Objectives In Xinjiang Uygur Autonomous Region, excessive phosphorus fertilizer input is common, leading to low P utilization efficiency and high cost in cotton production. We conducted localized field experiment to research the suitable phosphorus fertilizer dosage for acquiring high cotton yield and ecologically safe field phosphorous supply capacity.

Methods During 2018−2022, cotton field experiment was carried out in Shawan City, the main cotton producing area in Xinjiang. Six phosphorus (P₂O₅) application levels of P0 (kg/hm²), P50 (50 kg/hm²), P75 (75 kg/hm²), P100 (100 kg/hm²), P150 (150 kg/hm²) and P300 (300 kg/hm²) were set up. At harvest of each year, soil were samples for determination of water-soluble P, Olsen-P, and total P content; the cotton yields were recorded, and cotton seeds and flocs samples were collected to measure P content and then calculate the P output by cotton. All the cotton straws were returned to field every year.

Results Compared with P0, all the P fertilizer treatments increased soil Olsen-P content by 25.62%−64.69%, with an annual average increase of 0.89−2.55 mg/kg; increased soil water-soluble P content by 68.60%−112.98%, with an annual average of 0.67−1.10 mg/kg; increased soil total P content by 2.82%−9.37%, with an annual average of 0.01−0.03 g/kg. The linear fitting results showed that every extra 100 kg/hm² accumulation of soil P would result in an increment of Olsen-P and water-soluble P content by 1.93 mg/kg and 0.76 mg/kg, respectively. With the increase of P application rate, cotton yield first increased and then maintained stable. The fitted result of linear plus platform model showed that the P₂O₅ application rate for the maximum cotton yield was 124 kg/hm². The agronomic threshold of Olsen-P in cotton field was 19.23 mg/kg. P50−P150 treatment did not increase soil total P significantly until the fifth year, while P300 increased soil Olsen-P content close to the agronomic threshold in the first year of application.

Conclusions Continuous application of phosphorus can significantly increase the availability of soil phosphorus. For newly reclaimed cotton field, high amount of phosphorus fertilizer (300 kg/hm²) should be applied to increase Olsen-P to high level in the first year, then the phosphorus application rate should be controlled between 100−124 kg/hm² to reduce soil P surplus and meet the demand of phosphorus nutrition for high yield of cotton.