THE PHOSPHORUS COMPOSITION OF SOIL SOLUTIONS AND SOIL LEACHATES - INFLUENCE OF SOIL-SOLUTION RATIO

Compositional differences between soil solutions obtained by different methods have frequently been reported; variations in the soil:solutio n ratio may explain these results. In this study we compared the amoun t and composition of phosphorus (P) in soil leachates and soil solutio ns from a temperate grassland soil in northeast Scotland and determine d the influence of soil:solution ratio on P fractions in soil water ex tracts. Leachates were collected from intact soil cores over 6 months, the cores were then destructively sampled, and soil solutions obtaine d by centrifuging. Molybdate reactive P (MRP) represented 71% of the t otal dissolved P (TDP) in soil leachates but only 54% in soil solution s. The MRP component in soil water extracts increased from 71% to 92% as the soil:solution ratio increased from 1:1.5 to 1:15.4, while the d issolved organic P (DOP) component decreased from 26% to 6%. As the so il:solution ratio increased the amount of MRP extracted increased; by contrast the amount of DOP and dissolved condensed P (DCP) extracted r emained constant. While the MRP component is regulated by soil sorptio n processes, the supply and amount of DOP and DCP is probably related to biological activity. Dissolved organic carbon (DOG) extracted at wi de soil:solution ratios contained a smaller proportion of P than that extracted at narrower ratios. The results indicate differences in the behaviour of P fractions in the soil at various soil:solution ratios a nd that these are reflected in the P composition of soil solution and leachate.