure is found in subsoils. They are formed as a result of frequent erosion on prismatic structures.
3. Plate like structure. Soil particles are arranged in such a way that they form thin sheets lying horizontally on top of the other. This structure is found in the topsoils. It interferes with drainage and growth of plant roots.
4. Granular structure. In this type of soil structure, soil particles are joined together to form small rounded aggregates. This structure is found in topsoils that contain a lot of organic matter.
5. Crumb structure. This arrangement resembles the granular type but here, the aggregates are bigger more porous, and they readily crumble down when shaken to give aggregates with granular structure. This structure is found in the top soils where cultivation has been carried out.
6. Prismatic structure. In this case, soil aggregates are arranged as vertical columns with uniform cross section. This structure is found in subsoils.
Importance of a good soil structure
• It facilitates easy movement of air and water and also ensures that there is a good balance between water and air in the soil.
• It allows adequate retention of water for plant use.
• It prevents the buildup of carbon dioxide in the soil to toxic levels.
• It allows proper drainage of soil, thereby avoiding water logging.
• It allows plant roots to grow and spread widely within the soil.
• It does not allow soil to be eroded easily.
•Ensures a good balance between soil water and air that is good aeration ‘or proper plant growth.
•Facilitates good drainage avoiding water-logging.
•Ensures enough retention of water for plant use (water holding capacity).
•It determines the ease with which the soil can be ploughed workability).
•Controls soil temperature through its control of soil aeration.
•It determines the ease with which the soil can be eroded and leached.
•Eliminates buildup of Carbon dioxide (CO2) in the soil to toxic levels through proper aeration.
•It controls the ability of roots to penetrate deep into the soil.
Causes of loss of soil structure
Structure can be destroyed through any of the following:
•Use of heavy tillage machines for a long time causes compaction, making aeration poor. Also soil particles are broken into smaller particles which can be easily carried away.
•Soil erosion by either water or wind.
•Poor farming methods such as over cultivation, over stocking and ploughing up and down a slope.
•Bush burning destroys the organic matter and exposes soil to agents of erosion.
•Pollution decreases the population of microorganisms which break down organic matter. Hence the binding property of organic matte lowered due to absence of organisms to decompose plant remains
Maintenance of soil structure
Cover cropping; this reduces evaporation and controls soil erosion.
Addition of organic manure; manure has colloidal properties to bind particles together.
Minimum tillage; reduces on compaction and allows the soil to restore its structure.
Bush fallowing; the thick vegetation under fallow system controls erosion and adds organic matter to the soil.
Mulching; decaying organic mulches provide organic matter which binds the particles together. Also mulching controls erosion which affects structure.
Agro-forestry and afforestation; tree canopies control erosion, shed leaves which provide organic matter on decomposition.
Crop rotation and intercropping.
Drainage.
Erosion control methods.
EXAMPLES OF DIFFERENT SOIL STRUCTURE TYPES
SOIL TEXTURE
This refers to the percentage composition of different sized mineral particles like sand, clay and silt in a given soil. It can also be referred to as the degree of fineness or coarseness of soil particles when rubbed between the thumb and fingers.
Importance of soil texture
It determines the water-holding capacity of the soil.
It determines the aeration of the soil.
It determines the ease with which the soil can be ploughed (workability).
It determines the erodibility of the soil that is, how easy the soil can be washed away.
It determines the temperature of the soil.
It influences drainage of the soil.
It influences the penetration of roots in the soil.
Experiment to determine the texture of a given sample of soil using the finger feel method
Apparatus/Material
Samples of soil, Water
Method
Get a small lump of so from a given soil sample.
Add some water to moisten it.
Get a small lump and rub ft between fingers.
Observation
Sand feels rough/coarse/gritty.
Clay feels smooth and sticky.
Loam feels powdery.
Experiment to determine the texture of a given sample of soil using the rolling method
Samples of soil, Water.
Method
Get a small lump of soil from the soil sample.
Add some water to moisten it.
Make a paste out of ft.
Make a ribbon or ring out of it.
Observation
•Sand fails to make a paste and neither forms a ribbon nor a ring.
•Loam makes a paste, ribbon and a ring with cracks.
• Clay makes a paste, ribbon and a smooth ring without cracks.
Experiment to determine the texture using the mechanical analysis method
Materials/apparatus
Sample of soil, water, measuring cylinder and a millimetre sieve
Method
A 2 millimetre sieve is used to separate big soil particles (gravel) from small ones. The bigger soil particles remain on the sieve and small ones pass through. The fine particles are then put in the cylinder containing water. Shake the cylinder so that the particles break up. Allow it to stand overnight.
Observation
Formation of layers (sedimentation) takes place with big ones first followed by small ones.
Properties of Some of the Textural Classes of Soil
Sandy soils
• They are well drained, that is they have poor water holding capacity.
• They have a coarse texture.
• They are well-aerated.
• They have a high rate of leaching.
• They have a low nutrient content due to leaching.
• They have low capillarity.
• Their structure is less stable, and thus can be easily eroded.
• They are easy to cultivate, hence referred to as light soils.
• They are usually acidic.
• They warm up readily during the day and cool down very fast at night.
• They contain 80-95% sand, 5-20% silt and clay, and 0.1-1% organic matter.
Loam soils
• They have a well-balanced proportion of clay, silt and sand particles, and are rich in organic matter. On average, they contain 50-70% silt and clay, 30-40% sand, and 0.1-4 % organic matter.
• They have good chemical and physical properties as a result of good proportion of sand, clay and silt.
• They are fairly fine textured.
• They are moderately well-drained.
• They are moderately fertile.
• They are slightly acidic.
•They are easy to cultivate.
Clay soils
• They are fine textured.
• They contain a large amount of water, hence poorly
drained.
•They are very sticky when wet and very hard when dry.
• They are difficult to cultivate, hence referred to as heavy soils.
• They do not warm up easily due to large amounts of water they hold.
• They expand when wet and contract when dry.
• They are poorly aerated.
• They have high capillarity.
• Clay soils are fertile.
• They are slightly alkaline.
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