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Irrigated rice is less prone generally to phosphorus (P) deficiency than rainfed rice because redox reactions release P upon soil flooding. It is not known whether that is also true in highly weathered soils of Madagascar where the combination of high soil Fe and low P input may impede significant release of P. Soils and flag leaf samples were collected in 2010 in 38 irrigated rice and 46 rainfed rice fields belonging to private farmers. A critical flag leaf P content was derived from a P‐dosed pot trial study with three soils, and the results suggested 2.4 g P/kg as the critical value. Average flag leaf P was significantly larger in irrigated than in rainfed rice (2.2 compared with 1.7 g P/kg), and flag leaf P was below the critical value in 76% of irrigated rice fields while this fraction was 100% in rainfed rice. Nitrogen and K deficiencies were less prevalent. Flag leaf P increased with increasing soil pH and soil pH explained partially differences in leaf P between irrigated and rainfed rice. Flag leaf P was unrelated to soil organic matter, but increased with oxalate‐extractable soil P (Po). Multiple regression analysis revealed greater leaf P at equal soil Po and equal pH in irrigated compared with rainfed rice. Grain yield estimates (1‐m2 squares) increased with flag leaf P but not with leaf N and K. In a regression model, about 42 % of the yield variance was explained with soil Po and a rice‐growing system. The survey suggests that P is the main limiting nutrient for rice, and that soil P bioavailability is larger for irrigated than for rainfed rice in weathered soils of Madagascar.

A vast upland area in Madagascar remains uncultivated because of erratic rainfall and because of the low fertility of the soils that are highly weathered and depleted in available phosphorus (P). This study was set up to identify to what extent farmyard manure (FYM) can overcome P deficiency and increase the use efficiency of mineral P (TSP). Rainfed rice was grown with soybean in rotation (two fields) in three subsequent seasons with factorial supplies of FYM and TSP (both applied in planting hole) with blanket N&K doses. The low and unresponsive rice grain yields (<2 Mg ha -1 ) in the initial year were contrasted with large treatment responses cumulating in a grain yield of 5.8 Mg ha -1 in year 3 at highest rates, 3.6-fold above the no P and no FYM control with N&K and 11-fold above the absolute control. The above ground P uptake responded to total P application (TSP and FYM derived) and its slope significantly increased with FYM application. The fertilizer (TSP) P use efficiency in the above ground biomass, was 14% for the zero FYM dose increasing to 22% for the highest FYM dose of 10 Mg ha -1 at year 3 of study. The FYM benefits were likely unrelated to nutritional factors as revealed from tissue analyses and it is speculated that FYM alleviates moisture stress or Al toxicity. Dosing FYM only with no TSP did not alleviate P deficiency. This study illustrates the agronomic potential of the uncultivated area provided that the soil nutrients are capitalized.

Addition of organic matter (OM) to flooded soils stimulates reductive dissolution of Fe(III) minerals, thereby mobilizing associated phosphate (P). Hence, OM management has the potential to overcome P deficiency. This study assessed if OM applications increases soil or mineral fertilizer P availability to rice under anaerobic (flooded) condition and if that effect is different relative to that in aerobic (nonflooded) soils. Rice was grown in P-deficient soil treated with combinations of addition of mineral P (0, 26 mg P/kg), OM (0,~9 g OM/kg as rice straw + cattle manure) and water treatments (flooded vs nonflooded) in a factorial pot experiment. The OM was either freshly added just before flooding or incubated moist in soil for 6 months prior to flooding; blanket N and K was added in all treatments. Fresh addition of OM promoted reductive dissolution of Fe(III) minerals in flooded soils, whereas no such effect was found when OM had been incubated for 6 months before flooding. Yield and shoot P uptake largely increased with mineral P addition in all soils, whereas OM addition increased yield and P uptake only in flooded soils following fresh OM addition. The combination of mineral P and OM gave the largest yield and P uptake. Addition of OM just prior to soil flooding increased P uptake but was insufficient to overcome P deficiency in the absence of mineral P. Larger applications of OM are unlikely to be more successful in flooded soils due to side effects, such as Fe toxicity.

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