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Fertilizing alfalfa when milk prices drop

Fertilizing alfalfa when milk prices drop
Reducing potash application rates relative to University of Wisconsin recommendations can have consequences.

We have seen milk prices decline from record highs over the past several months and along with it alfalfa hay prices have dropped, too.  One thing that does not seem to come down in price much is fertilizer.  So, what can be done with alfalfa fertilization to maintain yield, keep the stand healthy and persistent and save a little cash?

Reducing potash application rates relative to University of Wisconsin recommendations can have consequences.

In order to decide if potassium application rates can be cut, a soil test is needed to determine where current soil test levels are at. Alfalfa requires about 60 pounds of K2O for every ton of dry matter removed. When soil test levels are low, the recommended rate of K20 is about 280 pounds per acre, assuming 4 tons of dry matter per acre yield. This rate not only replaces the potassium removed by the plant but also builds the amount of potassium in the low-testing soil. Of course, higher yields mean more K2O to replace.

If soils are testing in the optimum range and we assume the same 4 tons of dry matter per acre yield, the recommended rate for yield levels to be maintained is somewhere between 0 pounds and 100 pounds K2O per acre depending on soil type. When soils test excessively high, 0 pounds of K2O per acre is recommended as potassium levels can be drawn down without potassium deficiency being a problem. 

Current potash prices are being quoted around the $500 per ton range. Under these economic conditions producers may want to consider reducing the potash application rates for fields that test in the optimum, high, and very high categories to improve profitability until milk prices recover. 

Keep in mind that reducing potash application rates relative to University of Wisconsin recommendations can have consequences. Generally when crop removal of K exceeds the amount of K applied, soil test K levels will decrease. If soil test levels are above optimum, this is not all bad.  However, if soil test levels are at optimum, producers run the risk of soil test levels dropping into the low category which would require larger potash applications in the future.

Another potential consequence of reducing potash applications is winter survival and stand longevity. Maintaining a proper pH level of 6.5 to 6.8 throughout the life of the stand will help ensure higher yields and can help keep alfalfa stands healthier in the event of lower potash applications. UW research has shown that the amount of potash applied on soils testing optimum, provided soil pH is adequate for alfalfa production, does not greatly influence the final crown count. So, one year of a reduced application rate may not cause significant stand loss under these conditions. If soils test less than optimum for K and/or have a pH that is below 6.0, then a potash application is essential for maintaining the stand.

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Another factor to maximizing the potassium dollar is to apply potash at the proper time. The best time to apply potash is either after first- or third-crop harvest.  Following first-crop harvest, the potassium can be utilized for subsequent cuttings and provide enough nutrients for winter hardiness. An application following third crop allows for more potassium to be available for over-wintering.  A potash application should never be made at green up as alfalfa crowns can suffer significant damage at this time of year.  A split application should only be made if more than 500 pounds of potash are to be applied in a year. 

With potash, more fertilizer is not necessarily better in terms of yield when soil tests rise into the high and excessively high levels. However, more K fertilizer or excessive soil K levels can translate into luxury consumption of K by the plant and higher K plant tissue levels. Soil test K levels above 150 ppm can lead to high K plant tissue levels and are largely responsible for metabolic disorders like milk fever in dairy cattle.

Using a soil test to fine tune your soil test K levels into the 120 ppm to 150 ppm will help your alfalfa stands maintain yield, stay healthy and keep a little cash in your pocket.

Clark is the Chippewa County Extension crops and soils educator.

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