Figuring Out Your Fertilizer Plan

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Rebecca Vittetoe and Aaron Saeugling
Iowa State University
Extension Field Agronomists


Fertilization is just as important for forages as it is for row crops to maximize productivity. However, current fertilizer prices may have you seeing a lot of dollar signs.

Phosphorus, potassium, and lime considerations
To maximize fertilizer dollars, it is important to know what nutrients, particularly phosphorus (P), potassium (K), and lime, your forages really need. The only way to know is to soil test. See CROP 3108 “Take a Good Soil Sample to Help Make Good Fertilization Decisions” for more information on soil sampling.

Soils testing low or very low will benefit the most from P and K fertilization. Also, remember that forage harvest removes a lot of P and K. See Table 2 in PM1688 “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa” to estimate crop removal rates. We want to at least put back what we take off; however, if you cannot afford a full removal rate, put on what you can afford. If you must pick between P and K, prioritize the K since forages have a higher K removal rate.

Don’t forget soil pH as it also impacts forage productivity and affects nutrient availability. On your soil test results, the soil pH tells if we need to add lime, and the buffer pH tells how much lime is needed. 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. Use Table 16 in PM1688 “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa” to determine lime needs. We typically follow the recommendations for the two- or  three-inch depth when determining how much lime to apply in pastures.

Pelletized lime or ag lime: which one should you use if you need to apply lime? Both forms of lime are effective. However, the pelletized lime tends to work faster than the ag lime, which tends to take longer, but have more longevity.

Nitrogen considerations
If you take off a first cutting of hay prior to grazing (rather than resting  your pasture prior to turn out), you may want to be more aggressive with your nitrogen rate. If you have tall fescue, be cautious to not overfertilize with nitrogen. Suggested N application rates are found in Tables 1 and 2 in IBC 132 “Boosting Pasture Production."

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

Feel free to reach out to your local ISU Extension field agronomist if you have questions.

Date of Publication: 
March, 2022