- Introduction
- Soil Formation:
- Soil Profile
- Soils Formed in Situ and Soils Deposited
- Soil Depth
- Soil Constituents
- Soil Structure
- Soil Texture
- Soil Colour
- Soil pH
Introduction
- These are soil factors.
- Soil is the natural material that covers the surface of the earth,
- Made of weathered rock particles and decomposed animal and plant tissues, and on which plants grow.
Importance of Soil
- Provides anchorage to the plants by holding their roots firmly.
- Provides plants with mineral salts/ nutrients which are necessary for their growth.
- Provide the plants with water.
- Contains oxygen necessary for respiration of the plants and soil micro-organisms.
Soil Formation:
- Soil is formed through weathering process.
- Weathering is the breakdown and alteration of the parent rock near the surface of the earth to a stable substance.
- Weathering process is a combination of disintegration (breakdown) and synthesis (build up) process.
- Weathering process is continuous.
Types of Weathering
- Physical weathering
- Chemical weathering
- Biological weathering
Agents of Weathering
Physical Agents of Weathering
- Include wind, water, moving ice and temperature.
- Wind - carry materials which hit against each other to break into fragments.
- Water - intensity of rainfall causes breakdown of rock.
- Moving ice - has grinding effects which tear off rock particles.
- Extreme temperature cause rocks to expand and contract suddenly peeling off their surface.
Chemical Weathering
- Affects the chemical composition and structure of the rock.
- Involves processes such as ;
- Hydration - The process by which soluble minerals in the rocks absorb water and expand weakening the rock thus leading to disintegration.
- Hydrolysis; - The process whereby water dissolves soluble minerals in the rock weakening it.
- Oxidation - The reaction of rock minerals with oxygen to form oxides which break easily.
- Carbonation - The process whereby carbonic acids formed when rain water dissolves carbon dioxide, It reacts with calcium carbonates in limestone causing it to disintegrate.
Biological Weathering
This involves the action of living organisms, plants and animals on the rocks.
- Burrowing animals, for example, termites and moles bring soil particles to the surface exposing them to other agents of weathering.
- Big animals like, elephants, buffaloes, camels and cattle exert a lot of pressure on the rocks as they step on them due to their heavy weights causing the rocks to disintegrate.
- Earthworms take part in the decomposition of plant matter with the soil particles.
- Man's activities like, mining and quarrying expose rocks to the surface during excavation. These activities breakdown large rocks into smaller rock particles.
- Plant roots force their way through the cracks in the rocks thus widening and splitting them.
- Humic acids formed when plant tissues decompose react with the rocks weakening them further.
- Plant remains-decompose adding humus into the soil.
Factors Influencing Soil Formation
- Climate- (rainfall, temperature and wind)
- Biotic factors - living organisms.
- Parent material - Nature and properties of the original rock from which the soil is formed.
- Time - length of time during which the soil forming processes have taken place.
- Topography - influences the movement of disintegrated materials.
Soil Profile
- It is the vertical arrangement of different layers of soil from the ground surface to the bedrock.
- These layers are also referred to as horizons.
- The layers show differences in their contents and physical properties such as colour, texture and structure.
- The layers include: organic matter region, top soil, sub-soil, weathered rocks and parent material.
Organic Matter Region
- First layer of the soil found on the surface.
- Made up of leaves and other plant remains at various stages of decomposition.
- Some soil organisms may also be found here.
Top Soil
- Has a dark colour due to the presence of humus.
- Is rich in plant nutrients and well aerated.
- It is a zone of maximum leaching (zone of eluviations)
Sub-Soil
- It is compact and less aerated.
- It is a zone of accumulation of leached material (zone of aluviation) from the top layers.
- Deep rooted crops have their roots growing up to this region.
- Hard pans normally form in this layer
Weathered Rocks
- It is also called substratum.
- Rocks at various stages of disintegration are found in this zone.
- Most of the materials found in this zone originate from the parent rock.
Parent Rock
- It exists as a solid mass which is un-weathered.
- It is the source of the inorganic composition of the soil.
- The water table is on the surface of this rock.
Soils Formed in Situ and Soils Deposited
- Soil formed in the same place and remains there is said to be in situ.
- However, soil can be formed due to deposition of soil particles carried from its original site of formation to another area which is usually in the lower areas of slopes.
- Such soils are said to have been formed through deposition.
| Soil Formed in Situ | Soil Deposited |
| Has the colour of the parent rock | Has the characteristics of when: it came from. |
| Shallower | Deeper |
| Less rich in plant nutrients | Richer in plant nutrients |
| Easily eroded | Not easily eroded |
| Less silty | More silty |
| Have the same chemical composition | Differ in chemical composition from the as that of the underlying parent rock. underlying parent rock. |
Soil Depth
- This is the distance between top soil layer and the bottom soil layer in a profile.
