Should High Potash Prices Change Alfalfa Fertilization Practices?

Carrie Laboski, Dept. of Soil Science, UW-Madison

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Should high potash prices change alfalfa fertilization practices? This question has been on producers minds recently. An evaluation of previous research using current economics suggests that some producers might want to consider adjusting their potash fertilization rates.

A study assessing the effects of soil pH and potassium (K) application rate on alfalfa yield was conducted at Hancock, Marshfield, and Spooner Agricultural Research Stations from 1998-2001 (Table 1). The ideal soil pH for alfalfa is near 6.8. However, some producers may have soil pH levels that are somewhat lower than ideal. Thus, Table 1 presents the results of this study for soil pH levels in the 6.5-6.8 range and 6.0-6.3 range. At all locations the soil test level was in the optimum category where the recommended K₂O fertilizer rate is about equal to crop removal of K₂O. A second study completed at the Arlington Agricultural Research Stations from 1993-1997 assessed the effect of K application rate at various initial soil test K levels on yield of alfalfa (Table 2). In the Arlington study, initial soil test levels were in the low, optimum, and excessively high categories. The dry matter yield, tissue K content, and K removal data in Tables 1 and 2 are the average annual data over the study period.

For both studies, the economic return to applied potash was calculated based on $100/T of hay and $0.20/lb K₂O. When soil test levels were low (Table 2), the economic optimum K rate (EOKR) was 280 lb K₂O/a or approximately the rate recommended by UW in “Soil test recommendations for field, vegetable, and fruit crops” (UWEX bulletin A2809). When soils tested in the optimum range, the EOKR is somewhere between 0 and 100 lb K₂O/a which is roughly half the recommended rate for the yield levels attained. When the soils test excessively high, the EOKR was 0 lb K₂O/a which is the recommended rate. Under current 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. 

Reducing potash application rates relative to UW recommendations is not without consequence. Generally when crop removal of K exceeds the amount of K applied soil test K levels will decrease. This is shown in Table 2 where the 200 lb K₂O/a rate is approximately equal to crop removal and soil test levels remain relatively stable throughout the study period. At Arlington (Table 2) soil test K levels did not drop as much as expected when K removals exceeded applications. Kelling and Speth (1998) explained that this is an example of the subsoil supplying K such that topsoil was not depleted of K. Applying less potash than crop removal (~60 lb K₂O is removed per ton) can result in reducing soil test K levels. This is not necessary bad if soil test levels are above optimum. 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/stand longevity. The data in Table 1 show 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. The final crown counts provided in Table 2 are the result of four years of consecutive applications of a given rate of potash. 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 potash application is essential for maintaining stand.

Potash mines in Saskatchewan (our major supplier) are bringing more mining capacity online this year, which means we may see somewhat lower potash prices for next growing season. Thus, reducing or postponing potash applications could be a viable management tool for some producers provided the pros and cons are weighed against their tolerance to risk.

Table 1. Effect of topdressed potash and soil pH on average annual total dry matter yield, final crown count, average tissue K content, annual K₂O removal, final soil test K level, and economic return to potash application at Hancock, Marshfield, and Spooner (1998-2001).

From Peters et al. 2003.

Table 2. Effect of topdressed potash on average annual total dry matter yield, average tissue K content, annual K₂O removal, final soil test K level, and economic return to potash application at Arlington (1998-2001).

From Kelling and Speth, 1998.

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