Optimal phosphorus management strategies for wheat-rice cropping on a loamy sand

Knowledge about optimal P rates for wheat (Triticum aestivum L.)-rice (Oryz a sativa L.) cropping is insufficient because of nutrient availability diff erences between aerobic and anaerobic soil. We assessed P management strate gies in a wheat-rice rotation on a Typic Ustochrept at Ludhiana, India. Sev en P fertilizer treatments applied to wheat and rice, respectively, (P0-0, P0-26, P13-13, P26-0, P26-13, P39-0, and P26-26; treatment abbreviations us ed include P applied to wheat followed by P applied to rice, both in kg ha( -1)) were compared from 1990 to 1997. Grain yield and seasonal P accumulati on by wheat were highest for higher P rates and remained stable in treatmen ts with P applied to wheat. Phosphorus application to rice increased P accu mulation by rice, but did not consistently increase rice yields because flo oding decreased soil P sorption and increased P diffusion resulting in high er P supply to rice relative to wheat. Indigenous soil P supply measured in wheat was 5.8 to 8.0 kg P ha(-1), as compared with 14.9 to 18.1 kg P ha(-1 ) in rife. Phosphorus adsorbed by ion exchange resin capsules placed in sit u was five times greater under rice than under wheat. Applying only 26 kg P ha(-1) to wheat and no P to rice was not economical and led to a negative P balance and a decline in soil P. Applying 32 kg P ha(-1) to wheat and 15 kg P ha(-1) to rice was optimal for achieving short term economic and long- term agronomic goals when both grain and straw were removed from the field. These findings require further validation at other sites, at higher rice y ield levels, and for different straw management.