Phosphorus input-output on an ultisol cropped to upland rice in West Africa

Abstract

A long-term (1993-98) experiment was conducted in Man, Cote d'Ivoire, West Africa, to study the effect of four proved upland rice cultivars to direct phosphorus (P) in 1993 and to residual P in 1994, 1995, 1996 and 1998, applied at five rates only once in 1993. The soil at the site in the humid forest zone is an Ultisol. The four rice cultivars were WARDA-bred cultivars WAB56-125, WAB56-104 and WAB556-50 and check IDSA6, and the five rates of P fertilizers, applied as triple superphosphate were (0, 45, 90, 135 and 180 kg P/ha). All plots received a uniform application of N (applied as urea at 100 kg/ha) and K (applied as KCl at 100 kg/ha). At harvest, rice grain and straw yields were recorded and total P uptake in rice biomass was computed by analysing the P content of the grain and straw samples. The P input-output value (amount of P applied and amount of P taken up in the grain plus straw in five crops) was negative in the no-P treatment but positive in treatments where P fertilizer was added. The results showed that in five crops, only a small fraction of the applied P fertilizer was removed by the rice crop, which varied from 5.5-9.4%, and this value decreased with the increase in the rate of P applied. In 1998, however, after 5 years of successive cropping, available P in the soil was found to be similar and not affected by the rate of P fertilizer applied in 1993