Minnesota dairy producers have used alfalfa as a key component of the ration for many years.  Therefore, there is keen interest in the nutrient content of the alfalfa tissue.  Recently, attention of many producers has focused on the concentration of calcium (Ca) in the tissue.  There is concern because Ca concentrations have been reported to be reduced by high amounts of potassium (K) in either the soil or the fertilizer program.  Reduced levels of Ca are associated with animal health near calving.

The importance of adequate K has also been recognized as a key to profitable alfalfa production for many years.  A high yielding alfalfa crop will remove in excess of 200 lb. K per acre.  This amount must be supplied by either the soil or the fertilizer program.  So, K fertilization of alfalfa has a substantial effect on profitability.

Since this potential negative relationship between Ca and K was important to dairy producers, a study was established near Rochester in 2000 for the purpose of measuring the effect of various rates of fertilizer K on the Ca concentration in alfalfa tissue.   The properties of the soil at the experimental site are listed in Table 1.

Table 1.  Relevant soil properties (0-6 inches) at the alfalfa experimental site.

Lime was applied at the recommended rate and incorporated in the fall of 1999.  A leafhopper resistant variety was planted in the spring of 2000.  Prior to planting, 0-0-60 was broadcast and incorporated at rates to supply 75, 150, 225, and 300 lb K₂0 per acre.  These same rates were topdressed to the established stand in the spring of 2001.  Adequate rates of phosphate, sulfur, and boron were used each year.

Alfalfa yields were measured in 2000 and 2001.  The tissue from the first cutting was analyzed for K and Ca.  Some of the information collected in this study is summarized in the Tables and Figures that follow.

Alfalfa Yield
First cutting alfalfa yields in both years were increased by potash fertilization (see Table 2).  The broadcast application of 75 lb. K₂0 per acre produced the optimum yield in both years.  The increase of 0.3 ton dry matter per acre in 2000 and 0.4 ton dry matter per acre in 2001 is substantial.  If alfalfa is valued at $100 per ton dry matter and potash fertilizer is valued at $.12 per pound of K₂0, the value of the increased yield is $27 per acre more than the cost of the potash fertilizer.   This is certainly a good return on the investment in potash fertilizer.

Table 2.  The response of alfalfa grown on a sandy soil to the application of potash fertilizer.

Yields of the 2nd and 3rd cuttings were low because of dry weather.  Otherwise, larger increases in yield resulting from the use of potash fertilizer may have been measured.

Because the soil test for potassium was in the low range (57 ppm), a yield increase resulting from the use of potash fertilizer would be expected.  The optimum rate of 75 lb. K₂0 per acre indicates that the sandy soil at this site is supplying substantial amounts of K to the alfalfa crop.

Since 2000 was the establishment year, 1st cutting yields were substantially lower than the 1st cutting yields measured in 2001.

Potassium and Calcium in Alfalfa Tissue
The use of potash fertilizer increased the K concentration in alfalfa tissue each year. Each year, the highest K concentration was produced by the application of 225 lb. K₂0 per acre. The K concentration in alfalfa tissue, however, was not directly related to yield. Recall that optimum yield of the 1st cutting was produced by the application of 75 lb. K₂0 per acre. For the 1st cutting, application of high rates of potash (225 and 300 lb. K₂0 per acre) did not produce excessively high levels of K in the alfalfa tissue.

The application of potash fertilizer reduced the concentration of Ca in the alfalfa tissue each year (Figure 1). In 2000, the reduction caused by the highest rate of K₂0 used was approximately 0.2%. A larger reduction (approximately 0.5%) was measured in 2001. Considering lower rates of applied potash (75 and 150 lb. K₂0 per acre), the reduction in Ca concentration is in the range of 0.1%.

The importance of the reduction in Ca concentration remains open to interpretation. Any reduction in Ca concentration in alfalfa tissue is, of course, important for the approximate 2 week interval around calving. Rather than eliminate potash from the fertilizer program, it would seem that some adjustment in the ration to add more Ca for that two week period would be more appropriate and more economical for the total farm enterprise.

Potash Use for Alfalfa
The positive economic benefits of potash fertilization shown for the first cutting in this one study illustrate the importance of potash in a fertilizer program for alfalfa. As always, rate of potash supplied should be guided by the results of a soil test.

Appropriate timing of fertilization of alfalfa has always been a question. For 2002, the suggestion is to fertilize after harvest of the first cutting if fertilizer has not yet been applied. This management practice could improve economic returns from subsequent harvests.