Long-term effects of slurry and fertilizer applications on soil extractable potassium and the soil potassium balance for a temperate grassland

Abstract The potassium (K) balance is a key indicator of the suitability and sustainability of grassland nutrient management practices. K inputs can impact soil fertility, biomass yield and herbage quality. This study evaluated data from a long-term grassland experiment established in 1970 in Northern Ireland with eight treatments: control, mineral fertilizer (NPK), cow and pig slurry at three application rates (50, 100 and 200 m 3 /ha). The objectives were to investigate the nutrient treatment effects on: (i) soil extractable K (SEK) (using a ammonium acetate solution), (ii) herbage K concentration and K offtake and (iii) the long-term K balance. Cow slurry provided significantly greater K inputs than pig slurry. Thus, the concentrations of SEK increased for the cow slurry treatments, but decreased significantly for the pig slurry, NPK and control treatments. The decrease in SEK was greater with soil depth. Increasing slurry rate had a positive effect on the concentrations of SEK and herbage K. The low rate of pig slurry had a herbage K concentration less than the required critical level. High rates of cow slurry led to luxury consumption of K which increases the risk of herbage cationic imbalance that could trigger tetany for livestock. Assessment of the long-term partial K balance revealed a K deficit for the NPK and pig slurry treatments, however high rate of cow slurry had a large K surplus. Refinement of nutrient management should carefully balance K inputs and losses to avoid K deficiency and excessive K inputs to grasslands.