Abstract

Crops generally are unable to recover more th relatively small proportion of the phosphate applied in fertilizers even under optimum conditions of soil reaction, soil structure, moisture, drainage and other factors. This is in marked contrast to the situation with nitrogen.or potassium fertilizers where recovery is higher than 50%. Both fixation and positional unavailability have been regarded as the factors primarily accounting for low recovery of applied phosphorus by crops. Increased benefits have been obtained by placement of phosphate wherein extensive contact with the soil is reduced to an extent and the accessibility to roots has been increased (Stanford and Pierre, 1953). Early investigations by Coe (1926) with oats and wheat and by Duley (1930) on wheat clearly demonstrated the usefullness of drilling as compared to broadcast applications and as much as 50% reduction in dose to get a given yield increase have been reported (Reith, 1952; Smith. 1947). Further impetus has been given to research on placement methods as a result of developments in the use of radioisotopes. Until tagged phosphatic fertilizers came into use, it was not possible to evaluate clearly the relative or actual amounts of phosphorus obtained by the plant from various placements. This new development in research has greatly stimulated investigations along two main lines, 1) comparison of different types of placement for various crops and 2) evaluation of the factors or variables which may determine the relative value. of different placements.

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