Fertility management for pastures, meadows, and hay fields is a continuous process that is often only considered by producers during the establishment of the forage. Managing fertility for the maintenance of the stand and continued productivity is also extremely important. Forage crops are often grown on poorer soils and seldom managed as well as more marketable cash crops. Forage crops that contain grasses are easily sustained for years, and rarely need to be reestablished if fertility is maintained. This fact sheet presents information on the role of nutrients in forage crop growth and management to maintain adequate fertility for optimal production.
Nutrient Removal in Forages
If forages are grown without fertilizer, they will remove nutrients from the soil and soil test values will decrease. Over a period of time, the amount of forage produced will also decrease. The decrease in forage yields depends on the initial nutrients in the soil as indicated by soil test results, the nutrients removed in the harvested crop, the soil’s reserve nutrients, the release rate of those reserve nutrients, and the length of time from the last application of fertilizer.
Fields used as combination hay and grazing areas present some unique challenges when it comes to managing fertility. The nutritional needs of forages where hay is harvested are much higher than the needs of strictly grazed areas. In addition, the management of fertilizer blends and nitrogen sources can greatly affect the ability of plants to utilize nutrients.
It is estimated that each ton of tall grass or legume forage removes 13 pounds of phosphate (P2O5) and 50 pounds of potash (K2O). These nutrients must be replaced through commercial fertilizers, manure, or livestock nutrient cycling. Livestock are efficient recyclers of nutrients. However, if animals are existing on pasture alone, not all of the nutrients consumed will be replaced.
Livestock nutrient cycling occurs through urine and manure. If you count on the recycling of nutrients to supply all of the forage nutritional needs, great consideration should be given to developing an even manure distribution pattern. Significant amounts of nutrients are concentrated when animals loaf at water sources, in shade, or when allowed to graze in wood lots.
Soil pH has considerable influence on forage quality and plant growth. Soil may naturally become more acidic due to the leaching of basic cations, primarily calcium, magnesium, potassium and sodium. Furthermore, when the plant takes up these basic cations, they are replaced with hydrogen, contributing further to acidifying soils. Fertilizers containing ammonium nitrogen, which release hydrogen when nitrified, may also lower soil pH. Soil pH is of utmost importance in plant nutrition, as it has an influence on many crop nutrients (Figure 1). Most forage plants have optimum growth when soil pH is between 6.0 and 7.0.
|Figure 1. The relative availability of elements essential to plant growth at different pH levels for mineral soils.
Nitrogen availability in the soil is associated with microbial activity. Microbes are involved in organic matter decomposition, which helps convert organic nitrogen to forms that are available for plant uptake. As soil pH decreases below 5.5, bacteria become less adapted, decreasing the mineralization of organic nitrogen compounds. In addition, low pH is also detrimental to the rhizobial nitrogen fixation in legumes.
Phosphorus becomes less available as soil pH levels drop below 5.5. At this low pH, insoluble iron and aluminum phosphates form and phosphorus becomes essentially unavailable to plants.
Calcium (Ca) and pH are directly related since the soil calcium concentration primarily determines pH. When the soil base saturation of calcium is high (ex. 90 percent), the pH will usually be high (6.5 or greater). When the base saturation of calcium is low (ex. below 50 percent) the pH will usually be low (ex. 5.0). Gypsum can be used to supply only calcium without affecting soil pH.
Magnesium (Mg) is also a basic ion like calcium whose concentration is influenced by soil pH. When the soil pH is low, magnesium becomes more available and has the potential to leach. It is possible to have adequate pH levels when magnesium content of the soil is low. Fertilizer magnesium in these cases should be applied in a form other than dolomite to avoid over-liming. To supply magnesium without affecting soil pH, magnesium sulfate may be utilized.
Sulfur (S) is like nitrogen and its availability is greatly influenced by the activities of microorganisms. A soil pH that favors forage growth generally favors microbial activity and sulfur availability. Sulfur has great potential to leach along with many of the bases (calcium, magnesium and potassium), which leach out as sulfates.
Iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu) become increasingly available as soil pH drops. Soil pH levels of 7.0 or greater can result in low concentrations of these plant nutrients in forages. When relying heavily on forages as the primary source of nutrition, forage test to check for adequate levels. When the soil pH is below 5.0, aluminum availability increases to the point where it may become toxic to many common forage species.
Another important aspect of soil pH is its impact on herbicide carryover. Low soil pH results in triazine herbicide binding to the soil. If lime is applied for a forage crop just prior to seeding, the herbicide will be released and can have serious detrimental effects on forage seedlings. Triazine herbicide carryover effects are aggravated by the use of EPTC (Eptam), a pre-plant herbicide labeled for use in forage legume seedlings.
The most common way to increase the soil pH is to apply lime. Common agricultural liming material includes: burned lime, hydrated lime, carbonate lime, agricultural limestone, pulverized agricultural slag, etc. The two factors that greatly influence agricultural lime quality are: (1) its Total Neutralizing Power and (2) the fineness of the grindings. Both of these factors affect the neutralizing value of a product. Total Neutralizing Power (TNP) is a measure of the ability of a liming material to raise the pH. Pure calcium carbonate has a neutralizing power of 100. All other liming materials are compared on a percentage basis with pure calcium carbonate. The TNP of a liming material is affected by: percentage of calcium, percentage of magnesium, and impurities, such as silt and clay. The finer a given liming material is ground, the more rapidly it reacts with the soil to raise pH. Over time all the particle sizes will react to neutralize soil acidity, but a coarser grind will take more time to reach the desired results. Basically, if you apply a coarser ground product, then you will need to apply more of it to get the same results as agricultural ground limestone. The Ohio Agronomy Guide lists equivalent amounts of different liming materials based on TNP and fineness. Utilize this table when you are considering which liming material to purchase.
When financial constraints require a choice of using fertilizer or applying needed lime, usually lime is the best choice. The decision of whether to use dolomitic or calcitic lime should be based primarily on the amount of magnesium available as indicated by a soil test. The calcium to magnesium ratio is calculated on the basis of percent saturation of the soil cation exchange capacity (CEC) by each element. If the calcium to magnesium ratio is 1:1 (or less calcium than magnesium) a calcitic lime should be purchased. When both calcium and magnesium are needed, buy dolomitic lime.
Use of Nitrogen (N)
One way to save money and fertilizer is to establish and manage for legumes in meadows. Legumes have the ability to capture nitrogen from the atmosphere and fix it in nodules on the roots. Below ground nitrogen-fixing rhizobia make excess nitrogen available through the leaking of nitrogenous compounds and sloughing off and decay of nodules and roots. Clovers can provide more than 200 pounds of nitrogen per acre in a year by this method if soil pH, phosphorus, and potassium are at optimum levels and management favors legume growth. The amount of nitrogen fixed varies depending on the type of legume, stand density, initial soil fertility, and the amount of leaf surface on the legumes. Additional benefits of legume growth in meadows include: improved quality of harvested forage, better distribution of growth, and increased yields.
There are two main problems with adding nitrogen to a mixed stand of grasses and legumes. First, one way grasses respond to a nitrogen application is to grow leaves. This results in increased shading of legumes that typically occupy lower portions of the canopy. The greater competition for sunlight generally reduces the amount of legume in the stand. Second, nitrogen fertilization of mixed stands causes legumes to use nitrogen supplied by fertilizer and reduce fixation of nitrogen from the atmosphere.
The main decision that needs to be made is the percentage of legumes in the stand. Ohio State University fertilizer recommendations omit application of nitrogen fertilizers when the forage is made up of 35 to 40 percent or more legumes. When the percentage of legumes in the stand is below 20 percent, the field should be considered a grass field and nitrogen fertilizer should be applied accordingly. Between 20 and 35 percent reduced rates of nitrogen could be applied if the grass height is managed to limit its shading of the legumes. By managing grass height, legumes can be maintained in a stand even if high rates of nitrogen are applied.
