Suitability of rockphosphate for direct application in acid rice soils of Kerala

Abstract

An incubation study and a potculture experiment were conducted to assess the suitability of Rajastan rockphosphate (RRP) supplied from Rajastan State Mineral Development Corporation, in comparison with that of Mussooria rockphosphate (MRP) supplied from pyrites phosphate and chemicals Ltd. (U. P) and superphosphate (SP) in two acid rice soils of Kerala namely laterite (Kodakara, Trichur district) and kari (karumadi, Alleppey district) soils. Transformations of P fertilizers applied at the rate of 45 and 90 kg P2O5/ha in these soils under continuous submergence were studied in the incubation experiment. Soil samples were drawn at 15 days interval for the determination of various inorganic P fractions and available P. The direct and residual effects of the two rockphosphates in comparison with the water soluble SP were studied in the potculture experiment using rice (Jaya) as the test crop. The soils and levels of P applications were the same as in the incubation study. Application of N and K was done uniformly in all the treatments. Soil and plant samples were drawn at 15 days interval for the determination of available P and the uptake of major nutrients. The residual effect of phosphatic fertilizers was assed by continuing the experiment for the second season with out the addition of P fertilizers. However, for the second season a treatment receiving P at the rate of 45 kg P2O5/ha as SP in both the season was incorporated for comparison in both the soils. Application of phosphatic fertilizers irrespective of their water solubility increased the various inorganic fractions and available P content of the soil. The total native inorganic P increased in the presence of added P due to the enhanced mineralization of organic P. Among the various inorganic fractions, Fe-P was the dominant form accounting for 41 to 44 per cent of the total inorganic P. Second most abundant fraction was A1-P. The contents of reductant soluble-P, occluded-P and Ca-P were relatively less and that of saloid-P was negligible. The three sources of P did not differ significantly in increasing the various inorganic fractions of the soil except that of saloid-P. The contents of A1-P, Fe-P, reductant soluble-P and occluded-P were more in laterite soil compared to kari soil, while saloid-P and Ca-P were high in kari soil. Prediction equations were worked out to establish various inorganic P fractions and available-P at different periods of incubation. The peak values of saloid-P were observed during the seventh fortnight. Saloid-P was found to be positively correlated with Fe-P (r = 0.77*2*), AI-P (r = 0.73*1*) and Ca-P (r = 0.27*8*) and negatively correlated with reductant soluble-P (r = -0.249*) and occluded-P (r = -0.31*9*). Concentrations of AI-P and Fe-P were maximum during the twelfth period of sampling and they were negatively correlated with reductant soluble-P occluded-P. Highest values of reductant soluble-P and occluded-P were observed during the first period of incubation and the concentration of Ca-P was minimum in the twelfth fortnight. Forms and levels of applied P had little effect on increasing the available P content of the soil. The contribution of various inorganic fractions to available P was different. Direct and indirect effects of various inorganic P fractions on available P from RRP, MRP and SP were brought out by the path analysis. Saloid-P, Fe-P, AI-P and Ca-P were positively correlated with available P (Bray 1 and 2) and reductant soluble-P and occluded-P were negatively correlated. The extent of contribution of various inorganic fractions to available P was the same in all the three sources of P added to the soil. Application of SP at the rate of 90 kg P2O5/ha twice in two equal doses during the first and ninety-first day of incubation did not increase conspicuously the inorganic fractions and available P content of the soil compared to the initial application of the same quantity of P as SP and rockphosphate. In the potculture experiment in general, application of P fertilizers had resulted in a better utilization of major nutrients by the rice plant and this effect was more pronounced in kari soil during the first crop season. However, in the second crop season, the effect of P fertilizers on uptake of nutrients was pronounced in both the soils. During the first crop season, in the absence of added P, the uptake of nutrients and yield of straw and grain were more in laterite soil compared to kari soil while in the presence of added P, uptake of nutrients and yield were higher in kari soil. However, in the second crop season, both in the presence and absence of added P, the uptake of nutrients and yield were more in laterite soil. Uptake of N and K by the straw during the first crop season was significantly higher in treatments receiving SP compared to other treatments receiving rockphosphates though the uptake of P by the straw did not vary significantly with the variations in the source of P. However, in the second crop season, uptake of N, P and K by the straw was significantly higher in treatments receiving SP. Increasing the level of application of P from 45 to 90 kg P2O5/ha increased the uptake of N and K significantly while uptake of P by the straw and straw yield did not increase significantly with increasing the level of application. But in the second crop season, the uptake of N, P and K by the straw and straw yield increased markedly with increasing the level of application. Uptake of N and P by the grain of the first crop did not increase significantly over control in laterite soil, while that of K showed significant increase over control in both the soils. However, in the second crop, uptake of N, P and K by the grain increased conspicuously in both these soils. In the first crop season, total uptake of K was significantly higher over control in both the soils, while that of N and P was higher only in kari soil. But the total uptake of N, P and K increased over control in both the soils in the second crop season. In general, application of SP was found to be superior to the application of rockphosphates with respect of the uptake of N, P and K grain and their total uptake by rice plant in both the seasons of crop growth. The yield of grain by the application of phosphatic fertilizers showed significant increase over control only in kari soil during the first crop season. In the second crop season, both the soils were found to be responsive to the application of P fertilizers with respect to the increase in the yield of grain. In the treatments receiving rockphosphates as a source of P, the yield of grain was 93.18 and 92.67 per cent of that obtained in treatments receiving SP during the first crop and second crop seasons respectively. In both the seasons of crop growth, available P in the soil was significantly higher in treatments receiving SP than that in treatments receiving rockphosphates. Increasing the level of application of P from 45 to 90 kg P2O5/ha increased the available P content of the soil to the extent of 11.97 and 17.43 per cent over the lower level during the first and second crop seasons respectively. Application of SP at the rate of 90 kg P2O5/ha applied twice in two equal doses separately for the first crop and second crop seasons increased the uptake of nutrients, yield of grain and straw and available P content of the soil compared to the application of the same total quantity of P initially for the first crop only as SP and rockphosphate.