- It dictates root penetration and growth
- Deep soils are more suitable for crop growth since they contain more nutrients.
- Have a larger surface area for root expansion.
- Deep soils facilitate good drainage and aeration.
Soil Constituents
Organic Matter
- Dead and decaying plants and animal remains
Living Organisms
Soil organisms and plant roots.
- Micro-organisms (bacteria, protozoa and fungi)
- Invertebrates -termites,
- Earthworms and molluscs.
- Higher animals - rodents and others.
Inorganic or Mineral Matter
- Formed from the parent materials.
- Supply plant nutrients
- Form the skeleton and framework of the soil.
Air
- Found in the pore spaces of the soil.
-Used for root and organism respiration
- Used for germination of seeds.
- Helps in decomposition of organic matter.
- Regulates soil temperature.
- Regulates the movement of water through capillary action.
Water
- Dissolves mineral salts
- Maintain turgidity in plants.
- Used for germination of seeds
- Used by soil organisms.
- Regulate soil temperature
- Dictates the amount of air in the soil.
Water in the soil exists in three forms namely:
Superfluous/Gravitational Water
- Found in the large spaces (macro-pores) in the soil particles.
- Held by gravitation forces.
- When the pores are saturated, the soil is said to be waterlogged.
- It moves and may cause leaching.
Hygroscopic Water
- Water found in thin films on the soil particles.
- Held by strong adhesive forces between water and soil particles.
- Does not move and hence not available for plant use.
Capillary Water
- Occupy micro-pores in the soil particles.
- Held by cohesive forces between water molecules.
- Moves through capillary action
- Available to plants for use.
Soil Structure
- This is the arrangement of soil particles in a soil horizon.
- Types of Soil Structure –
- Single-grained
- Crumby
- Granular
- Prismatic
- Columnar
- Platy
- Blocky
Importance of Soil Structure on Crop Production
Soil Structure Influences
- Soil aeration
- Soil drainage and water holding capacity.
- Plants root penetrability and anchorage.
- Microbial activities in the soil.
- Circulation of gases in the soil.
Farming Practices which Improve the Soil Structure
- Application of inorganic manure into the soil.
- Tilling the land at the right moisture content.
- Crop rotation.
- Minimum tillage.
- Cover cropping.
- Mulching.
Soil Texture
- It refers to the relative proportion of the various sizes of the mineral particles of soil.
Importance of Soil Texture on Crop Production;
- Influences soil fertility
- Affects the organic matter content
- Influences the drainage of the soil.
- Influences soil aeration.
- Influences water holding capacity.
- Influences the capillarity or movement of water in the soil.
Soil Textural Classes
Sandy Soils
- Made up largely of sand particles.
- Have large pore spaces hence poor in water retention.
- Easy to till (light soils).
- Freely draining.
- Low fertility due to leaching of minerals.
- Easily erodible.
Clay Soils
- Made up largely of clay particles.
- Have small pore spaces hence good in moisture retention.
- Difficult to till (heavy soils).
- Poorly 'drained.
- Expand when wet, crack when dry.
- High capillary.
- Rich in plant nutrients.
Loam Soils
- About equal amounts of sand and clay.
- Moderately good in both moisture and air retention.
- Fertile soils.
Soil Colour
- This depends on the, mineral composition of the parent rock and the organic matter content.
- Soils containing a lot of iron are brownish, yellowing and reddish in colour.
- Soils with a lot of silica are white.
- Soils with a lot of humus are dark or grey.
Soil pH
- This refers to the acidity or alkalinity of the soil solution/the concentration of hydrogen ions in the soil solution.
- Soil pH is determined by the concentration of hydrogen ions (H+) or the hydroxyl ions (OH) in the soil solution.
- A pH of less than 7 means that the soil is acidic.
- A pH of more than 7 means that the soil is alkaline.
- As the hydroxyl ions (OH) in the soil increase the soil becomes more alkaline.
Influence of Soil pH Crop Growth
- It determines the type of crop to be grown in a particular area.
- Most crops are affected by either very acidic or very basic soil pH.
- Soil pH affects the choice of fertilizers and the availability of nutrients to crops.
- At low pH the concentration of available iron and aluminium in the soil solution may increase to toxic levels, which is harmful to plants.
- Very acidic or low pH inhibit the activity of soil micro-organisms.
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