Help! My alfalfa field may be slowly starving!

Phil Kaatz<kaatz@msu.edu>, Michigan State University Extension - July 11, 2024

Because potassium top dressing for alfalfa was less than recommended for many during times of high fertilizer prices in previous years, return on investment may be positive for potassium on alfalfa fields.

Discolored alflalfa leaves. — Figure 1. Potassium deficient alfalfa plants. Photos by (L) Utah State University, AGUFG.01; (R) University of Minnesota Extension.

The hay season is off to a robust start and yields in 2024 have started strong. The question to ask is will my forage fields be able to get the nutrients they need, or will they slowly starve? Now may be the time to reinvest into alfalfa, grass or grass/alfalfa mixed fields with increased fertilizer applications.

Since 2022, soil test recommendations and the cost for fertilizing forage fields with potassium (K) may have been greater than what producers were willing to spend. Some of the volatility of K fertilizer prices has lessened along with supply chain issues. The result is a drop in price for potash of approximately 39.2% since 2022, according to the United States Department of Agriculture (USDA) Illinois Production Cost Report for May 22, 2024.

Reinvesting into a forage fertility program, specifically K, may be more affordable than producers think. Consider K as a long-term investment strategy. The reduction in K fertilizer prices (Table 1) provides an added incentive to revisit recommendations for K needs on your farm. Those recommendations can mean the difference between a healthy and profitable alfalfa crop and a low yielding, unthrifty crop.

Table 1: Changes in potassium (K) cost and replacing K when 1 ton of dry matter alfalfa is removed. Source: USDA Production Cost Report (Bi-weekly), Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa
Year	Price/ton potash (0-0-62)	% Change from previous May	Cost to replace K removed from 1 ton of DM alfalfa
May 2020	$360	---	$ 14.22
May 2021	$441	23 %	$ 17.42
May 2022	$859	95 %	$ 33.93
May 2023	$613	- 29 %	$ 24.21
May 2024	$522	- 15 %	$ 20.62

It’s important to remember that forages are a large user of K and deficiencies may not be visible (Figure 1). Potassium in growing alfalfa is needed to facilitate the formation of starch from sugars and protein from amino acids. Sufficient K activates over 60 important enzymes in forage plants. Potassium is also arguably the most important soil nutrient for stress tolerance and winterhardiness. Over time, K is often the issue with forage stand decline. This makes K one of the most important macronutrients for alfalfa production.

Michigan State University Extension K recommendations are based on the “build and maintain nutrient levels model” for soil testing. The recommended frequency for soil testing forage fields is every three to four years. The critical levels for K will vary to some extent based on soil texture and thus the cation exchange capacity (CEC) of the soil. Generally, sandy soils (CEC < 5) have a critical level of 100 parts per million (ppm) while finer-textured soils (CEC > 5) have a critical level of 120 ppm. Based on the Tri-State Fertilizer Recommendations, every ton of harvested dry matter forage removes approximately 49 pounds of potassium oxide (K₂O) during the growing season. High yielding alfalfa fields can produce 6–8 tons of dry matter per acre per year. So, it is easy to see why soil test levels in fields that begin in the maintenance range can quickly move into a deficient range without any fertilizer being applied. After establishment, split applications of K are recommended to avoid luxury consumption, normally after first and third cutting.

This article is focused on K, however, producers should also consider other important nutrients necessary for a healthy alfalfa crop. In a Purdue Extension study, researchers found that alfalfa stands receiving phosphorus (P) fertilizer but not K fertilizer yielded less than stands that were not fertilized at all.

Many alfalfa producers depend on manure applications to provide the necessary nutrients for a growing crop. Producers should ask the question, does adding manure before planting provide the necessary K a thriving alfalfa stand needs? Although manure can be a valuable resource, it is usually only part of the equation for supplying the necessary K needed by an alfalfa field with high yields.

Each farm will have varying amounts of nutrients in their manure. The following farm manure samples (Table 2) were analyzed by dairy producers and the results were striking. By example, if Farm 9 applied 10,000 gallons of liquid manure per acre compared to Farm 5, there would be an applied difference of 296 pounds of K₂O per acre.

Table 2. Nutrient results in liquid manure for potassium from 12 Michigan farms (unpublished results).
	Farm 1	Farm 2	Farm 3	Farm 4	Farm 5	Farm 6
lbs. K₂O/1000 gallons	23.0	24.3	30.0	8.9	37.8	21.1
	Farm 7	Farm 8	Farm 9	Farm 10	Farm 11	Farm 12
lbs. K₂O/1000 gallons	30.5	14.0	8.2	25.1	16.0	15.0

For the best results, farmers should sample their own manure for nutrients before application. This provides better understanding of the amount of nutrient applied. It’s important to remember that a high producing field of alfalfa can utilize all the K₂O applied from manure in one season.

In conclusion, if you want to keep your alfalfa stands healthy instead of being deficient in soil nutrients, pay attention to the details of your fertility program. The results usually give you better profitability and higher productivity.

For more information, contact Phil Kaatz, Michigan State University Extension, at kaatz@msu.edu or 810-338-5242.

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