When grass is the predominant forage species, nitrogen fertilization is extremely important. Economic returns have been demonstrated when a total of 150 to 175 pounds of nitrogen per acre per year are split applied three times during the growing season. According to the Ohio Agronomy Guide, the general recommendation is for 40 pounds of nitrogen per ton of forage removed, or 70 to 80 pounds per acre in early spring when grasses first green up and 50 pounds per acre after each cutting. It should be noted that with an aggressive nitrogen fertilization program, there is a potential to mismanage potassium, which may create magnesium deficiencies in plants and animals consuming these forages. Determine the risk of grass tetany through soil and forage testing. It may be necessary to apply fertilizer after the first cutting to avoid problems associated with high potassium levels.
Grazing animals excrete 70 percent of the nitrogen they ingest in urine and 30 percent of the nitrogen they ingest in manure. The nitrogen in urine exists mainly as urea and amino acids. These are rapidly converted to ammonium and nitrate by soil microorganisms and after conversion are readily available to forage plants. However, these organic forms of nitrogen are not immediately available to growing plants, and some nitrogen will be lost to denitrification and volatilization. Soil pH and temperature significantly affect the rate of urea conversion to ammonia gas. Soil with a pH greater than 6.5 and a temperature above 50 degrees F increases the rate of urea volatilization. Greater nitrogen losses can be expected under these conditions. For these reasons, a meadow without legumes could not be expected to generate all of the nitrogen needs through animal waste recycling.
Applications of urea-based fertilizers in summer may result in significant losses of nitrogen for the same reasons. If 0.5 inch of rain is not received in three to four days after an application of urea, then volatilization of the urea can be significant. Due to soil pH’s effect on nitrogen volatilization, surface applications of urea forms of nitrogen fertilizer are not recommended within one year of a lime application when lime is not incorporated. If urea-based fertilizer blends are utilized as the nitrogen source (46 percent N), they can be best utilized when placed in direct contact with the soil. Otherwise, consider purchasing sulfur-coated urea when surface applications are made during dry and hot periods. Ammonium nitrate (34-0-0) fertilizer blends can remain relatively stable with surface applications and would be a good choice when volatilization is expected. However, when the soil is wet and is likely to leach, the ammonium N quickly converts to nitrate N and is lost as nitrogen gas (N2). The denitrification of nitrate is associated with increased microbial activity. Diammonium phosphate—DAP (18-46-0) and ammonium sulfate (21-0-0) are also non-volatile sources of nitrogen and can be applied during hot and dry weather without significant losses. DAP should only be used on meadows where phosphorus is needed. Ammonium sulfate experiences very little surface volatilization and is a good source of sulfur. The disadvantage of ammonium sulfate is that it is very acidifying to soil, requiring two to three times as much lime to neutralize the same amount of acidity as formed by other common nitrogen carriers. Regardless of the nitrogen carrier used, there is little difference in the effect of the various nitrogen fertilizers on plant growth when each is used correctly and in equivalent amounts.
Strategic Use of Nitrogen
Besides the general recommendation for hay, nitrogen can also be used strategically in grazing situations. A light application of nitrogen (20-40 pounds N/A) in March will jump-start spring growth and allow for earlier grazing. The total acreage for this application should be limited because the seasonal distribution of grasses would be out of balance and the potential for grass tetany is increased. Generally one acre of pasture per two cows should be fertilized and never more than a third of the total pasture acreage.
The other time nitrogen could be applied strategically to pasture is to help offset the summer slump of cool season grasses. Moderate amounts of nitrogen (30-50 pounds N/A) could be applied in late June or early July after the spring flush and reproductive stages of the cool season grasses is over. This application could be used to stimulate growth that can be stockpiled for use when pasture growth slows. Again, this application should be limited in acreage.
