Improving phosphate rock solubility and uptake and yields of lowland rice grown on an acidic soil amended with legume green manure.

Abstract

The study investigates the effect on rice yields of rock phosphate (PR) combined with a pre-rice green manure (GM) (Aeschynomeme afraspera) as compared with its application directly to the rice crop grown in rotation in Côte d'Ivoire. Ten days after incorporation of the legume residues into flooded potted soil, the anaerobic decomposition of relatively large amounts of applied organic matter, resulted in intense gas emissions from the pots and formation of copper-coloured film in the water surface. A few days later, rice seedlings started to die, presumably due to the release of toxic compounds formed during the GM decomposition. Grain yield increase averaged over GM was 1.6- (PR) and two-fold (triple superphosphate, TSP) above the no-P control treatments. Similarly, mean grain yield over fertilizer P sources was significantly greater following GM than following bare fallow. The relative agronomic effectiveness (RAE) was 78% in the absence of GM. Applying TSP to the legume resulted in 62% more rice dry biomass as compared to P applied alone to rice. Combined PR+GM resulted in a 46% increase over PR alone. Incorporation of unfertilized GM improved rice N nutrition by 62 mg N/pot but was not significantly different from that of the control (no-P and no-GM) treatment. The increase in rice P uptake in response to PR application in the presence of GM and in the absence of GM were 32.3 and 15.5 mg P/pot, respectively. Applying PR with GM contributed to increasing 1.2-fold the amounts of extractable Bray-1 P in soil at rice maturity, as compared to PR applied alone.