DIRECT AND RESIDUAL FERTILIZER PHOSPHORUS EFFECTS ON YIELD AND PHOSPHORUS EFFICIENCY OF UPLAND RICE IN AN ULTISOL

Phosphorus (P) deficiency is one of the major limiting factors for cro p production in highly weathered soils in the humid zone of West Afric a. Very few studies have evaluated the residual value of fertilizer P to rice in these soils. Field experiments were conducted for three yea rs (1993-1995) to determine the response of four upland rice cultivars to fertilizer P applied at 0, 45, 90, 145 and 180 kg P ha(-1) only on ce in 1993, and to fertilizer P residues in 1994 and 1995. The soil at the experimental site, in the humid forest zone of Ivory Coast (West Africa), was an Ultisol, low in available P. Grain yields of the rice cultivars were significantly increased by fertilizer P in 1993, and by the fertilizer P residues in 1994 and 1995 although the magnitude of response decreased with time since the fertilizer was applied. The cul tivars differed in cumulative agronomic and physiological efficiencies , and the efficiencies were higher at the lower rates of P. The amount s of total P removed in three successive crops were similar for all th e four rice cultivars although P harvest index was 10-12% higher in th e P efficient than inefficient cultivars. The results suggest that the differences observed in the P efficiency of rice cultivars are due to differences in the internal efficiency of P.