Effects of Grassland Ley Sward Diversity on Soil Potassium and Magnesium Forms in Two Contrasting Sites

Although multispecies grassland leys are known to influence nutrient dynamics in soils, little is known about the soil potassium (K) and magnesium (Mg) quantities in such systems. In this study, we quantified soluble, active, and exchangeable forms of K and Mg in two contrasting sites differing in soil types: Cambisols and Luvisols. These measurements were conducted in grassland ley swards differing in the species composition of their sown mixtures. The grassland ley experiment included six species belonging to three functional groups: grasses (G1: Lolium perenne, G2: Phleum pratense), legumes (L1: Trifolium repens, L2: Trifolium pratense), and herbs (H1: Cichorium intybus, H2: Plantago lanceolata). Thirty-three plant communities were established following a simplex design approach, with sown proportions ranging from 100% (monocultures) to 50%, 33%, 25%, 16.7%, and 0% across the different mixture combinations. Plant diversity only had a slightly negative trend for potassium on Cambisols. Grass-dominated mixtures maintained higher soil K levels, while legume-rich swards exhibited lower concentrations, consistent with legumes’ greater K demand. Notably, the different effects of extractants on K were most evident in Cambisols, indicating stronger cation retention. This indicates the need to use the CaCl2 and NH4OAc extraction methods to determine the potassium content in this type of soil, and that these methods should be considered for evaluation of soil fertility.