A current soil test is the best guide for a sound fertilization program. Make sure to request the current Extension bulletin 974 Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat and Alfalfa (Indiana, Michigan, and Ohio) from your soil testing lab. Forages are very responsive to good fertility, and forages remove significantly more phosphorus and potassium from the soil compared with corn, soybean, and wheat grown for grain removal. Adequate levels of phosphorus and potassium are important for high productivity and persistence of legumes, especially alfalfa. Forage fertilization should be based on soil-test levels and realistic yield goals. Under hayland management, forages should be topdressed annually to replace crop removal and maintain soil nutrient levels. Each ton of tall grass or legume forage removes approximately 12 pounds of P2O5 and 49 pounds of K2O. These nutrients need to be replaced, preferably in the ratio of one part phosphate to four parts potassium. Phosphorus and potassium recommendations for forages are given in Tables 7.5 to 7.7. Exceeding the recommended levels for potassium fertilization is especially of concern. Luxury consumption of potassium by the plant will result in high forage potassium concentrations, which can lead to serious animal health problems in some classes of livestock.
Timing Topdress Phosphorus and Potassium Applications
The timing of phosphorus and potassium applications is not critical when soil-test levels are optimum. Avoid applications with heavy equipment when the soil is not firm. Soil conditions are frequently most conducive to fertilizer applications immediately following the first cutting or in late summer to early fall. Split applications may result in more efficient use of fertilizer nutrients when high rates of fertilizer are recommended. For example, apply half of the recommended fertilizer after the first cutting and half in late summer to early fall. If soil-test levels are marginal to low, fall fertilization is especially important to provide such nutrients as potassium to improve winter survival.
Nitrogen Fertilization
Nitrogen fertilization is extremely important for good production where grasses are the sole or predominant forage. Economic returns are usually obtained with 150 to 175 pounds of nitrogen per acre per year, split three times during the year―70 to 80 pounds per acre in early spring on hay fields when grasses first green up and 50 pounds per acre after each cutting. Legumes fix atmospheric nitrogen. Where the forage stand is more than 35% legumes, nitrogen should not be applied (Table 7.9). In pastures, nitrogen application can be used to strategically increase forage production when it will be most needed. This is discussed in more detail in Chapter 9. "Grazing and Pasture Management."
When applying nitrogen in the summer, keep in mind that many forms are subject to surface volatilization resulting in loss of available nitrogen. Ammonium nitrate is the best source choice because surface volatilization losses are minimized; however, this formulation is virtually unavailable now. For more information on nitrogen forms and volatilization losses, refer to bulletin 974 Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.
Sulfur Fertilization
Although sulfur deficiency in forages grown in Ohio is still quite rare, we have begun to see cases of deficiency in alfalfa on some Ohio soils. Sulfur may be needed when alfalfa and clover are grown on low organic-matter soils and course soils when yield levels are high. Sulfurdeficient plants exhibit a general pale green or yellowing color, with weaker growth along with lower crude protein content. The symptoms resemble a mild nitrogen deficiency and are more apparent in new growth than in old growth. We are beginning to see sulfur deficiencies in alfalfa on some soils in Ohio. For a positive diagnosis, a tissue test should be taken, as the soil test for sulfur is not reliable since sulfate is water soluble and leaches through the soil profile in a similar manner to nitrogen. The upper third of the alfalfa plants should be collected and sent to a commercial soil and tissue testing lab to analyze the sulfur content in the plant. If the sulfur content is below 0.25% in the upper third of the plant, it is likely sulfurdeficient and should respond to additional sulfur. Use a sulfate form of sulfur if the application is made in the spring (for rapid uptake by the plant), and an elemental form of sulfur for fall applications (elemental sulfur requires at least two months to become available to the plant in the soil solution).
Micronutrients
Micronutrient deficiencies are rare in most Ohio mineral soils. Micronutrient fertilization should be based on demonstrated need through soil testing and/or tissue testing. Boron may be needed when alfalfa and clover are grown on sandy soils and highly weathered soils low in organic matter. If the soil test is one part per million (ppm) or less of boron (B), or a plant tissue test shows 30 ppm or less boron, then apply a fertilizer containing two pounds of boron per acre. Refer to bulletin 974 Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa for more details on micronutrient fertilization.
Grass Tetany
Grass tetany occurs in animals when their demands for magnesium exceed the supply. It most often occurs in the spring, when high-producing animals are consuming primarily grass forage. High soil potassium tends to reduce uptake of magnesium by plants. The risk of grass tetany is reduced by not applying potash in early spring to grasses, because grasses take up more potassium than needed for growth (luxury consumption). After the first harvest, apply needed fertilizer to maintain a balanced soil-fertility program. It may also be helpful to feed livestock a high-magnesium supplement during spring.