Soil and soil fertility

Soil as a living medium

Soil is the loose upper layer of the earth's crust, formed by the weathering of rocks and the action of climate, organisms and time. A typical good soil contains about 45% mineral matter, 5% organic matter, 25% water and 25% air by volume.

Physical properties

• Texture is the relative proportion of sand (2–0.05 mm), silt (0.05–0.002 mm) and clay (below 0.002 mm). Loam, with a balanced mixture, is best for most crops. Sandy soils drain fast and hold little water; clay soils hold water but drain poorly.
• Structure is the arrangement of particles into aggregates (crumb, granular, blocky, platy). Crumb and granular structures are ideal because they allow good aeration and root growth.
• Tilth is the physical condition of the soil in relation to plant growth: a well-tilled, crumbly, moist seedbed is said to have good tilth.

Chemical properties

Soil pH measures acidity or alkalinity. Most crops grow well at pH 6.5–7.5. Acid soils (pH below 6.5) are corrected with lime; alkaline soils (pH above 8.5) with gypsum. Soil fertility is the inherent capacity of the soil to supply nutrients in adequate amounts and proper balance. Soil productivity is the ability to produce crops; a fertile soil may still be unproductive if water or management is poor. Soil health is the continued capacity of soil to function as a living ecosystem.

Soil organisms

Bacteria, fungi, actinomycetes, earthworms and other fauna decompose organic matter, release nutrients and improve structure. Earthworms are called "farmers' friends".

Major soil types of India

• Alluvial: Indo-Gangetic plains; most extensive and fertile.
• Black (regur): Deccan plateau; clayey, high water-holding, swells and cracks; good for cotton.
• Red and yellow: iron oxide gives colour; Tamil Nadu, Odisha, Chhattisgarh.
• Laterite: heavy rainfall areas; leached and acidic.
• Desert (arid) and saline/alkaline soils.

Key points

• Texture = particle size proportions; structure = arrangement of particles.
• Clay particles are smaller than 0.002 mm.
• Lime corrects acidity; gypsum corrects alkalinity.
• Black soil is best known for cotton.
• Fertility is not the same as productivity.

Common mistakes

• Mixing up texture and structure.
• Thinking a fertile soil is always productive.