Phosphorus potassium and magnesium

These elements are determined at pH 3.6 according to the Egner-Riehm dou-ble-lactate extraction method (Egner et al., 1960; VDLUFA, 1991; sections A. and A. The solubility of phosphorus in the soil is influenced by its biological activity and humus content. Soil microflora excrete organic acids such as acetic (weak), lactic (medium) and citric (strong), and these are chosen as extractants. The acetic acid-sodium acetate extraction is that of Morgan (Lund et al., 1950), using 5 g soil plus 25 ml extractant at pH 4.8. Soil P is readily soluble in acetic acid, plant-available P is extracted by lactic acid, while the citric acid extraction indicates the potential reserves of P in the soil. This latter extractant is 2% citric acid, using 5 g soil plus 50 ml citric acid solution (VDLUFA, 1995). In a biologically active soil, the ratios of P extracted by acetic, lactic and citric acids should be about 1:3:9. Lower biological activity could widen the lactic:citric extractable P ratio to 2:10, and the biological processes should be stimulated by suitable organic fertilization using, e.g. green manures.

Potassium is determined in the acetic acid as well as the lactic acid extract. The potassium values for biologically managed fields lie between 100 and 200 mg K kg-1 soil, and are less than those recommended for conventional agriculture. Light soils normally have low values, and clay soils, which bind more potassium, have higher K levels. Potash fixation is also assessed according to Schlichting and Blume (1961, p. 84).

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