The most universally beneficial use of nitrogen in grazing situations is during late summer. Grasses fertilized in August can be stockpiled and grazed in late fall or early winter. The general recommendation is 30-50 pounds of N/A for most grasses and 50-60 pounds N/A for tall fescue when choosing to stockpile for winter grazing. When the decision is made to apply fertilizer, consider the field that would make the best utilization of applied nutrients. High producing clean fescue and orchardgrass meadows would be good choices. Also consider soil water holding capacity—apply N in late summer to those meadows least likely to be under drought stress. In grazing situations, there is no point in applying nitrogen to grow more grass than your animals can utilize.
Losses of phosphorus under grazing conditions are minimal. Generally speaking, livestock remove only small quantities of phosphorus in livestock products, with the exception of dairy. About 25 percent of the phosphorus eaten in pasture is removed through cows’ milk. Cattle manure and urine contain about 3.5 percent phosphorus. About 80 percent of that phosphorus is immediately available to plants, while the rest is in an organic form that must be broken down. Like nitrogen, phosphorus can be concentrated in grazing situations under shade trees and around water sources. Phosphorus is not mobile in the soil and can result in high concentrations over time. Grazing management is needed to insure an even manure distribution to help cycle phosphorus where it can be used to produce forages efficiently and evenly.
Since phosphorus is tightly held by soil colloids and not removed in excess quantities under grazing conditions, forage analysis levels of 0.35 to 0.40 percent contain adequate amounts of phosphorus for most stages of animal production. When harvesting hay, maintain the critical soil test level of 15-25 parts per million (ppm) phosphorus for Bray P1 test results or 28-40 ppm phosphorus for Mehlich III test results, and apply supplemental phosphorus according to soil analysis recommendations. The critical level is basically the soil test level above which the soil can supply adequate amounts of a nutrient to support optimum economic growth. Grasses have a critical soil test level for phosphorus at 15 ppm Bray P1 or 28 ppm Mehlich III and legumes at 25 ppm Bray P1 or 40 ppm Mehlich III.
Phosphorus is essential for seedling growth. Therefore, it is extremely important that soil levels be built up to the optimum range prior to establishment of a new stand. Grasses are efficient scavengers of phosphorus while legumes have a higher requirement to maintain the nitrogen fixation process. Increased phosphorus levels will assist legumes in the competition for available P.
The common sources of phosphorus as a fertilizer are: triple super phosphate (0-46-0), diammonium phosphate (18-46-0), monoammonium phosphate (11-55-0), and ammonium polyphosphate (10-34-0). All of these sources contain readily available phosphorus. Triple super phosphate is the best source for legumes because it does not contain nitrogen.
Potassium differs from other plant nutrients since it does not enter the structure of the plant. Potassium remains in the plant sap and is leached from dead plant tissue. Animals remove relatively small quantities of potassium. If meadows are not managed for even manure and urine distribution, potassium shortages and excesses will occur throughout the grazing area. Ninety percent of excreted potassium is contained in urine with the balance found in manure. The appearance of clover dominance in urine patches can indicate a potassium deficiency in a pasture. Major losses of soil potassium are through forage removal and leaching. The critical soil test levels for potassium are 125-200 ppm. Plants have the ability to take up more potassium than they need. This is called luxury consumption. This can occur when there are high soil levels of potassium. High concentrations of potassium can also affect magnesium uptake by plants. This cannot only affect the plant physiology but can also cause metabolic imbalances in animals that consume mainly forages. The metabolic imbalance in animals is usually referred to as grass tetany or hypomagnesemia. If the ratio of magnesium to potassium (Mg:K) is less than 2:1, on a percent exchangeable basis, then magnesium is recommended.
The most common source of potassium fertilizer is muriate of potash or potassium chloride (0-0-60). It is a readily available source of potassium. Muriate of potash does have a relatively high salt index, which at high rates can cause salt injury to the crop. Ohio State University fertilizer recommendations limit muriate of potash applications to 300 pounds per acre in a year.
