(a) What is soil consistency? (b) Explain how each of the following soil properties influences crop growth: (i) soil texture; [2 marks] (ii) soil pH. [4 mar...

(a) Soil consistency

Soil consistency is the degree of resistance of the soil to deformation or rupture, that is the strength with which soil particles hold together (cohesion) and stick to other objects (adhesion) at different moisture contents. It is described as loose, friable, firm or hard when dry, and as sticky and plastic when wet.

(b) Influence of soil properties on crop growth

(i) Soil texture: texture refers to the relative proportion of sand, silt and clay. It determines how much water and air the soil holds and how easily roots penetrate. Sandy (coarse) soils drain fast and dry quickly, holding few nutrients, while clayey (fine) soils hold much water and nutrients but may become waterlogged and poorly aerated. A well balanced loam gives the best water, air and nutrient supply for crop growth.

(ii) Soil pH: pH measures the acidity or alkalinity of the soil. It controls the availability of plant nutrients; most crops grow best at slightly acidic to neutral pH (about 6 to 7). Very acidic or very alkaline soils lock up nutrients, may release toxic elements, and reduce the activity of beneficial soil micro-organisms, thereby limiting crop growth.

(c)(i) Four methods of maintaining soil fertility

1. Application of organic manure
2. Application of inorganic (chemical) fertilisers
3. Crop rotation
4. Cover cropping/green manuring (mixed farming and fallowing are also acceptable)

(c)(ii) How each method maintains soil fertility

• Organic manure: adds nutrients to the soil, and improves soil structure, aeration and water-holding capacity.
• Inorganic fertiliser: supplies specific nutrients (N, P, K) quickly, and corrects identified nutrient deficiencies to boost yield.
• Crop rotation: alternating deep- and shallow-rooted crops uses nutrients from different soil depths, and including legumes fixes nitrogen while breaking the pest and disease cycle.
• Cover cropping/green manuring: protects the soil from erosion and adds organic matter, and legume cover crops fix atmospheric nitrogen into the soil.