POTASSIUM RELEASE KINETICS AND AVAILABILITY IN UNFERTILIZED VERTISOLSOF SOUTHWESTERN SPAIN

There are few studies about K status and K exchange equilibrium for Ve rtisols of southwestern Spain, and the K requirements for crop product ion in this region are not well defined. Moreover, there is no informa tion on the relationship between the kinetics of K release and plant K uptake for these soils. The objectives of this work were (i) to inves tigate the K status in these soils, (ii) to investigate the kinetics o f K release, and (iii) to compare the effectiveness of different extra ction techniques for the prediction of R-supplying capacities. Kinetic s of soil K release by successive extractions with 0.01 M CaCl2, extra ctable K using different soil extractants, and total K were studied in surface samples of 19 Vertisols in southwestern Spain. Total K uptake in three cuts of Agrostis tennuis grown in the greenhouse was used to measure plant-available soil K. The following procedures extracted in creasingly higher average amounts of soil K: 2 M NaCl (181 mg K kg(-1) ), 0.1 M HNO3 (285 mg K Kg(-1)) 1 M NH4OAc (371 mg K kg(-1)), and boil ing 1. M HNO3 (1328 mg K kg(-1)). Potassium release kinetics were desc ribed best with the Elovich equation, which showed the best fit of the four models tested. Elovich b values were most closely associated wit h K uptake by Agrostis tennuis (r = 0.942). Potassium extracted by 2 M NaCl and by 0.1 M HNO3 showed higher correlation with K uptake by the crop (r = 0.905 and r = 0.899, respectively) than did 1 M NH4OAc (r = 0.724), which is used to extract K in the soils of the region studied . A multiple regression model with K extracted by 2 M NaCl and by 0.1 M HNO3 as independent variables also explained 91% of the total variat ion in K uptake.