PHOSPHATE SORPTION IN VERTISOLS OF SOUTHWESTERN SPAIN

Low phosphorus availability is a critical impediment to agricultural u se of Vertisols. Several studies have explored the P sorption characte ristics of Vertisols, but few have investigated the relationship betwe en soil physicochemical and the P sorption parameters calculated from uniform-surface and two-surface Langmuir equations, Phosphate sorption was investigated in surface-samples of 19 Vertisols in southwestern S pain. The phosphate sorption maxima (PAM) deduced from the uniform-sur face Langmuir equation ranged from 150 to 2566 mg P kg(-1), with a mea n value of 1115 mg P kg(-1). The calculated sorption maxima (TPAM) for the two-surface Langmuir model ranged from 410 to 15406 mg P kg(-1), and the mean value (4296 mg P kg(-1)) increased about 3 times compared with the basic Langmuir equation, The two-surface Langmuir model fit the sorption data better than the uniform layer model, Therefore, an u nderestimation of the P-sorption was observed when the traditional Lan gmuir equation was used,Total surface area was highly correlated with sorption maxima: n = 0.708** for PAM and 0.900*** for TPAM, For the t wo-surface equation, correlation analysis suggested that the ''active' ' CaCO3 (ACCE) was the most active sorbent of P on the high energy sit es, whereas citrate-dithionite-bicarbonate extractable Fe (Fe-d) contr ibuted to P sorption on the low energy sites. The prediction of the PA M and TPAM was improved by combining all or several of following soil properties: Fe-d, ACCE, clay, and total surface area in a multiple-reg ression analysis. The equations obtained could offer a rapid estimatio n of P-sorption in Vertisols of southwestern Spain. Moreover, the resu lts obtained from a greenhouse experiment with Agrostis tennuis and tw o levels of P (0 and 218 mg P kg(-1)) showed that the amount of availa ble soil P was a positive function of Fed but a negative function for ACCE.