The effect of soil organic matter on potassium fixation
One of three primary nutrients required by crops, potassium (K) is found in several pools in the soil. Plants can readily take up K from the cation exchange sites of soil particles, but in the case of certain clay minerals, namely vermiculite, K can become fixed in the clay interlayer where it is slowly available to plants over longer timescales. Soil organic matter can bond to the surface of the clays and reduce K fixation. It was hypothesized that removal of organic matter could increase the K fixation potential. To test this, an experiment determined the K fixation capacity for eight whole soils, and again with the organic matter removed. However, this study found that on average, nearly 60% of the K fixation capacity was lost when organic matter was removed. It is possible the procedure, in addition to removing the organic matter, oxidized iron in the clay minerals from ferrous to ferric iron, effectively reducing the net negative charge of the clays and therefore the K fixation potential. Further research is needed to determine organic matter's effect on K fixation without affecting the charge of the soil's mineral fraction.