Distribution of phosphorus in particulate, colloidal and molecular-sized fractions of soil solution

The total P in soil solution can be divided into three procedurally defined components namely molybdate reactive P (MRP), organic P (OP) and condensed P (CP). The amounts of these components in different size fractions of soil solution from a peaty podzol was determined using 1.6- and 1.2-μm filters to remove particles, and 0.45- and 0.22-μm filters to remove colloids. For MRP, 55% was associated with particles >1.2 μm and a further 23% with colloids >0.22 μm. For OP, 34% was associated with particles >1.2 μm and a further 46% with colloids >0.22 μm in size. CP behaved differently with no significant amount (P≥0.05) associated with particles >1.2 μm, and around 54% associated with colloids >0.22 μm. In a separate experiment, the distribution of MRP, OP and CP in different molecular sized fractions of 0.45-μm filtered soil solution was determined using 100-, 10- and 1-kD ultrafilters in a stirred cell, part filtering the soil solution to minimize compositional changes in the retained fraction, and adding NaCl to minimize electrostatic exclusion. The 0.45-μm filtrate contained 116 of MRP, 212 of OP and 332 μg dm−3 of CP. Ultrafiltration showed 32% of the MRP, 95% of the OP and 90% of the CP was >10 kD. During ultrafiltration a gel, containing P mostly as OP, accumulated on the filter. The results highlight the quantitative importance of P associated with particulate, colloidal and high molecular weight materials in soil solution from an organic soil.