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

Abstract. 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.