Dipping vigorous seedling roots in phosphorus-enriched slurry at transplanting efficiently increases lowland rice yields

P-dipping refers to the placement of phosphorus (P) fertilizer at the root system during transplanting of rice by adhering P-enriched slurry to the seedling roots. This approach is beneficial for smallholder farmers in sub-Saharan Africa who apply small amounts of P to highly P-fixing soils. This study aimed to identify the optimum seedling age for maximizing the impact of P-dipping. Pot experiments revealed that the adhered amounts of slurry to the roots with P-dipping increased in a sigmoidal pattern against seedling age. Correspondingly, the effect of P-dipping on the initial biomass was enlarged with older seedlings in a sigmoidal pattern, increasing slowly during the young seedling age (2.9–4.5 leaves), sharply during the intermediate seedling age (4.5–6.3 leaves), and plateauing during the old seedling age (6.3–7.0 leaves). Combining P-dipping with much older seedlings (> 7.0 leaves) resulted in severe transplanting shock and plant death. On-farm trials on 90 fields in Madagascar demonstrated a significant interaction between seedling age and P treatment on grain yield. The highest yield gains over the control from P-dipping were observed in seedlings with intermediate age (1.0 ​t ​ha⁻¹), followed by old (0.7 ​t ​ha⁻¹) and young (0.6 ​t ​ha⁻¹) seedlings. These results suggested that vigorous and intermediate seedlings with higher slurry adherence than young seedlings and a lower risk of transplanting shock than old seedlings benefited most from P-dipping. This finding provides smallholder farmers with practical knowledge on how to apply the P-dipping more efficiently for achieving improved P management for sustainable rice production.