Potassium Dynamics in Western Uruguayan Agricultural Mollisols

ABSTRACT

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.

KEYWORDS:

• Illite
• nonexchangeable potassium
• potassium reserves
• sodium tetraphenylboron

Acknowledgements

The authors acknowledge Carlos Bautes and Juan Molfino for their help with soil selection; Wilfredo Mesa, Emiliano Barolín, Héctor Vergara, and Damián Janavel for their assistance with soil collection; Daniel Bassahún, Raquel Musselli, and Rosario Oten for their help with laboratory analyses; and Agustina Cabrera for her help with greenhouse management. Thanks are also due to Félix Gutierrez for providing ryegrass seed and Wilfredo Ibañez for help with statistical analysis. We especially acknowledge Dr. Daniel Poiré (CIG La Plata, Argentina) for performing the mineralogical analyses.