Timing of Phosphorus and Potassium Applications
Applications of phosphorus and potassium should be made prior to establishing a new seeding and applied according to soil test results. For maintenance, phosphorus and potassium can be applied at any time during the growing season when soil test levels are above the critical level. However, research shows that if one application of phosphorus and potassium is being done, then fall is the best time for the application. By applying phosphorus and potassium in September or October, plants develop a healthier root system and improve winter survival. This results in a plant better able to withstand drought the following year. If high rates of phosphorus and potassium are recommended, then there is an advantage to splitting the application. Some of the recommended fertilizer should be applied after the first harvest, with the balance being applied in the fall. This will help reduce the luxury consumption of potassium by the plants and improve the efficiency of potassium use. There is no advantage to splitting fertilizer applications when the soil test levels are above the critical level.
Forages need relatively large amounts of calcium. The requirement for calcium is nearly as large as the requirement for phosphorus and potassium. Calcium is a key component of cell wall structure and is important for stabilization. Legumes generally contain twice as much calcium as grasses. Grazing animals have a high calcium requirement. Animals can suffer from calcium deficiencies even when grazing on plants that are not deficient in calcium for growth. Because they are both cations, high levels of calcium can effect the uptake of magnesium, where large amounts of available calcium is present at moderate levels of magnesium.
|Table 1. Sufficient Nutrient Concentrations of Forage Tissue
|Top 6" sampled
|Top 6" sampled
|Sufficiency Range 1
|1 Range is only valid for crop, plant part, and stage indicated.
A soil test is the best guide for determining the calcium needs of a soil. Most soils with a pH of 6.0 or greater contain enough calcium for growing plants. Calcium is related to soil acidity, but a soil’s pH does not necessarily indicate calcium level. Calcium content mainly depends on the parent material from which the soil was formed. Although rare in Ohio, if calcium is needed without affecting soil pH, gypsum can be used. The least expensive way to apply calcium is through the use of agricultural liming materials.
|Table 2. Soil Test Values for Forages
|Tall Grass/Legume Mix
|Alfalfa and Other Legumes
|Bray P1 soil test2
|Mehlich III soil test
|The critical level for ppm K = 75 + (2.5 x CEC) for all crops.
|Soil pH Recommendations and Lime Test Index (LTI) for Forage Crops
|Mineral Soils with subsoil pH
|> pH 6 < pH 6
|1 When soil test values fall below the lower limits of the listed ranges for available P they reach the “critical level.” When soils are below the critical level the soil is not able to supply the requirement of the crop. If soil test levels are above these values follow OSU fertility recommendations for crop removal.
2 There are two different extraction methods used by soil test labs. While both provide valid results, the values returned are not equal. Make sure you know which method was used on your soil test report.
3 These limits vary widely depending on cation exchange, calcium to magnesium ratio, and percent base saturation.
4 Sandy soils with low organic matter may experience boron deficiencies.
5 Birdsfoot trefoil should be limed to pH 6.0.
Magnesium is needed in the process of photosynthesis and is contained in chlorophyll. Magnesium content of a soil is dependent on the soil’s parent material. Legumes usually contain more magnesium than non-legumes regardless of the magnesium levels in the soil. Forage containing less than 0.2 percent magnesium is likely to cause grass tetany problems in lactating animals that are under mineral stress. These conditions are likely to occur during heavy growth, or after heavy applications of nitrogen or potassium.
Magnesium deficient soils tend to be sandy. Liming with only gypsum or calcium carbonate can pronounce this deficiency. Magnesium can be applied through applications of dolomitic limestone, magnesium oxide, or magnesium sulfate, etc. Dolomitic limestone is the most economical source of magnesium. A soil test is the best guide for determining the magnesium needs of a soil.
