1/28/2010 - By Harrell's

Managing Your Soil Potassium

G. H. Snyder and J. L. Cisar explain potassium's important role in contributing to overall soil quality.

Potassium (K) is second only to nitrogen in terms of plant requirements. Potassium plays a critical role in the control of and regulation of various minerals, water relations, promotion of meristematic tissue, and rooting. In addition, K also activates enzymes associated with respiration and photosynthesis. Potassium is known as the “health” element because insufficient soil K increases susceptibility to cold, heat, drought, diseases, and wear.

Potassium, a monovalent cation, is poorly retained in soils with low cation exchange capacity (CEC) due to the lack of electrostatic attraction necessary for K retention. Therefore, it is difficult to maintain adequate soil K in sandy soils. Moreover, problems related to K retention in the high sand content, low organic matter conditions typical of putting green root zones, particularly when newly established, is exacerbated. For example, in a field study, conducted on sand based putting greens with and without peat in which K was applied weekly (0.5 lb K 1000 sq ft^-1 wk^-1) during grow-in (12 wks) approximately 30 and 70 % of the applied K leached from the root zone with and without peat, respectively (Snyder et al., 2003). Soil-test data from the same study further demonstrate the inability to “bank” K despite repeat applications at high application rates described above (figure 1).

Figure 1

A sand soil might have a CEC (ability to retain positively-charged nutrients of only 1 or 2 cmoles kg-1. Furthermore, in most cases only 4 to 7 % of the CEC retains K (Wolf, 1999) because the rest is occupied by the more strongly adsorbed calcium and magnesium ions. Thus, a soil with a CEC of 2 cmoles kg-1 and 5 % K saturation will, theoretically, retain less than 40 lbs K per acre in an exchangeable (plant-available form, and lower values frequently are observed. Low soil-test K is common in irrigated sand soils, and generally can not be increased appreciably over time by K fertilization.

Because K is so mobile, turf managers who understand the importance of K nutrition generally address this problem by applying K frequently. This method of management, however, can lead to a peak and valley effect ranging from sufficient to deficient soil K within a matter of 30 days, and fully reverting to pre-fertilization levels 60 days following application of soluble muriate of potash (KCl) (Fig. 2). In addition, while K loading is not of serious consequence to the environment, the time and labor required to reapply K is of serious value.

Figure 2

An alternative to the use of soluble K sources such as muriate of potash and sulfate of potash might be to use controlled-release K fertilizers. Controlled-release of K generally is accomplished by coating soluble K sources with materials such as sulfur, plastic, or resins. The use of controlled-release K fertilizers either alone or in combination with soluble K sources will reduce the peaks and valleys associated with soluble K sources at the same time reducing time and labor costs.

Several studies conducted at the University of Florida demonstrated the potential of controlled-release K sources to reduce K loss after fertilization relative to soluble K sources. In a glasshouse study conducted in Arredondo fine sand, Sartain observed that soluble muriate of potash leached large amounts of K after only one inch of leaching occurred. Sulfate of potash required two inches in order to leach appreciable K, indicating less potential of sulfate of potash to leach relative to muriate of potash. Sulfur coating sulfate of potash reduced K leaching by 90% relative to soluble K sources. After seven inches of leaching occurred only a small fraction of the applied K had leached from the coated K source.

In a field study on a sand soil in Ft. Lauderdale, bermudagrass tissue contained more potassium over a six-month period when fertilized with slow-release K (coated) sources than when fertilized with the water-soluble sources muriate of potash (KCl) and sulfate of potash (K₂SO₄) (Table 1), illustrating the benefits of a controlled-release K source.

Table 1. Potassium content of bermudagrass clippings following fertilization with water-soluble and slow-release potassium fertilizers.

Potassium Source	Months after fertilization
	0	2	4	6
None	0.95%	0.60%	0.96%	0.61%
KCl (soluble)	0.74%	1.08%	1.28%	0.88%
K2SO4 (soluble)	0.74%	0.99%	1.18%	0.90%
Resin Coated	0.92%	1.36%	1.83%	1.45%
Sulfur Coated	1.00%	1.23%	1.44%	1.03%

Source:  Snyder, G. H., and J. L. Cisar. Amer. Soc. Hort. Sci. 117:411-414(1992).Wolf, B. 1999. The Fertile Triangle. Hawthorne Press, Inc. New York, NY. p. 183.