Soil Fertility (Hardbound - 2010)

Soil fertility is the ability of soils to supply the nutrients to crops in the form, proportion and amount favourable for their growth and yield. Nutrient cations occur in the soil solution and are loosely held by clay/humic micelle as exchangeable cations and are also fixed by clay minerals in the form of non-exchangeable cations. Nitrate, sulphate and phosphate anions are loosely adsorbed on the surface of hydrous oxides of iron and aluminium in acidic soils and on the surface of calcite in alkaline soils and are also fixed in the non-exchangeable form.

Roots absorb nutrients ions from the soil solution and it results in the reduction of their concentration in the soil solution. Exchangeable ions loosely held by clay/humic micelles on their surfaces then move to the soil solution. Some fractions of nutrient ions which had been fixed by the soil solid constituents move to their surface where they are held as exchangeable cations. Therefore, the sum total of the nutrient ions occurring in the soil solution, those loosely held on the surface of the soil solid constituents and some fraction of the nutrient ions fixed by some soil solid constituents may be considered to be absorbed by the roots of crops during their growth period.

Plant nutrients occur in the soil in different forms each of which differs from the others in its availability for the nutrition of the crop. Usually, the highly available form of the nutrient occurs in small amount in the soil. It favours crop growth to the maximum possible extent. The yield of the crop increases if the soil contains high amounts of this particular nutrient.

Crop yield is governed by the quantity of the nutrient that is present in the soil in the least abundance. For instance, if the soil is deficient in nitrogen, the crop yield can be gradually increased if the supply of nitrogen to the soil is gradually increased till the amount of some other nutrient, e.g. phosphorus, begins to limit the yield of the crop.

When crops are being grown, the application of the increasing amounts of nutrient(s) to them usually results in a decreasing rate of yield response. This means if all other factors are not changed, and nutrients are applied to crops in units of equal size, each increment in yield response is smaller than the preceding increment.

Soils inherit their fertility from their parent material from which they have originated. Their fertility is subsequently modified by climate and topography. For instance, basic parent material basalt has been weathered in relatively semi-arid climate of Maharashtra and Madhya Pradesh to form black soil and in relatively humid climate of the same states to form red soils. The former is relatively fertile while the latter is relatively infertile.

A.K. Kolay, M.Sc. (Agriculture), Ph.D., is a retired professor from the School of Agricultural Sciences & Rural Development, North Eastern Hill University, Shillong. Earlier he served at the Soil Conservation Department of the Government of Himachal Pradesh and also at the National Bureau of Soil Survey and Land Use Planning, New Delhi. Presently, he is engaged in writing books on soil science for students and teachers of agricultural universities, institutions and colleges.