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Potash has been a key component of fertilizer programs in southeast Minnesota for many years and rate and placement guidelines have been in place for some time. Until recently, however, research and extension programs that focus on potash fertilization have not been extremely active.

Beginning in the mid-1990’s several trials were conducted in fields of cooperating farmers to take a renewed look at potash fertilization practices for corn. It was especially important because of higher yields, to evaluate the effectiveness of current fertilizer recommendations. These trials focused on yield response in relation to soil test values for potassium (K).

A summary of corn yields from three locations is provided in Table 1. At all locations, soil samples collected from 0 to 6 inches were collected before the fertilizer was broadcast. There was no history of manure for any field. The various rates of potash supplied as 0-0-60, were broadcast and incorporated before planting.

Table 1. Corn yield in southeastern Minnesota as affected by a broadcast application of potash fertilizer.

Using current University of Minnesota definitions the soil test K values are considered to be medium, high, and low at the Goodhue, Dodge, and Fillmore County sites respectively. There is a very good probability that potash fertilizer will increase yields when soil test K is low. Without adequate potash, yield was reduced by about 40% at the Fillmore County site. When soil test K is in the medium range, use of potash fertilizer may increase corn yield. Without potash use at the Goodhue County site, yield was reduced by about 7%. A 6% reduction in yield was measured at the Dodge County site where a high soil test K would indicate a low probability of a yield increase. Excellent yields were measured at all sites and this may have affected the response at the Dodge County site.

The importance of adequate potash fertilization is also illustrated in Table 2. Corn was grown at a site in Fillmore County having a low soil test value for K (63 ppm). A broadcast rate of 150 lb. K₂O per acre provided for near optimum yield. Using a value of $2.75 per bu. of corn and a price of $.12 per pound of K₂O, the value of the additional corn after subtracting the cost of the fertilizer was $127 per acre. Yields were reduced by about 35% if potash was not applied.

The soil test values for potassium measured after harvest also changed with potash fertilization. After subtracting the amount of K removed by the crop, the soil test for K changed by 1 ppm for each 7 lb. K₂O applied per acre. Measured changes in soil test values were relatively small. Much higher rates of potash fertilizer would be needed to produce higher values following a high corn yield.

Table 2. Corn yield and soil test values (0-6 inches) for potassium as affected by rate of potash applied. Fillmore County.

Potassium is considered to be an immobile nutrient is soils and, therefore, efficiency of use should be influenced by contact between fertilizer and soil. In reducing fixation or tie-up, banding should be expected to be more efficient than broadcast applications. Placement comparisons at a research site in Goodhue County have confirmed the efficiency of the banded application (Table 3). The initial soil test value for K at this site was 77 ppm. Clearly, the reduced rate applied in a band was just as effective, if not more effective, than a higher rate of potash that was broadcast.

Table 3. Corn yield as affected by rate and placement of potash fertilizer.

The effectiveness of banded potash fertilizer has also been demonstrated in fields of cooperating farmers in southeast Minnesota (Table 4). As with the previous research conducted in Goodhue County, the efficiency of the banded potash is obvious.

Table 4. A comparison of banded and broadcast applications of potash fertilizer for corn production in southeast Minnesota.

A study of the results of the various projects that have involved potash fertilization of corn in southeast Minnesota leads to the following conclusions.

- Highest corn yields are not always associated with highest soil test values for K. With adequate rates of potash fertilizer, high yields can be attained even though soil test values for K might be low (see Table 1).

- When soil test values for K are low and yields are high, relatively high rates of potash fertilizer are needed to produce substantial changes in soil test values for K.

- Banding of potash fertilizer at planting is as efficient as broadcasting at higher rates. Therefore, the practice of banding can save money. It is important to include some potash in the band. For example, use of 10-34-0 in a band at planting will not be as effective as use of 7-21-7. Avoid, however, use of high rates of 7-21-7 in contact with the seed.

The various research projects summarized in the previous paragraphs show the importance of potash fertilization for corn production in southeast Minnesota. There is, however, more research that is needed. Time and dollars can be saved if potash fertilizer can be applied only once in a crop rotation. Therefore, studies are needed that will compare the effects of a single or multiple applications of potash over a number of years. There is also very little information that documents the residual effects of large applications prior to the planting of either corn or alfalfa. If time and resources are available, research to address these tow concerns will be started in the near future.