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Potassium in an Arid Loessial Soil: Changes in Availability as Related to Cropping and Fertilization¹

ABSTRACT

Two K fertilizer experiments were initiated in 1961 in arid central Washington on a Shano silt loam soil site that had been leveled to accommodate irrigation. The experiments were continued for 3 years and utilized rates of K fertilizer up to 320 kg K/ha in a randomized complete block design. By 1964 there existed a wide range of Kst (soil test K) as a result of the fixed random K treatments and the partial exposure of K-deficient subsoils. That year the experiments were altered to completely randomized designs using both Kst and K fertilizer rates as controlled variables. The experiments were continued through 1970, providing an evaluation of the effects of crop management and K fertilization on soil K reserves.

High Kst, both indigenous and fertilizer induced, decreased rapidly as a result of crop removal, with the rate of decline decreasing to zero as Kst fell below 100 ppm. The decrease in Kst was strongly modified by the type of crop (rooting habit and total K uptake) and by the method of crop residue disposition. Forage crops maximized the changes, whereas grain crops (with seed only removed) had a minimal effect, causing an apparent increase in K availability as a result of surface soil enrichment with K from the subsoil via the stubble. Decreases in Kst were stopped or reversed by K fertilization. Large amounts of K were removed by crops when Kst was low and static.

At low levels of Kst soil mineral K (Km) had a dominating influence, strongly buffering Kst changes involved with K removal (cropping) and K addition (fertilization). Whereas Kst serves as a good first approximation of soil K supply potential, supplemental information is required to fully describe the intensity and capacity factors in the K-soil-plant system. At comparable levels of Kst, the Shano silt loam profile is essentially homogeneous in regard to K availability. Therefore, special efforts to sample subsoils need not be made in routine fertility diagnosis of this soil series.

¹ Scientific Paper no. 4391, Project 1990. College of Agric. Research 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. This paper was presented in Div. S-4 Soil Sci. Soc. Am., Las Vegas, Nevada, 14 Nov. 1973.

² Former Associate Soil Scientist (now Professor of Soils, Utah State Univ., Logan 84322); Research Technologist III; Associate Soil Scientist, Irrigated Agric. Research and Extension Center, Prosser, WA.; and Former Associate Soil Scientist, now visiting Associate Professor of Soil Science, North Carolina State Univ., Raleigh, respectively.

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