Dipping rice seedlings in P-enriched slurry increases grain yield and shortens days to heading on P-deficient lowlands in the central highlands of Madagascar

Phosphorus (P) deficiency is a major yield constraint for lowland rice production on highly weathered soil in Sub-Saharan Africa. To overcome this constraint, we examined the effect of dipping seedling roots into a P-enriched slurry before transplanting (P-dipping) on yield and P use efficiency in the typical P-deficient lowlands in the central highlands of Madagascar. Experiment 1 was conducted at a high-elevation site (Site1) and consisted of three levels of P treatments, 0 P (no P applied), 60BP (60 kg ha–1 of P2O5 broadcasted), and 30DP (30 kg ha–1 of P2O5 applied via P dipping), combined with two levels of N treatments, −N (no N applied) and +N (total of 60 kg ha–1 applied). Experiment 2 was conducted continuously at Site1 and additionally at middle-elevation site (Site 2) following the same design but adding the 30BP treatment (30 kg ha–1 of P2O5 broadcasted). The 30DP treatment significantly and consistently accelerated initial biomass production with high crop growth rate (CGR) and shortened days to heading by 14 days compared to 0 P and by 6 to 9 days compared to 30BP and 60BP. The 30DP treatment increased grain yields by 59–171% relative to 0 P and, more importantly, by 9–35% relative to 30BP and 60BP, with a significant site × P-treatment interaction. The effect of P-dipping was consistent between the N treatments. The effect of P-dipping was particularly significant at a high-elevation and cool climate site (Site 1), which was partly attributable to the improved grain fertility by avoiding cold stresses at the reproductive stage. Furthermore, P-dipping achieved remarkably high agronomic P use efficiency (increase in grain yield per kilogram of P applied): 85–198 kg kg–1 across the sites. A set of on-farm trials revealed that P-dipping can increase both grain yields and P use efficiency, even with shorter growth durations, than equivalent or doubled P application rates via broadcasting. Because lowland rice production in Sub-Saharan Africa is widely subjected to environmental stresses and to highly P-deficient soils, P-dipping could be an efficient P fertilization technique for resource-limited farmers in the region. Further participatory farmers' trials are expected to assess the farmers' appreciation and potential constraints for adopting this technique.