Four surface soils from farms located on hills in different parts of India, at elevations around 2000 m, were sterilized with gamma ray doses of 0.25, 0.5, 1.0 and 5.0 Mrad. Afterwards, single superphosphate, labelled with P-32 @ 1500 ppm P2O5 was incorporated into these as also into control soils by slow equilibration on incubating at 25°C or by rapid equilibration through repeated wetting and steam-bath evaporation cycles. The treated soils were analysed for various phosphorus fractions. Potato plants were grown on the treated soils employing a micro-eulture technique and the pathways of the applied, as well as the mineralized phosphate in the soils were traced by radio-assays.
Slow equilibration at a low temperature promoted mineralization of much of the organo-phosphate present, especially in soils with a wide C/P ratio. Both radiation-sterilization and heatsterilization, reduced the rate of mineralization substantially. A considerable part of the applied phosphate found its way into the organic milieu mainly through the agency of the enzymes, which were found responsible for the greater part of the turn over of the soil organic matter.
Radiation-sterilization resulted in the diversion of a good proportion of the applied phosphate to the sub-fraction identifiable with calcium phosphates and further, stimulated the plants to draw on larger quantities of phosphorus from the soils, with much of it derived from the applied phosphate. Heat-sterilization reduced the proportion of the iron phosphate sub-fraction and also adersely affected the growth of plants, without curtaliling the phosphorus uptake. In the ferruginous Ootacamund soils, heating produced a lower plant uptake of fertilizer phosphorus. ‘Available’ phosphorus values too supported these findings.
