Objectives The objectives are to clarify the characteristics and main driving factor leading to the variation of soil available phosphorus content (AP) and phosphorus use efficiency (PUE) by crops, and provide scientific basis for phosphorus nutrient management in the Loess Plateau.

Methods Yongji County, located in the southeast Loess Plateau, was selected as the research region. The soil available phosphorus content and the relevant agricultural production data (crop yield, fertilization, livestock and poultry breeding, etc.) of the cultivated land that were published in 1983, 2008, and 2023, as well as the past 40 years (1983−2022), were searched, and the methods of geostatistics, ArcGIS, Random forest were used to analysis their periodical and spatial variation characteristics in the past 40 years.

Results Soil AP at the topsoil (0−20 cm) in Yongji County showed a trend of first rapid increasing and then slow decreasing from 1983 to 2023. The soil AP increased from 4.69 mg/kg in 1983 to 14.56 mg/kg in 2008, and then decreased to 13.37 mg/kg in 2023. The overall pattern of soil AP in 1983 was higher in the central and western regions but lower in the eastern region; Compared to 1983, the soil AP in the eastern region of Yongji County increased significantly in 2008, the farmland AP became higher in the east but lower in the west in 2023. In the past 40 years, chemical phosphorous fertilizer acted as the main source of phosphorus input to farmland, accounting for 74.8%-94.6% of the total phosphorus input in Yongji County. The western region input more phosphorus in the 1980s and 1990s, with an average input of P₂O₅ 159.0 kg/(hm²·a). During the 2000s and 2010s, the phosphorus input amount became higher in the central region with the unit area phosphorus input in towns reaching 250.2 kg/(hm²·a). Over the past 40 years, the phosphorus amount taken out of farmlands by crops continuously increased from 10.42 kg/(hm²·a) in 1983 to 25.2 kg/(hm²·a) in 2023, thereby the PUE showed a trend of first decreasing and then increasing, the current average annual PUE was 24.03%. In the 1980s, 1990s and 2010s, the PUE in the eastern region was higher than the central and western regions. Fertilization is the main factor affecting the available phosphorus content in cultivated land in Yongji County, and the relative importance of phosphorus application to the change of soilAP was 44.7%. In addition, the relative importance of elevation, annual rainfall and soil pH for the variation of soil AP were 21.6%, 20.8% and 12.9%, respectively.

Conclusions We suggested that the low phosphorus input towns in the central and western regions (such as Chengxi Town and Puzhou Town) should appropriately apply more phosphate fertilizer and organic fertilizer to increase the soil available phosphorus content in, and the regions with relatively low phosphorus use efficiency (such as Hanyang Town) need to optimize fertilization methods and pay attention to the rational application of nitrogen, phosphorus and potassium fertilizers.