Effect of residual phosphate on yield of lowland rice in North Sumatera

Greenhouse experiments were conducted at three locations to study the residual effect of phosphate on rice, grown successively for four seasons in North Sumatera, Indonesia. The soils used at the three locations were high in phosphate from continuous applications with phosphorus (P) fertilizers for 15 years. Rice (Oryza sativa), variety IR-64, was grown receiving treatments with 0, 50, 100, 150, and 200 kg ha(-1) superphosphate. These treatments, replicated three times, were applied only to the first crop. At the end of each growing period, rice yield was determined and the soil analyzed for available and acid extractable P content by the Olsen and HCl extraction methods, respectively. The results indicated that the high concentration in available or acid extractable P content in the first season had resulted in the highest rice yields. Rice yield of the second crop decreased and remained constant at this level through the fourth crop. The phosphate levels in soil appeared to show a similar trend as rice yields. Both available and HCl extractable P decreased in soils in the second and third growing season to level off at the fourth season. Apparently the plants were able to satisfy their P requirements from these residual P contents in soil during the second through the fourth growing season. It can be concluded that the use of P fertilizers can be reduced substantially both in total and frequency of applications.