Soil Formation, Properties and Conservation

What you need to know

• Soil is the loose top layer of the earth's crust in which plants grow, formed from weathered parent rock mixed with decayed organic matter, water, air and living organisms. By volume a good agricultural soil is roughly 45 per cent mineral matter, 5 per cent organic matter, 25 per cent water and 25 per cent air.
• Soil forms by the weathering of parent rock, which may be physical, chemical or biological, followed by the addition and decay of plant and animal remains. The five factors of soil formation are climate, living organisms, relief or topography, parent material and time.
• A soil profile is the vertical section from the surface down to the parent rock, showing distinct horizons. The O horizon is the surface litter, the A horizon is the dark topsoil rich in humus and roots, the E horizon is a leached or eluviated layer, the B horizon is the subsoil where leached clay and iron accumulate, the C horizon is weathered parent material, and the R layer is the solid bedrock.
• Soil texture is the proportion of sand, silt and clay particles. Using the international scheme, sand particles measure from 0.02 to 2.0 mm, silt from 0.002 to 0.02 mm and clay below 0.002 mm. Texture is permanent and cannot be changed by ordinary farm practice.
• Sandy soil has large particles and wide pores, so it drains quickly, warms early, is easy to till and is good for carrot, groundnut and cassava, but it holds little water and leaches nutrients fast. Clay soil has tiny particles, holds water and nutrients well and suits rice and swamp crops, but it is sticky when wet, cracks when dry, drains poorly and is heavy to work.
• Loam is the best agricultural soil because it combines the two: it retains water and nutrients like clay but drains and aerates like sand. A typical loam is about 40 per cent sand, 40 per cent silt and 20 per cent clay.
• Soil structure is the way individual particles group into aggregates or crumbs. Crumb and granular structures are the best for crops because they give good aeration, drainage and root penetration, while platy and massive structures restrict root growth and water movement. Unlike texture, structure can be improved by adding organic matter and by avoiding tillage when the soil is wet.
• Organic matter and humus hold water, improve structure, supply nitrogen, phosphorus and sulphur as they decompose, raise cation exchange capacity, buffer soil pH, darken the soil so it warms faster and feed soil organisms. This is why farmyard manure, compost and green manure raise yields even where chemical fertiliser is available.
• Soil pH measures acidity and alkalinity on a scale from 0 to 14, with 7 as neutral. Most crops grow best between pH 6.0 and 7.0 because that is the range in which nitrogen, phosphorus and potassium are most available. Acid soils are corrected by liming with calcium trioxocarbonate (IV), quicklime or dolomite, and alkaline soils are corrected with sulphur, gypsum or heavy additions of organic matter.
• Macronutrients needed in large amounts are nitrogen, phosphorus, potassium, calcium, magnesium and sulphur; micronutrients needed in traces are iron, manganese, zinc, copper, boron, molybdenum and chlorine. Nitrogen deficiency shows as yellowing of the older lower leaves and stunted growth; phosphorus deficiency shows as purple leaves and poor root development; potassium deficiency shows as scorching along leaf margins and weak stalks that lodge.
• Nigerian farmers commonly use NPK 15:15:15 and NPK 20:10:10 compound fertilisers, urea at 46 per cent nitrogen, single superphosphate and muriate of potash. The figures on the bag are the percentages of nitrogen, phosphorus and potassium in that order.
• Soil fertility is lost through continuous cropping without rest, leaching by heavy rain, erosion of the topsoil, bush burning which destroys organic matter and soil organisms, overgrazing, crop removal without replacement, and acidification.
• Soil erosion in Nigeria takes four forms: splash erosion from raindrop impact, sheet erosion which removes a thin uniform layer and is hardest to notice, rill erosion which cuts small finger channels, and gully erosion which cuts deep ravines. The gullies of Nanka, Agulu and Oko in Anambra State and those around Auchi and Abia are the national examples; wind erosion is the dominant form in Sokoto, Katsina, Yobe and Borno.
• Conservation measures are mulching, cover cropping with melon or sweet potato, crop rotation, planting along the contour, terracing and bench terracing on slopes, strip cropping, construction of drainage channels, afforestation and shelter belts, minimum tillage, agroforestry, addition of organic manure, controlled grazing and the avoidance of bush burning.