Direct and residual phosphorus effects on grain yieldphosphorus uptake relationships in upland rice on an ultisol in West Africa

Phosphorus (P) deficiency is a major constraint to crop production on highly weathered, lowactivity clay soils in the humid zone of West Africa. Past research suggested a linear relationship between grain yield and P uptake over a range of fertilizer P applied to upland rice cultivars. However, there is lack of information on how these relationships are affected by the long-term fertilizer P effects, although such information is needed for developing P management strategies because phosphate fertilization effects last for several seasons. Results from a long-term field experiment (1993-1998) conducted to determine the response of four improved upland rice cultivars to fertilizer P (0, 45, 90, 135 and 180 kg P ha-1) applied only once in 1993 and to its residues in 1994, 1995, 1996 and 1998, were used to determine grain yield and P uptake relationships. The soil at the experimental site, in the humid forest zone of Côte d’Ivoire (West Africa), was an Ultisol with acidic pH and low in available P. Significant linear relationships (R varying between 0.796 and 0.956) were observed between grain yield and total P uptake for each of the crops grown during 1993-1998. The results indicate that P uptake based models can be used to determine P requirements of rice cultivars under direct and residual P.