Soil is the uppermost layer of the earth that supports the growth of higher plants, mainly by providing a medium for plant roots and supplying elements that are essential to the entire plant. the soil is the regulator of water supplies in rivers, lakes, and underground aquifers; it recycles raw materials such as humus and is a habitat for soil organisms, and parasites.
The soil type, soil fertility, and the present and potential systems of soil management have considerable influence on agricultural production. the regional variations in soil types often influence the prospect of growing particular crops in different parts of the countries around the world.
The Physical Soil Factors Affecting Crop Production
The physical soil factors which affect crop production include the soil texture, structure, porosity, and bulk density as further explained in details below;
1. Soil Texture
The soil texture is defined as the relative proportions of the sand, silt, clay, and gravel/stone particles and provides a useful guide to a soil’s potential for agricultural crop production since it exerts a major influence on soil characteristics.
The soil texture influences the water-holding capacity (through the clay type and content and capillary conductivity), temperature, drainage, and nutrient retention capacity of the soil. Also, soil texture influences the efficacy of soil-applied pre-emergence herbicides and other pesticides.
Soils are classified into (sandy, workable), medium (loamy, most workable) or heavy types, (clay, unworkable) based on soil texture, due to their close relationship to the workability (the ease of working the soil with machinery) of the soil.
2. Soil Structure
This is defined as the arrangement of the particles (sand, silt, clay) in the soil. it influences the soil tilth, root growth and development, gaseous exchange/aeration, drainage, water infiltration into the soil, and efficiency of water and nutrient uptake by plants (through capillary conductivity).
Structural stability is the ability of the soil to resist deformation when wet. It is influenced by the clay content, presence of lime, iron oxides, and humus. However, soil structure is not a stable soil property, and therefore changes with time and weather.
Poorly stable soil aggregates slake (collapse) easily while good aggregate structure maintains the shape when wet for a short time and gradually pieces off thereafter. Good structural stability is essential to prevent soil degradation and limited crop growth.
Soil structure can be improved by the addition of decomposable Organic Matter (OM) e.g. farmyard manure (FYM), crop roots, and crop residues. Heavy machinery causes damage to soil structure in wet soil, especially heavy clay soils.
3. Soil Porosity
This is defined as the percentage volume filled with air when the soil is fully drained of saturated water. The pore sizes include micropores (smallest pores containing only water which rarely dries out and is unavailable for crop uptake); mesopores (middle-sized pores containing water available to plants and which allow free aeration of the soil); and macropores (pores greater than 0.1 mm in diameter, can drain easily to allow in air after full wetting of the soil).
Soil porosity influences the infiltration of water into the soil, water-holding capacity, drainage, and aeration of the soil aggregates; these properties have a significant influence on the soil organic matter (SOM) status.
Ecologically, soil aeration plays a significant role in organic residue decomposition; oxidation-reduction of elements, especially nutrients, plant growth; nutrient and water uptake; soil composition; soil structure; and soil cultivation.
Aeration capacity is very high in sandy soils, optimal in loamy soils, and very low in clay soils. However, organic matter additions (which increase the number of meso and macro-pores) can improve the aeration capacity of clay soils.
4. Soil Bulk Density
This is the mass of soil per unit volume of the soil. It is determined by the volume of pore spaces in the soil; the more the pore spaces, the lower the bulk density, and vice versa for high bulk density or soil compaction.
Soil bulk density affects the workability of the soil, especially with respect to mechanical cultivation, and especially in dry weather. No-tillage or minimum tillage is also strongly affected by soil compaction.
5. Soil Water
Water is held in the soil in three forms, namely;
i. Capillary water (water held by surface tension forces as a continuous film around the particles and in the capillary pore spaces of the soil);
ii. Gravitational water (water held to the soil particles against gravitational forces and suction force of the roots, and which drains under the influence of gravity); and
iii. Hygroscopic water (water absorbed from an atmosphere of water vapor as a result of attractive forces on the surface of the soil particles and aggregates).
Soil water is very critical to root absorption of essential nutrients from the soil, soil temperature, microbial and microbial soil activities, organic matter decomposition, etc. The farm soil needs to be at field capacity always to ensure optimal growth and development.