A lot of nitrogen fertilizer is generally put down on hay fields and pastures. For every ton of grass harvested about 40 pounds of nitrogen is removed and every ton of legume removes 80 pounds of nitrogen. Fields that contain only grasses can use 150-200 lb/acre of nitrogen throughout the growing season. Generally nitrogen applications are split 75-100 lb/acre at spring green up, and 50 lb/acre after each cutting. Fields with legumes that are less than 50% of the stand usually respond to between 30 and 75 lb.acre of nitrogen throughout the growing season. Nitrogen application applications are generally not needed for pastures or fields that have more than 75% legumes, unless irrigation is used and there is high yield potential (greater than 4 tons per acre per year), Table 1.
Table 1. Nitrogen Application Rates to Forage Fields Based on Legume Content and Yield Potential
Adapted from Utah State University Extension
Nitrification inhibitors slow the growth of Nitrosomonas bacteria which change ammonium (NH4+) to nitrate (NO3-). N-Serve (DowAgroSciences) and Guardian (Conklin) are widely used for this and generally provide 4-10 weeks of protection for fall or spring applied nitrogen for grain crops. They are not labeled for pasture, hay, silage use.
Urease inhibitors prevent ammonium from volatizing into the atmosphere. Agrotain (Koch Agronomic Services) is the most widely used, and generally provides 10-14 days of protection. This is ideal for nitrogen applications to pastures, hay, and silage in the spring because of the unpredictability of rainfall that would incorporate surface applied urea or other nitrogen fertilizers.
Slow release nitrogen fertilizers have a protective outer coating that requires microbial activity and moisture to break them down over time to release the nitrogen. Sulfur coated urea or ESN (Agrium), Osmocote (Scotts), and IBDU (Nu-Gro) all have been shown to be effective slow release fertilizers for many crops including pastures and forage fields. These products should only be used as a partial supply for forage nitrogen needs as they often are not immediately available to the plant. Combining a slow release nitrogen fertilizer with regular nitrogen fertilizer (urea, UAN, etc.) treated with a urease inhibitor would maximize nitrogen protection while still having some nitrogen available to the plant. Ammonium sulfate is not as vulnerable to nitrogen losses as urea and other fertilizers and would be a good choice for fertilizer pastures, hay and silage fields.
Phosphorous (P2O5 or P)
Responses to phosphorous fertilizers application are highly dependent on soil test levels. If soil tests are in the "very low", "low", "medium", or "high" ranges for phosphorous, fertilizer should be applied. Soil tests and phosphorous fertilizers are almost always reported as P2O5 and not actual phosphorous. One pound of P2O5 is equal to 0.44 pounds of actual phosphorous. Original methods of analysis were only capable of measuring the oxide forms of phosphorous (P2O5) and potassium (K2O). This form of reporting analyses has continued to this day and most recommendations are made using P2O5 and K2O (not actual phosphorous and potassium content). Most phosphorous is applied as MAP (monoammonium phosphate), DAP (diammonium phosphate), super-phosphate, or a blended fertilizer. The amount of phosphorous fertilizer that is needed increases as soil test levels of phosphorous decreases and are described in Table 2 for established forage stands.
Table 2: Phosphorous Application Rates to Forage Fields Based on Soil Test Levels
Newly established pastures, hay fields, and silage stands should have an additional 20-30 lb/acre P2O5 for all soil test levels including "very high". Additionally if growing alfalfa in a high yield potential environment (>5 ton DM/acre) the phosphorus application rates in the above table should be doubled.
Potassium (K2O or K)
Forage stands, especially those with legumes, require high levels of potassium to remain productive and potassium recommendations are often based on the percentage of legumes in the stand. Additionally alfalfa stands require more potassium than clover or trefoil fields. Furthermore, different soil types have a large range in their capacity to supply potassium to crops. As a result, potassium applications can range from 0 to over 250 pounds per acre of K2O. A pound of K2O is 0.83 pounds of actual potassium. Table 3 describes the application rate of potassium based on soil tests and yield goals for alfalfa.
Table 3: Potassium Application to Alfalfa Fields Based on Soil Tests and Yield Goals
Adapted from Alfalfa Fertilization
If an alfalfa stand is more than three years old, potassium application should be increased by 20% to help prevent winter-kill Potassium applications to alfalfa are often split after first and third cuttings in order to reduce tissue K test levels to help prevent milk fever, reduce fertilizer loss (potassium leaches), and prevent salt injury. Potassium applications to grass and clover fields should be reduced by at least half of the recommendation amounts for alfalfa in Table 3 because alfalfa has much higher potassium requirements compared to other forage plants. Different soil types also vary greatly in their ability to supply potassium to forage plants. Sandy soils often need twice as much potassium as clay soils because they have lower cation exchange capacity. Loamy soils fall between sands and clays in their ability to supply potassium to soils.
The air is simply cleaner now than it was 20 years ago, Figure 1. This means that most pastures, hay fields, and silage stands in the Northeast and Midwest will respond to 15-25 pounds per acre of actual sulfur. Each ton of grass or legume removes about 5 pounds of sulfur. Ammonium sulfate, potassium sulfate, and gypsum are excellent sources of sulfur during the growing season because the sulfur is immediately available to the plants. Elemental sulfur (S2) cannot be absorbed by plants and must be converted to sulfate (SO4-2) before plants can use it. However, when establishing an alfalfa stand either elemental sulfur or the other fertilizers will provide the sulfur needed in the 2nd and 3rd years after establishment (University of Wisconsin).
Figure 1: Change in Sulfur Deposition in the United States 1994 to 2011
While only needed in small amounts, boron can greatly increase the establishment and persistence of legumes. On sandy soils 1 lb/acre per year of actual boron should be applied, while on clays and loams only 2 lb/acre over the course of 3 years. Response to boron application will also be more likely in dry years and where no manure is applied to field.
The likelihood of a response in yield or quality to other micro-nutrients in pastures, hay fields, or silage stands will be very low in most situations. Fields with high soil pH (>7.0), low pH (<6.0), low organic matter (<1%), muck soils, sandy soils, and fields without a history of manure application may respond to micro-nutrient applications when grain crops are grown, but legumes usually only respond to sulfur, boron, and occasionally small amounts of molybdenum and grasses respond to sulfur. Manganese can be applied as dolomitic lime if the soil levels are low, but is generally not a concern.
Managing the soil pH is critical to managing nutrients in forage systems. Generally soil pH should be maintained between 6.0 and 7.0. Keeping soils in this range will maximize the availability of nutrients to grasses and legumes, Figure 2.
Figure 2. Availability of Soil Nutrients Based on pH
From Pennsylvania State University
1. Adding nutrients to soils are critical for harvesting high yields of pasture, hay, and silage.
2. Applications of nitrogen, phosphorous, potassium, sulfur, lime, and for legumes boron are often necessary for forages to maximize yields.
Ketih Kelling, University of Wisconsin
Cornell Cooperative Extension's Guide for Integrated Field Crop Management
Forage Production Section-Jerry Cherney, Cornell University
Fertilizer Management for Grass and Grass-Legume Mixtures
Rich Koenig, Mark Nelson, James Barnhill, & Dean Miner, Utah State University Cooperative Extension
National Atmospheric Deposition Program
University of Illinois
Will Alfalfa Respond to Sulfur in Wisconsin?
John Peters and Keith Kelling, University of Wisconsin