Soil phosphorus storage capacity as affected by repeated phosphorus addition in an Ultisol

Soil phosphorus (P) loss risk and environmental impact can be assessed by different soil test indicators such as soil P storage capacity (SPSC), a site-specific and reliable measurement of soil P status. In this study, the changing trends of soil P saturation ratio (PSR) and SPSC in red soil (equivalent to Ultisol) were discussed after 10 consecutive additions of P solution with 20 mg P L-1, and the relationships between Olsen-P, CaCl2-P, and PSR, SPSC also were evaluated. Results showed that the soil PSR was increased to 0.22 at the end of P addition, and the accumulated P was high to 1625 mg kg(-1). The threshold PSR of the red soil was determined to be 0.10, and the SPSC significantly decreased from 242.0 mg kg(-1)to -382.1 mg kg(-1)with the increase of P addition times. When SPSC = 0, the accumulated P in red soil was 487.7 mg kg(-1). Therefore, there existed four stages for the soil P management in red soil according to the changes of SPSC and Olsen-P: short supply stage of soil P (SPSC>88 mg kg(-1)and Olsen-P <= 20 mg kg(-1)), stable supply stage of soil P (68 mg kg(-1)< SPSC <= 88 mg kg(-1)and 20 mg kg(-1)< Olsen-P <= 40 mg kg(-1)); over supply stage of soil P (0< SPSC <= 68 mg kg(-1)and 40 mg kg(-1)< Olsen-P <= 108 mg kg(-1)), and the high loss risk stage of soil P (SPSC <= 0 and Olsen-P > 108 mg kg(-1)). In the last stage, P fertilization must be stopped and environmental protection measures must be taken immediately.