Optimizing the Phosphorus Concentration and Duration of Seedling Dipping in Soil Slurry for Accelerating the Initial Growth of Transplanted Rice

Given the finite nature of P fertilizer resources, it is imperative to investigate effective P management practices in order to achieve sustainable rice production. This study was conducted (1) to assess the effect of dipping rice seedlings in P-enriched slurry before transplanting (P-dipping, hereafter) on initial plant growth and (2) to determine the optimum P concentration and dipping duration. In the P-dipping treatments, four P2O5 concentrations in the slurry (4.3%, 5.0%, 6.0%, and 7.5%) and four dipping durations (0.5 h, 2 h, 4 h, and 8 h) were investigated. After the treatments, the seedlings were transplanted into 1/5000 Wagner pots and grown under flooded conditions for 42 days and they were compared with plants under conventional P incorporation at the rate of 300 mg P2O5 pot(-1) and with plants under no P application. The amount of P2O5 attached to P-dipped seedlings, or locally applied in the rhizosphere at transplanting, increased with higher P concentrations in the slurry, ranging from 87.5 to 112.2 mg pot(-1). Shoot biomass at 42 days after transplanting (DAT) was greatly increased in plants under the P-dipping treatments, compared to that in plants with no P application and was comparable to or greater than that in plants under conventional P incorporation, even when P levels were 2.5 to 3 times lower. Among the P-dipping treatments, we observed some significant effects of P concentrations and dipping durations on seedling P uptake and shoot biomass, without any interaction between these variables. Seedling P uptake and biomass tended to be higher with higher P concentrations in slurry and longer dipping durations. Conversely, the shoot biomass at 42 DAT was significantly lower in plants under the highest P concentration treatment (7.5% P2O5) compared to that in other plants and tended to be lower with longer dipping durations (4 h and 8 h). These negative effects can be attributed to the slow recovery from transplanting shock because of the chemical damage of seedlings exposed to higher salt concentrations for longer durations. The present study highlights that (1) P-dipping could be an effective approach to increase transplanted rice production with minimal P inputs, and (2) this effect could be higher with a low P-concentration in the slurry (4.3% P2O5) and a short dipping duration (0.5 h). Based on the obtained results, further on-farm trials are expected to assess farmers' appreciation and the potential constraints of adopting this technique.