Potassium fertilizer management in alfalfa

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

With the low milk prices, some dairy producers may be tempted to eliminate or significantly reduce the amount of potassium fertilizer applied to alfalfa fields this season. This decision requires careful analysis as potassium plays a critical role in achieving high yields, preventing disease and increasing stand persistence. This article will guide you through this decision.

As in all other nutrient management decisions, the first step is to determine the amount of available potassium in each of your fields (soil test reports). The next step is to determine the critical soil test level for each your fields and compare this to your soil test levels. The critical soil test level for potassium is where the plants are able to reach 95 to 97 percent of their yield potential without additional fertilizer applications. Whenever the soil test level is above the critical level and up to 30 ppm higher than the critical level, potassium is recommended at a rate equal to crop removal (50 lbs. of K2O per acre per ton). Table 1 shows how the critical soil test level is affected by the cation exchange capacity of the soil.

Table 1. Effect of CEC on critical soil test levels for potassium.
table 1

Eliminating potassium fertilizer applications on fields where the potassium soil test levels are less than the critical level will definitely result in yield losses. Apply potassium fertilizers at application rates that are at least equal to crop removal rates to these fields.

The greatest potential for reducing or eliminating potassium fertilizer applications to alfalfa this season occurs when the soil test potassium levels are above the critical levels. However, it is important to remember that the soil test level will decrease whenever fertilizer application rates are lower than crop removal rates. Determining how fast the soil test levels will decrease is an important step.

Table 2. Pounds of K2O removed from the soil to reduce soil test levels by 1 ppm.
Table 2

Using the information from Table 2, calculate how fast the soil test levels will decline if we decide not to apply fertilizer in the following example.

CEC = 12

Yield = 5 tons per acre

Potassium soil test level is 130 ppm at the beginning of the season.

5 tons/acre x 50 lbs. K2O per ton = 250 lbs. of K2O per acre removed by the crop.

From Table 2, we see that it takes 9 lbs. of K2O of crop removal to change the soil test by 1 ppm. 250 ÷ 9 = 27.8 ppm decrease in the soil test level.

130 – 27.8 = 102.2 ppm is the final soil test level. By not applying potassium fertilizer in one season, the soil test changed from being well above the critical level to being just below it.

The following recommendations will ensure that you maximize your return on your investment in potassium fertilizer.

  • Topdress at the optimum times (immediately following first and or third cutting is ideal).
  • Avoid applying potash in October on soils having cation exchange capacities less than 6 meq/100 g to prevent leaching losses.
  • Split potash applications when more than 200 pounds of K2O per acre is recommended to prevent leaching losses and luxury consumption. Luxury consumption occurs when the plants take up more potassium than needed to produce high yields so fertilizer efficiency is reduced. Luxury consumption also increases the potential for milk fever to occur in cows shortly after calving.
  • Never apply more than 300 pounds of K2O per acre per year.
  • Maintain soil pH levels between 6.8 and 7.0.
  • Keep soil test levels for phosphorus above the critical level (30 ppm).
  • Monitor and control alfalfa weevils and potato leafhoppers.
  • Apply 1 to 2 lbs. of actual boron per acre per year to fields having cation exchange capacities less than 8 meq/100 g.

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