WATER-SOLUBLE ORGANIC-MATTER IN FOREST SOILS .2. INTERFERENCE WITH PLANT CATION UPTAKE

Soil culture experiments were conducted to examine the effects of wate r-soluble organic matter (WSOM), isolated from the A(h) horizon of a f orest soil on cation uptake by seedlings of Agrostis capillaris and Si lene dioica. In contrast to the large effects on soil equilibria, cati on uptake was only slightly affected by WSOM. Solubilized Al, Fe, Cu a nd Pb, were not found in higher amounts neither in the Toot nor in the shoot of the test species. Significant effects were only found for Cd , Mg, Mn and Zn, where uptake was either enhanced or decreased by WSOM . An experiment with several standard low molecular weight organic aci ds affirmed the results obtained with WSOM and supported the hypothesi s that cations are taken up predominantly in the free ionic form and n ot as organic complexes. Results are discussed in relation to the effe cts often found for fulvic and humic acids.