Crop yields and phosphorus fertilizer transformations after 25 years of applications to a subtropical soil under groundnut-based cropping systems

In Subtropical regions where two crops are grown each year, effects of fertilizer P applications to summer, winter or both crops each year were evaluated on the basis of crop yields and transformations of fertilizer P into different soil P pools. Summer-grown groundnut showed small and inconsistent response to fertilizer P. Winter-grown crops responded significantly and consistently suggesting 40 and 60 kg P2O5 ha(-1) as adequate for rapeseed-mustard and wheat, respectively. Application of P to winter crops produced yields comparable to those when applied to both summer and winter crops. After 25 years of cropping, while Olsen-P decreased slightly in no-P control plots, organic P declined by 39% of the 8-year value. In fertilizer-treated plots, organic P increased by 18-22% of the 8-year value, whereas Olsen-P increased from initial 11.6 to 14-34 kg P ha(-1) after 3 years, 16-58 kg P ha(-1) after 8 years and 18-59 kg P ha(-1) after 25 years. Negligible increase in Olsen-P was observed in between 8 and 25 years. This was probably due to reduction in the applied P rates and raising five crops in between rotational changeovers without applying fertilizer P. Of the total fertilizer P added in 25 years, crops removed 21-54% whereas 33-64% accumulated in plough layer soil leaving 12-32% unaccounted for. This was probably leached to lower soil layers. The residual fertilizer P was transformed into labile (14-18%), moderately labile (28-35%) and non-labile (47-57%) pools of soil P. Results of this study with groundnut-based cropping systems in a subtropical region reveal that (a) long-term applications of fertilizer P to both crops each year significantly increased residual fertilizer P accumulation in soil and accelerated its conversion to non-labile P forms. (b) While application of fertilizer P to winter crops is essential, P requirement of summer-grown groundnut can be met from soil-derived and residual fertilizer P; this practice helps enhance fertilizer use efficiency (42-54% as compared to 27-40% in cumulative P treatment), saves costly fertilizer (50%), and reduces P accumulation by 44-76%. (c) The Olsen-P test alone is not adequate to reflect the depletion/accumulation of bio-available P. Therefore, the status of different soil P pools in long-term fertilized soils need to be determined to assess and make use of accumulated fertilizer P for sustainable crop production and environmental safety. (C) 2003 Elsevier B.V. All rights reserved.