How to Address Fertility Needs of Drought Damaged Pasture

Resources available to improve pasture fertility and yield

March 26, 2024, 9:00 am | Rebecca Vittetoe

AMES, Iowa – Many pastures and hayfields across the state continue to recover from the dry conditions of 2023. One way to help boost forage production and help forages recover from these dry conditions is by addressing fertility needs.

Soil sampling in a pasture.Rebecca Vittetoe, field agronomist with Iowa State University Extension and Outreach, said that while fertilization for perennial forages is often overlooked, it's just as vital as it is for row crops to maximize productivity.

"It is important to know what nutrients – particularly phosphorus, potassium and lime – your forages really need," she said. "And the only way to know is to soil test."

Nutrient determinations

Two publications from ISU Extension and Outreach provide some useful guidance. "Take a Good Soil Sample to Help Make Good Fertilization Decisions" provides more information on soil sampling.

Soils that test low or very low will benefit the most from P and K fertilization, Vittetoe said, and the publication “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa" can help you interpret your soil test results.

“For example, if your soil test for an alfalfa-grass pasture comes back as 20 ppm P (Bray P1) and 130 ppm K (dry), we can look at Table 10 in the crop nutrient publication and see that it would be recommended to apply 90 pounds P205 per acre and 250 pounds K20 per acre, because both the P and K are in the low testing category,” she said.

Also, remember that forage harvest removes a lot of P and K. Table 2 in the crop nutrient publication provides information to estimate crop removal rates. You want to put back at least what you take off; however, if you cannot afford a full removal rate, put on what you can afford. If you must choose between P and K, prioritize the K because forages have a higher K removal rate.

If soils are very low or low in P or K, the recommendation is to apply P and K either in the early spring or in the fall to help boost forage production. For soils that test in the optimum category, the timing of P and K applications is more flexible.

Vittetoe reminded producers not to forget about soil pH because it also impacts forage productivity and nutrient availability.

"In your soil test results, the soil pH indicates if we need to add lime, and the buffer pH tells how much lime is needed," she said. "A soil pH of around 6.0 is recommended for grass-based hayfields and pastures. To encourage and maintain legumes, try to maintain a pH of 6.5 for clovers and birdsfoot trefoil and a pH of 6.9 for alfalfa."

Also in the crop nutrient publication, producers should use Table 16 to determine lime needs, and follow the typical recommendations for the 2-inch or 3-inch depth when determining how much lime to apply in pastures.

"Producers often ask if pelletized lime or ag lime should be used. Both forms of lime are effective," Vittetoe said. "However, pelletized lime tends to work faster than the ag lime, which tends to take longer, but has more longevity. Like P and K, application is typically recommended in early spring or fall."

Nitrogen considerations

If producers take a first cutting of hay off prior to grazing, they may want a more aggressive nitrogen rate compared to resting their pasture prior to turn out. If you have tall fescue, be cautious to not over-fertilize with nitrogen.

Suggested N application rates are found in tables 1 and 2 in the publication “Boosting Pasture Production.” From a timing perspective, N can be applied either once annually or split-applied. Single applications typically are made in early spring, March or April. If split-applying N, apply in the early spring and again in August.

To minimize nitrogen losses, using ammonium sulfate or urea coated with a urease inhibitor is often preferred. Liquid nitrogen can work well if producers want to apply herbicide with the fertilizer, Vittetoe said. However, be aware you may see nitrogen burn on the forage. Let the grass recover from this prior to baling or grazing.

Producers are encouraged to contact their local ISU Extension and Outreach field agronomist with any questions or concerns.

Shareable photo: Soil sampling in a pasture.

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