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Soil fertility is the ability of the soil to supply nutrients required by plants in adequate quantities and correct proportions. Plants require at least 16 elements to complete their life cycle. They are: C, H, O, N, P, K, Ca, Mg, S, Fe, Mn, Cu, Zn, Mo, B, Cl. Some of the lower plants in addition to the above elements require Co, V, Si. Among these C, H, O, N, P, K, Ca, Mg and S are required in large quantities and therefore called macronutrients and the others called micronutrients. The elements C, H, O are obtained mainly from air and water and the rest from the soil.

In a fertile soil, production is high at the start but diminish rapidly later due to exhaustion of the soil reserve of nutrients. In order to continue the high production of the pond, fertilizers containing the nutrients need to be applied frequently to the ponds. A good fraction of the fertilizers applied do not act directly on the organisms or water but often gets adsorbed on to the soil which releases the nutrients little at a time for a long period. As a result the fertilizers have prolonged action.

The amounts and kinds of fertilizers that need to be applied to the pond depend on the natural fertility of the soil. Fertility status of the soil can be determined by analysing samples of soils for nutrients in standard soil laboratories. For example the phosphorus fertility status of a soil can be determined by extracting phosphorus from the soil by a solution containing 0.03N NH4F and 0.1 N HCl (Bray and Kurtz, No. 1 extractant) or by any other standard methods (Black et al., 1965b). If the phosphorus extracted by the Bray and Kurtz No. 1 method is less than 10 ppm (10 μg P per g soil) the soil is considered to be low in fertility and large quantity of phosphorus fertilizers need to be applied to the soil; if the value is 10 – 20 ppm, the soil is moderate with respect to phosphorus fertility and moderate quantity of phosphorus fertilizers need to be applied; if the soil has more than 20 ppm phosphorus, the soil is considered to be high in available phosphorus and no need to apply phosphorus fertilizers during the first year.

In a similar way, the fertility levels of other nutrients can be determined in the laboratory using appropriate methods for the elements concerned and rates of fertilizer requirements for the ponds can be determined.

Generalised ratings of the nutrient levels in agricultural soils are given in Table VI. The ratings may change slighly depending on the type of soil, ecology and the crop to be cultivated. However, they can be used as guides in interpreting soil test values. As there are no known ratings of soil nutrient values for aquaculture, the values given in Table VI. can be broadly used in determining the fertilizer needs of ponds.

Types of fertilizers, their characterististics and methods of applications are discussed in a seperate chapter in this book. Nutrient cycles and transformations in the soil - water - organisms - atmosphere systems are also discussed elsewhere in this book.

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