Potassium Uptake by Grass from a Clay and a Silt Soil in Relation to Soil Tests

Abstract

In a pot experiment with five harvests of ryegrass the potassium (K) uptake from soil was studied in a clay and a silt soil, with (K1) and without (K0) K fertilizer. Ammonium acetate lactate-extractable K (K-AL) was rapidly depleted, and in K0 the K-AL level stabilized at 30 and 100 mg K kg -1 in the silt and clay soil, respectively. Corrected for different ''minimum'' values, the K-AL value predicted the K uptake by ryegrass from AL extractable K very well. In the silt soil the K release from reserve K (total K release from soil minus K release from K-AL) was small, whereas in the clay soil there was a substantial release from reserve K. Part of the reserve K in the clay soil was easily releasable and contributed to luxury consumption of K in the first crop. Acid-soluble K (K-HNO 3 minus K-AL) was a good parameter by which to assess the ability of the soil to supply ryegrass with reserve K. The results were compared with the results of the field experiments from which the soils were collected. The difference in K release between silt and clay soil was larger in the pot than in the field experiments, but without K fertilization the K-AL values levelled off at the same values in field and pot experiments.