Ability of some Norwegian soils to supply grass with potassium (K) ‐ soil analyses as predictors of K supply from soil

Uptake in grass crops of ammonium acetate lactate extractable K (K AL ) and reserve K (interlayer K + structural K) in soil was studied in 16 field experiments at different locations on a range of mineral soil types in Norway. The K uptake from soil, both from K AL and reserve K, was considerable, often even at the highest level of K fertilizer. During three years, only on the sandy soils with a low level of acid soluble K (K HNO3 minus K AL ) was there a yield response to K fertilization. The K AL values declined rapidly and flattened off at a ‘minimum level’ which differed with soil type. This minimum level for K AL is a useful parameter in fertilizer planning, because the grass usually took up the K in excess of the minimum level over two years. The minimum value of K AL was significantly correlated with the content of clay + silt in soil. The decrease in K AL during the growing season was closely correlated to the K AL value in spring minus the minimum value and, therefore, the amount of K supplied to the grass from the K AL fraction can be calculated. Furthermore, the K AL value for the following spring may be estimated. The release from reserve K was partially related to acid soluble K.