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Potassium in an Arid Loessial Soil: Characterization by Equilibrium Release-Absorption to Strong Salt Solutions¹

ABSTRACT

Shano silt loam soil developed from loessial and volcanic materials of recent origin under mild weathering conditions. It contains < 5% of clay, and 60% or more of silt. The rate of release of mineral K (Km) in this soil to 1N Cl solutions decreased in the order Na > Ca > Mg >> NH₄. Release of Km may have involved some framework silicates, but for practical purposes the layer silicates controlled Km activity. Release of exchangeable K (Kx) graded imperceptably into Km release and apparently depended on the relative accessibility of edge-interlattice adsorption sites. The classical method for estimating Kx (three extractions with NH₄) evidently released some Km because the conformation of the cumulative solution K (Ks) curves was the same for initial NH₄ extractions as for NH₄ extractions following removal of Kx with Ca, Mg or Na. Based on the proportions and K-releasing abilities of several soil size fractions, it was concluded that the preponderance of Km release activity originated in the silt fraction. A lesser amount of Km activity was associated with the sand with only a small portion of the Km release attributable to the clay. Equilibrium between Km, Kx, and Ks in the Shano soil was essentially complete within 24 hours.

It appears that Km may be more important than Kx in this soil as pertaining to K intensity and capacity. In neutral and calcareous soils Ca and Mg are present in large amounts as exchangeable ions and as soluble and/or slightly soluble salts. Therefore, the activity ratio Ks/Nas potentially has greater significance in neutral to calcareous soils than do the potassium adsorption ratio K/[(Ca + Mg)/2] or quantity-intensity relations, Ks vs. [K/(Ca + Mg/2)].

¹ Scientific Paper no. 4390, Project 1990, College of Agric. Res. Center, Washington State Univ., Pullman. A grant-in-aid, gratefully acknowledged as partial support of this research, was made by the Potash Institute of North America.

² Former Associate Soil Scientist (now Professor of Soils, Utah State Univ., Logan 84322) and Research Technologist III, Irrigated Agric. Research & Extension Center, Prosser, WA 99350.

Received for publication March 31, 1975. Accepted for publication August 8, 1975.