Efficiency of rock phosphate in the acid rice soils of Kerala

Abstract

Investigations were conducted to assess the suitability of Rajasthan rock phosphate for rice in two acid rice soils viz. Vellayani kayal soil and Moncompu soil respectively. The study also included a study on fractionation of the soil P collected at periodical intervals from these two locations for the different forms of P. The results are summarized below: 1. At both the locations a significant increase in height of plants, number of tillers, panicle length, thousand grain weight etc. was obtained by the application of phosphatic fertilizers over control. But between the different phosphatic forms no significant difference could not be observed. 2. In both the locations application of rock phosphate both Rajasthan rock phosphate as well as Mussoorie rock phosphate has been found to be as good as super phosphate in enhancing the yield of rice grain. 3. In both the locations the treatments with different forms of P increased the P content of grain and straw. This indicates that, the rock phosphate applied in acid soils while increasing the yield significantly increase the P content in plants also. In general, the treatments with phosphate fertilizers increase both the Ca and Mg content of grain and straw. The increase is much more marked in the case of calcium. 4. At both the locations the total P content of the soil is found to be significantly enhanced by the phosphatic fertilizer treatments. The highest total P content was recorded 20 days after transplanting and there is a progressive decrease in total P with time. 5. At Vellayani, super phosphate is found to be significantly superior to the other two rock phosphate in enhancing the available P status of the soil. But there is no significant difference between the two forms of rock phosphate in enhancing the available P. At Moncompu all the phosphatic treatments ie. super phosphate, mussoorie rock phosphate and Rajasthan rock phosphate were found to be equally good in enhancing the available P. The difference in response might be due to the difference in pH of the two soils. The highest available P content was recorded at 40th day at Vellayani and on 60th day at Moncompu.Thus there is variation in the period of submergence required for the attainment of peak value in available P in the two soils. Soil to soil variation is partly responsible for this. 6. Fe- P fraction of the soils treated with phosphatic fertilizers increase with period. Maximum Fe- P was recorded by super phosphate. However between the two rock phosphate there is no significant difference. Maximum Fe- P was recorded at 40th day of water logging. At Moncompu all the phosphatic fertilizer treatments significantly enhance the Fe- P content. All the phosphatic fertilizer forms significantly increase the Al- P fraction over control. But there is considerable variation in the behaviour at the two locations. The treatments significantly enhance the Al- P level at Moncompu than at Vellayani. The peak Al- P was recorded at 40th day at both the locations. There is progressive decrease in reductant soluble P and occluded P as the period progresses. The pattern was followed irrespective of soil types. The progressive decrease in reductant soluble P is accelerated by phosphatic fertilizer treatment in kayal and Moncompu soils. Same is the case with occluded P. The treatments with phosphatic fertilizers increase the Calcium P fractions in all the soils. Application of Mussoorie rock phosphate and Rajasthan rock phosphate as well as super phosphate enhance the Ca- P fraction. There is not much difference between the locations in enhancing the Ca-P fraction with respect to the treatments. Saloid fraction reach a peak value by 60 days of water logging. The results of the present study indicate that Rajasthan rock phosphate compares very well with super phosphate and Mussoorie rock phosphate under water logged situations. Its lower cost will substantially contribute towards reducing the input cost of phosphates under acid rice soil conditions. The studies on the pattern of dissolution give an indication that its solubilization and conversion to more available forms can be put to greater advantage in the growing of a rice crop by synchronizing the peak period of utilization of phosphate by the rice crop with the peak period of availability by shifting the basal application of rock phosphate by about 10 to 15 days perior to transplanting or sowing, in situations where such a practice is feasible.