Phosphorus fertilizer recommendation based on minimum soil surplus for cotton growing in salt-affected soils

Phosphorus (P) is a non-renewable resource that will become an economic limiting factor for food production in the future. Cotton is one of the most salt tolerant cash crops and therefore is widely grown on saline soil in China. Farmers often apply excessive P to obtain high yield. This has led to large P accumulation in soils and lower P use efficiency (PUE) as the world progresses towards "peak phosphorus" in the coming years. The objective of this study was to develop an optimum strategy that balances P supply with demand in cotton grown in saline soils in northwestern China. In order to understand a balanced P management regime based on targeted-yield-demand, a seven-year (2014–2020) field trail with six treatments of P0 (no P), P33 (33 kg P ha−1 chemical P, targeted-yield-demand rate), P65 (65 kg P ha−1 chemical P, current recommended rate), P131 (131 kg P ha−1 chemical P, excessive P rate), S18 (18 kg P ha−1 straw P, straw retention) and S36 (36 kg P ha−1 straw P, doubled straw retention) was conducted in Xinjiang, China. The soil salinity at the site was classified as moderate at the beginning of the study with an optimal soil available P of 16.3 mg kg−1. This study investigated the effect of different management strategies on improving P efficiency, including PUE, P recovery efficiency (PRE) and agronomic efficiency (AE-P). The results showed that the mean lint yields of treatments ranged from 1890 to 2250 kg ha−1, which is a marginal medium yielding level in this region. All P input treatments, except for S18, had higher yields compared to the P0 treatment, and the P33 treatment gave the highest yields. The P input-output balance was nearly zero in the P33 and S36 treatments, while maintaining the soil Olsen-P concentrations at 18.5 mg kg−1. The PRE in the P33 and S36 treatments was 103% and 94%, respectively. The P33 treatment gave the highest AE-P and economic return, suggesting the average P uptake of 33 kg P ha−1 at targeted lint yield of 2250 kg ha−1 should be the recommended P fertilizer rate in this region. Applying P based on P demand for target cotton yields maintained soil available P concentrations at a critical level and resulted in nearly 100% utilization of P fertilizer. This P management strategy is economic, environmentally responsible, and sustainable for cotton production. These results suggest that balancing P input with P demand will lead to more sustainable soil P management.