Knowing phosphorus, potassium and lime level needs of forage is important.

March 25, 2024

3 Min Read

Many pastures and hayfields continue to recover from the dry conditions experienced during 2023. One way to help boost forage production and help forages recover from these dry conditions is by addressing fertility needs. Iowa State University extension field agronomist Rebecca Vittetoe says that while fertilization for perennial forages is often overlooked, it's just as vital as it is for row crops to maximize productivity.

Phosphorus, potassium, and lime considerations

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

Iowa State extension publication, "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 notes, and the Iowa State resource PM1688 “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa," can to interpret 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 PM1688 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 says.

Also, remember that forage harvest removes a lot of P and K. Table 2 in PM1688 provides information to estimate crop removal rates. Vittetoe says producers want to put back at least what was removed; however, if a full removal rate is not affordable, apply at least what is. If having to choose between P and K, prioritize the K because forages have a higher K removal rate, she says.

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 reminds 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 says "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 PM1688, producers should use Table 16 to determine lime needs, and follow the typical recommendations for the two-inch or three-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 says. "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 growing tall fescue, be cautious to not over-fertilize with nitrogen, Vittetoe warns. Suggested N application rates are found in IBC132 “Boosting Pasture Production” in Tables 1 and 2. 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 says. However, be aware that nitrogen burn may be seen on the forage. Let the grass recover from this prior to baling or grazing.

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