| heal.abstract |
Surface and subsurface samples from 26 representative agricultural soils (13 Entisols, 9 Alfisols and 4 Vertisols) from Central and South Western Greece were equilibrated for a week with P solutions ranging from zero to 800 mg P kg-1 soil. The Freundlich and in the cases of high fine carbonate content (> 6%) the linear equations described the sorption isotherms in a satisfactory way. Sequential fractionation indicated that a 0.1 M NaOH plus 1 M NaCl + 0.3 M Na-citrate + 1 M NaHCO3 (buffer) solution extracted more than 60% of the P retained by Entisols containing low to medium fine carbonates and by Vertisols, and less than 60% by Entisols with a high fine carbonate content. In all Entisols, this percentage decreased significantly at P additions higher than 150 mg kg-1. The opposite was observed in Vertisols, in which this fraction was the most important part of the retained P up to the maximum P addition. In contrast, in Alfisols this fraction reached 100%, but in the case of high Bray-P (> 50 ppm) it was more than 100% of the retained P. The second fraction extracted with the 0.3 M Na-citrate + 1 M NaHCO3 buffer plus Na-dithionite was found to be much smaller than the first fraction, increasing with P additions in absolute amounts but remaining constant as a percentage of the retained P, ranging from 15-35% among the studied samples. The third fraction obtained with 1 M HCl solution showed a peculiar behaviour. It was close to zero of the retained P in samples containing carbonates (Entisols and Vertisols) at P additions up to 300 mg kg-1 soil, but increased significantly at higher P additions and almost doubled in Entisols. Finally, the P fraction obtained after seven successive extractions with 0.01 M CaCl2, correlated well by a polynomial of 2nd degree equation with NaOH+CB-P and linearly with Olsen-P and Bray-P. © WILEY-VCH Verlag GmbH, 1998. |
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