Micro‐dose placement of phosphorus induces deep rooting of upland rice

Upland rice production is often constrained by phosphorus deficiency (P) and drought events. Methods are needed to maximize P use efficiency, while promoting deep root development to mitigate drought. This study evaluates micro-dose P placement as a technique to enhance drought resilience of upland rice, thereby hypothesizing that P placement enhances deep root development, stimulated by the local P supply in the planting hole, compared to broadcast P. Two pot experiments were conducted using P deficient upland soil in Tanzania (Expt.1) and Madagascar (Expt.2), with factorial combinations of P doses (control and two P levels), application method (placement versus broadcast), rice varieties (DJ123 & NERICA4) and water regimes (field capacity and drying periods). Micro-dose P placement strongly boosted the establishment of upland rice and enhanced P recovery rates (4- to 5-fold) and fertilizer use efficiency (9- to 14-fold) compared to broadcast P. Micro-dose P placements significantly enhanced the fraction of roots found at largest depth (> 30 cm, Expt. 1; >15 cm, Expt. 2) by 22- to 33 %. Surprisingly, shoot P concentrations were markedly lower under P placement than under broadcast at equivalent P doses or equivalent biomass, indicating a Piper-Steenbjerg effect. This study is first in showing the enhanced deep rooting after basal P placement, and suggests replication at field scale where subsurface moisture may yield stronger benefits than in pots. The depleted shoot P concentrations induced by vigorous plant establishment under P placement may, however, counteract benefits at later growth stages and need further attention.