Potassium Removed from Some Southern Brazilian Soils by Exhaustive Cropping and Chemical Extraction Methods

Abstract

Abstract Soils from the orders Alfisol, Inceptisol, Mollisol, Ultisol, and Oxisol contained exchangeable K (neutral 1 N NH 4 OAc) and boiling 1 N HNO 3 extractable K varying from 31 to 358 ppm and 62 to 652 ppm, respectively. Total K varied from 1,780 to 14,200 ppm. Quantities absorbed from each soil by 7 cuttings of perennial ryegrass ( Lolium perenne ) in the greenhouse ranged up to 13 times that of exchangeable K and up to 5 times that of HNO 3 ‐extractable K. Total plant uptake represented 3.5 to 29.7% of total soil K. In no soil did K become limiting in the first four cuttings. However, except for the Mollisols, uptake decreased abruptly in the later cuttings, indicating very little “slowly” available K. Even though exchangeable K and K removed by strong acids were highly correlated with plant uptake ( r = 0.744** to 0.881**), all extractants greatly underestimated actual plant‐available K under intensive cropping. Total soil K did not significantly correlate with plant uptake ( r = 0.211).