Effects of continuous use of cattle manure and fertilizer phosphorus on crop yields and soil organic phosphorus in a Vertisol

Soil organic phosphorus plays a significant role in phosphorus (P) nutrition of crops especially in high-P-fixing soils of the tropics. Soil management practices, which favour the accretion of organic P, would, therefore, help in improving soil P fertility over time. In a six-year field experiment with soybean-wheat rotation on a Vertisol, the effects of continuous additions of four rates of animal manure (0, 4, 8 and 16 t ha(-1) yr(-1)) with and without fertilizer P (22 kg P ha(-1)) on crop yields and soil organic P were examined. Soybean and wheat yields, and P uptakes increased significantly with the addition of manure and fertilizer P. For the similar amount of P input, the yield increases were larger with manure P than with fertilizer P. Combined use of manure and fertilizer P resulted in a greater crop yield and P uptake than their solitary application. Irrespective of the treatment, P content in different organic P fractions followed the order: moderately-resistant organic P (MROP) > moderately-labile organic P (MLOP) > highly-resistant organic P (HROP) > labile organic P (LOP), with their relative proportion being in the ratio of 22:8:5:1. Continuous annual application of manure increased the content of all the soil organic-P fractions, except the HROP, over the control. Addition of fertilizer P together with manure promoted the accretion of soil organic P. Generally the increases in organic P fractions due to manure additions were strongly associated with a concomitant increase in the soil organic carbon. Cropping without manure and fertilizer P depleted soil organic P, while regular additions of manure and fertilizer P favoured its accumulation. The magnitude of depletion/build-up was strikingly larger in MLOP and MROP fractions compared to others, indicating that these two fractions are major sources and sinks for plant-available P in soil. (C) 2000 Elsevier Science Ltd. All rights reserved.