Farmyard manure application has little effect on yield or phosphorus supply to irrigated rice growing on highly weathered soils

It is well established that organic matter application can enhance soil phosphorus (P) bioavailability in acid, weathered soils. It is yet unclear to what extents this also holds for irrigated rice in flooded soils where, in principle, organic matter may unlock soil P due to reductive dissolution of Fe-(III) minerals. This study was set up to identify if farmyard manure (FYM) can enhance the use efficiency of soil-P and mineral fertilizer-P in irrigated rice after excluding N and K effects. Field trials with factorial supplies of FYM and banded mineral P fertilizer (Triple Superphosphate or TSP) were set up at three irrigated rice fields in Madagascar with equal annual applications during three consecutive years. All treatments received a blanket N and K application. The FYM rates were within local agronomic rates and TSP rates were maximally 80 kg P ha−1. Total P application from FYM and TSP increased rice grain yields of 4.2 Mg ha−1 at the +NK-control to, on average for all years and fields, 5.5 Mg ha−1 at highest P doses. Grain yield generally responded to TSP but not to FYM. Total P uptake in grain and straw increased with total P application. Positive effects of FYM on P uptake were mainly related to its P application to irrigated rice soils. The soil P balances amended with FYM only, i.e. the common local farmers practice, were negative. At adequate N and K supply, effect of FYM application on increasing soil or fertilizer P use efficiency was not detectable in irrigated rice; it mainly recycles P.