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Use of Sorption-Isotherms for Evaluating Potassium Requirements of Plants¹

ABSTRACT

A sorption-isotherm technique was used to evaluate plant-available K for potatoes (Solanum tuberosum var. ‘Russet Burbank’), celery (Apium graveolens ‘Utah Pascal No. 15’), tomatoes (Lycopersicon esculentum var. ‘Owyhee’) and beans (Phaseolus lunatus var. ‘Fordhook’) in a greenhouse study. Sorption equilibrium was reached after 24 hours. Desorption equilibrium was reached after 1 hour. A 95% yield of beans, tomatoes, celery, and potatoes was achieved with 7.2, 7.4, 11.1, and 13.6 µg/ml K in sorption-equilibrium solution and 2.6, 2.7, 3.5, and 4.7 µg/ml K in desorption-equilibrium solution, respectively. The energies of exchange, E_k, for these sorption equilibrium solution K levels were –2,500, –2,485, –2,360, and –2,300 calories/M, respectively. Increase in the equilibrium-K levels beyond the level needed for 95% yield resulted in little or no change in the growth of these crops. The desorption-equilibrium solution K levels for maximum yields in this study are in close agreement with the solution K levels of nutrient-culture studies reported earlier. Tissue-K concentrations for 95% yield were 1.78, 3.30, 3.60, and 4.65% in beans, celery, potatoes, and tomatoes, respectively. Potential use of this technique for determination of K requirement is discussed.

¹ Contribution from the Dep. of Plant & Soil Sciences, Idaho Agric. Exp. Sta., Univ. of Idaho, Moscow. Research Paper no. 74728.

² Graduate Research Assistant and Associate Professor of Soils, respectively.

Received for publication December 16, 1974. Accepted for publication May 12, 1975.