Relative efficiency of ammonium polyphosphate and orthophosphates for wheat and their residual effects on succeeding cowpea fodder

The relative efficiency of ortho and polyphosphates as P sources for wheat were studied in a field experiment with five sources—TSP, DAP, NP, APP (solid) and APP (Liquid) at three levels—13, 26 and 39 kg P ha^–1 and the residual effect was studied on a succeeding cowpea (fodder). Both the crops were grown each year at same location. Wheat responded upto 39 kg P ha^–1 in the first year but only upto 26 kg P ha^–1 in the second year. The growth parameters—plant height and dry matter production and yield attributes-ears m^–1, grains ear^–1 and test weight were favourably influenced by P application. The rate of P uptake initially exceeded the dry matter production. Wheat when fertilized with 26 or 39 kg P ha^–1 left behind a significant residual effect to raise the soil P availability and dry matter yield of cowpea. The grain yield of wheat was higher with APP's than with NP or DAP but the residual effect was the highest with NP. The magnitude of build up of soil available P with NP and APP's were similar and higher than DAP or TSP.