Effect of three years of pig slurry application on the distribution and contents of exchangeable K in three different soil types in the plain of Piacenza

Variations in exchangeable K (Kexch), amount of recovery of K in the soil, of the uptake and of K recovery in the crops, and the amounts of unaccounted-for K in the soil-plant system, have been determined for three soil types with different textures, CEC, pH, and lime content. These soils were fertilized with pig slurry for three consecutive years, in different periods, at a constant rate of 60 t N ha-1 year-1 in experimental plots cropped with maize (Zea Mais L.). The increases of soil Kexch observed for soil 3 (clay with ΔmaxKexch estimated 162.6 mg kg-1) showed values significantly (P<0.01) greater than the other two soils (soil 2 silty-clay-loam with a value estimated for ΔmaxKexch 72.6 mg kg-1 and soil 1, silt-loam with estimated ΔmaxKexch 38.1 mg kg-1). The difference in the estimated cumulated amounts of Kexch among the three soil types is due to the different clay content, C.E.C. and to the higher slope values present in soils 1 and 2 respect to soil 3 too. For soil 3 only a significant inverse correlation (P<0.01 and r = -0.945) was observed between ΔKexch and K applied with slurry. The amounts of K taken up by the crops were only slightly correlated with the applied K: soil 2 (r = 0.804; tot. rec. = 33.3 % K), soil 1 (r = 0.785; tot. rec. = 35.6 % K) and soil 3 (r = 0.581; tot. rec. = 39.8 % K).The values of unaccounted-for K were, on average, significantly lower (P<0.01) for soil 3 (17.4 %) compared to soil 1 (58.3 %) and soil 2 (59.5 %). Only for soil 3 a significant correlation (P<0.05) was found with the amount of applied K (r = 0.957).