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Potassium Removed from Some Southern Brazilian Soils by Exhaustive Cropping and Chemical Extraction Methods¹

ABSTRACT

Soils from the orders Alfisol, Inceptisol, Mollisol, Ultisol, and Oxisol contained exchangeable K (neutral 1N NH₄OAc) and boiling 1N HNO₃ 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 HNO₃-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).

¹ Contribution from the Departments of Soil Science, Federal University of Rio Grande do Sul (Brazil), and University of Wisconsin, Madison, 53706, as part of USAID contract 1A-147. Part of dissertation submitted by the senior author in partial fulfillment of the requirements of the M.S. degree, Federal University of Rio Grande do Sul. Presented before Div. S-4, Soil Science Society of America. Aug. 24, 1970 at Tucson, Arizona.

² Former Research Assistant at the Federal University of Rio Grande do Sul, former Assistant Professor, and Professor at the University of Wisconsin, respectively. The first author is presently Assistant Professor at the Federal University of Rio Grande do Sul and the second author is Assistant Professor, Colorado State University, Ft. Collins 80521.

Received for publication September 2, 1970. Accepted for publication June 18, 1971.