Available phosphorus status and critical threshold for leaching in greenhouse soils influenced by different fertilizer sources

In this paper, laboratory soil incubation experiment was conducted to investigate soil available P (Olsen-P) and calcium chloride extracts P (CaCl2-P) contents influenced by manure and inorganic P sources at different rates in three soils with different planting years in Raoyang County, Hebei Province. The three soils included soil next to the greenhouse without planting vegetables (control, S1), plastic greenhouse soil with 30 years of planting vegetables (plastic greenhouse soil, S2）and solar greenhouse soil with 4 years of planting vegetables (solar greenhouse soil, S3). All three kinds of soil are taken from 0~20 cm topsoil. Organic fertilizer is represented by M, and inorganic fertilizer is represented by F. Six phosphorus levels were set (P): ① control (no fertilization, CK); ② 50 mg·kg-1 (dry soil) (M50 and F50); ③ 100 mg·kg-1 (dry soil) (M100 and F100); ④ 150 mg·kg-1 (dry soil) (M150 and F150); ⑤ 300 mg·kg-1 (dry soil) (M300 and F300); ⑥ 600 mg·kg-1 (dry soil) (M600 and F600). The organic fertilizer is air dried and decomposed pig manure, and the inorganic phosphorus fertilizer is KH2PO4. Each plastic bottle mouth is covered with a film, and there are two holes on the film to ensure air circulation. Then place the plastic bottle in the incubator at (25 ± 1) ℃ for constant temperature culture. The water content shall be controlled at 50% of the soil saturated water content. After 4 days of air drying, water shall be added to about 50% of the saturated water capacity. As above, the soil shall be air dried after 4 days of incubation. In this way, four times of alternating drying and wetting shall be carried out, so that the phosphorus adsorption and analysis in the soil can reach equilibrium. After 28 days of the test, the soil shall be air dried for testing. Olsen-P and CaCl2-P contents in soil were determined by molybdenum antimony resistance colorimetry after extraction with 0.5 mol · L-1 NaHCO3 (pH 8.5, soil water ratio 1 ∶ 20) and 0.01 mol · L-1 CaCl2 (soil water ratio 1:5), respectively. WPS 2016 and Sigma Plot (12.5, SystatSoftware Inc.) are used for data and chart processing. Data ANOVA was conducted using SAS (8.0, SAS Institute). The threshold value of soil phosphorus was simulated and analyzed by using the two-stage linear model of Sigma Plot 12.5 software to determine the threshold value of soil phosphorus leaching.