Potassium Dynamics in Western Uruguayan Agricultural Mollisols

Knowledge of soil potassium (K) dynamics and quantification of plant-available K reserves are essential for the correct management of this nutrient. The objectives of this study, conducted in six Uruguayan mollisols, were to (i) determine the contribution of plant-available nonexchangeable K to plant nutrition, (ii) compare the ability of the ammonium acetate (NH₄OAc) method and the sodium tetraphenylboron (NaBPh₄) method to determine plant-available K, and (iii) quantify the effect of K fixation and release processes on the ability of both methods to estimate changes in K availability due to different K balances. In a greenhouse experiment, perennial ryegrass (Lolium perenne, cv. Horizon) was grown over a period of 320 days in six soils fertilized with 0, 100, 200, and 400 mg K kg^–1. We measured plant K uptake and the changes in soil K status related to different K balances using NH₄OAc and NaBPh₄. Use of NaBPh₄ resulted in a more accurate determination of plant-available K (R² = 0.97 vs. 0.78) and soil K balance (R² = 0.73 vs. 0.63), but neither of the methods was suitable for identifying positive K balances. However, when positive balances were established by K addition and incubation without plant growth, both methods related well with K balances. Again, NaBPh₄ was better than NH₄OAc (R² = 0.98 and 0.88, respectively). The more accurate determination of plant K uptake and the strongest relationship with K balance of NaBPh₄ was due to the extraction of exchangeable K plus a proportion of plant-available nonexchangeable K. Soil–plant interactions enhance soil K fixation in forms that are available to plant absorption but are not extractable by the chemical methods considered in this study.