Transformations of Inorganic-P Fractions in Soil and their Bio-Availability to Plants in Raya-Soybean (B. Juncea L.-Glycinemax L.) Crop Rotation.
N.S. Dhillon and Varinderpal Singh. Dept of Soils, Punjab Agricultural Univ, Ludhiana, India
Transformations of applied and residual phosphorus govern the availability of P to crops. Very limited information is available on P transformations in soils under specific cropping sequences. Therefore, P transformations in soil under raya-soybean cropping sequence were investigated using sequential P fractionation procedure in a screen house experiment. Surface soil samples of selected treatments (100% NPK- P both as SSP and DAP, and 100% NPK + FYM), from a long-term fertilizer field experiment after 32 years of cropping under maize-wheat-cowpea fodder cropping sequence were used for investigation. During the first year, soybean (Glycinemax L.) was grown in the soils supplied with fresh application of graded levels of phosphorus. In the proceeding season raya (BrassicaJuncea L.) was grown in the same pots on residual P fertility. Simultaneously, raya was grown in freshly filled pots supplied with different P doses. After the harvest of raya, soybean was grown in same pots on residual P fertility. A set of un-cropped pots was maintained to isolate the effects of cropping, and P transformations in soil were studied in un-cropped and cropped pots after each harvest. Inorganic P fractions analyzed after harvest of soybean, in soybean-raya rotation (when P was applied to soybean) indicated decreased saloid-P and Al-P at all the levels of applied P, whereas Ca-P and Fe-P content decreased only in control treatment and increased with P additions. Cropping of raya on residual soil P (after the harvest of soybean) decreased Ca-P drastically, whereas Al-P and Fe-P contents remained unaffected or showed little increase. P transformations after harvest of raya grown on freshly applied P also followed same trend. However, P-transformations after cropping of soybean on residual soil P behaved differently from transformations observed after cropping of soybean on freshly applied P. Build up of Ca-P and Fe-P was observed after harvest of soybean grown on freshly applied P whereas soybean grown on residual P fertility decreased all the inorganic P fractions in the soil. The results explicitly pointed out the ability of soybean to mine different inorganic P fractions when grown on residual soil fertility. Raya was also efficient to utilize residual soil P, but it does so by mobilizing only Ca-P. Possibly, the release of H+ ions from the raya roots mobilized P precipitated as sparingly soluble calcium phosphates. However, combined effect of high temperature, development of endomycorrhizal associations and excretion of organic acids from the soybean roots might be responsible for mobilization of different pools of inorganic-P in the soil. Percent recovery of applied P by raya and soybean grown in alternative rotation to each other was higher when both the crops were fed on residual soil fertility. Comparing P transformations data with P uptake, recovery percentage of applied P and agronomic performance of crops grown in rotation to utilize applied and residual soil P, lead to a conclusion that both raya and soybean are efficient to sustain P uptake from residual sources. Thus, the myth that winter crops in general have higher P requirement does not mean that these crops should accordingly be specifically fertilized with copious amount of fertilizer P. Raya was found to be highly efficient winter crop to utilize residual soil P and identification of such crops or development of P-efficient germplasm for desired crop species is the thrust area that should be exploited for developing best P fertilizer management strategies.