Phosphate Sorption to Macropore Wall Materials and Bulk Soil

Preferential flow through macropores may allow reactive solutes to travel long distances in soil. The amount of solute transported is expected to depend on the ability of the macropore wall materials to retain the solute from the bypassing solution. From a loamy sand soil, samples of bulk Ap-horizon, bulk Btg-horizon, earthworm burrow lining, and both iron-depleted and iron-enriched materials from glossic fracture walls were obtained. The different soil materials were shaken for 5 min, 2 hr and 7 d in 0.01 M CaCl2 with initial P concentrations from 0 to 3.3 mg H2PO4--P L-1 at pH 5. The resulting changes in solution P-concentration were interpreted in terms of P-desorption or P-sorption. Burrow lining and bulk-Ap were poor P sorbents, especially at short contact times (5 min and 2 hr). They were unable to sorb P at concentrations below approximately 1 mg PO4-P L-1. In contrast, fracture wall materials and bulk-Btg were much stronger sorbents. They removed P from solutions having P-concentrations of only about 0.03 mg PO4-P L-1. The results of the study suggest that environmentally critical concentrations of dissolved Pi will be leached more easily through earthworm burrows than fractures, and that sorption characteristics of bulk soil may deviate strongly from sorption characteristics of macropore wall materials.