Phosphorus Availability and Sorption as Affected by Long-Term Fertilization

Udic Ferrosols derived from the red clay of the Quaternary are characterized by a high P sorption capacity (PSC) and low P availability, which are mostly correlated with various inorganic phosphate (Pi) fractions that exist in soils. However, the direct cause and effect relationships between Pi and PSC or available P (Olsen P) can't be explained by a simple correlation analysis. Therefore, the objectives of this study were to determine which Pi fraction exerts the greatest influence on the PSC and Olsen P. A total of 21 soil samples collected from seven fertilizer treatments and with different available P contents and P adsorption capacities were obtained from a long-term fertilization experiment in Liujiazhan, Jiangxi Province, China, The PSC can be represented using the phosphate sorption index (PSI), so the PSI was determined for all samples by a single-point sorption method. Additionally, Pi fractions were determined according to Chang and Jackson's method. Multiple regression analysis and path analysis were used to discuss the direction and degree of influence of P-i on Olsen P or PSI. The results showed that various inorganic phosphates were positively correlated with Olsen P and negatively correlated with PSI. Nevertheless, only the Al phosphate (Al-P) and Fe phosphate (Fe-P) fractions were stable influential factors and critical intermediates for other inorganic phosphates' effects on Olsen P. Significant linear positive correlations existed between the Al-P/Fe-P ratio and Olsen P. and changes in the Al-P/Fe-P ratio of red soils is an important indicator of soil P availability.