Investigating the Effects of Soil Type and Potassium Fertiliser Timing on Potassium Leaching: A Five-Soil Lysimeter Study

Potassium (K) is essential for grassland productivity, but soil K leaching can reduce fertiliser use efficiency, increasing environmental losses. International evidence suggests soil type and K fertiliser timing influence K leaching, yet limited data exist for Ireland’s diverse soil types. This study investigated the effects of K fertiliser timing (autumn, winter, and spring) and soil type on K leaching using a controlled lysimeter facility with five representative Irish soils sown with perennial ryegrass. Potassium fertiliser (125 kg K ha−1) was applied in October, December, or February, with leachate collected from October to April. Soil type affected cumulative K leaching (1.4–9.8 kg ha−1; p ≤ 0.001), with the greatest losses observed in sandy soils. Potassium and nitrogen uptake in spring-harvested grass were also influenced by soil type (p ≤ 0.05), with strong positive correlation between the two nutrients (R2 = 0.78; p ≤ 0.001). Temporally, significant interactions (p ≤ 0.05) between K application timing and sampling date were found for K leachate in three of the five soils tested. Autumn and winter applications tended to increase cumulative leaching risk, especially on coarser-textured soils such as the Oakpark soil (p ≤ 0.05). The study indicates that applying K in early spring will tend to reduce leaching K losses, particularly on sandy soils.