Sulfur is needed by plants for several functions. The formation of certain amino acids, vitamins, and chlorophyll are a few of the functions that involve sulfur. Sulfur deficiency is not a common problem in Ohio due to deposits from air pollution, and use of sulfur-containing nitrogen and phosphorus fertilizers. With greater reduction in air pollution and the reliance on higher analysis fertilizer for nitrogen and phosphorus, sulfur may become deficient. Decomposing organic matter is the main source of sulfur. Sulfur exists in soil mainly in the organic form (S), but plants absorb it as sulfate (SO4). The conversion to sulfate is an acid-producing reaction and can lower the soil’s pH.
The current soil tests for sulfur are not reliable. The best way to determine sulfur adequacy is through plant tissue analysis. The amount of sulfur required by plants can be related to nitrogen and phosphorus requirements. Plants require generally as much sulfur as phosphorus. The percentage nitrogen to percentage sulfur ratio (N:S) is nearly constant at all stages of growth and is a useful indication of sulfur adequacy. The N:S ratio for optimum plant growth ranges from 14:1 to 16:1 when the nitrogen content of the plant is adequate. Elemental sulfur, gypsum, sul-po-mag and ammonium sulfate are commonly used sources of sulfur.
Occasionally metallic micronutrient deficiencies can be found in forages utilized for hay and grazing. The solubility of manganese (Mn), zinc (Zn), copper (Cu) and iron (Fe) decreases rapidly as pH increases above 6.0. Conversely, a soil pH above 6.0 increases the availability of molybdenum, creating copper deficiencies. Low forage copper levels are a serious problem for grazing animals in southern Ohio. Do not attempt to fertilize soil for micronutrients without first testing forage tissue levels and soil concentrations. Sufficiency ranges for most nutrients can be seen in Tables 1 and 2. In several cases, micronutrient levels in forages may be sufficient for plant growth but deficient for animal production. In those cases, it is more economical to supplement livestock with minerals rather than fertilize for these nutrients.
When applying fertilizer, select the formulation based on your soil test results. Only purchase the nutrients you need. When making surface applications of fertilizers one should consider the nitrogen source. It is not always a value if it is lost over the next week. Split applications of fertilizer may maximize crop response. Apply crop nutrients when the plants are actively growing and remember that late summer may be the most beneficial time to apply needed forage nutrients for late season grazing. A maximum growth response will be obtained from fields that are well drained and have southeast facing slopes. When grazing high producing meadows, allow animals who benefit the most from higher levels of nutrition to graze first, i.e., growing calves, lactating cows, young lambs, etc. The cost of fertilizer needs to be recouped in the form of hay or increased animal performance. For example, if you invest $25.00 per acre in fertilizer, then you would need approximately 30 pounds of additional gain for a cow and calf grazing on one acre when the calves are valued at 80 cents per pound.
- Bartholomew, H. and R. H. Leep. Soil Fertility, “The Basis From Which Plants, Animals, and We Benefit.” Ohio Grazing School Handout.
- Folliett, R. F. and Wilkinson, S. R. (1985). “Soil Fertility and Fertilization of Forages” in Forages: the Science of Grassland Agriculture. Heath, M. E., Barnes, R. F. and Metcalfe, D. S. eds. Iowa State University Press, Ames, Iowa.
- Michigan State University, The Ohio State University, Purdue University. “Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat & Alfalfa.” Extension Bulletin E-2567.
- Penn State Cooperative Extension. “Soil Fertility Management for Forage Crops: Maintenance.” Agronomy Facts 31-C.
- Potash & Phosphate Institute. (1996). Soil Fertility Manual. Norcross, Georgia.
- Rayburn, E. (1996). “Forage Management: Forage Fertilization Based on Yield and Management Goals.” West Virginia University Extension Service.
- Sperow, C. and Baker, B. (1996). “Forage Management: Nitrogen Fertilization for Early Pasture.” West Virginia University Extension Service.
- The Fertilizer Institute. (1982). The Fertilizer Handbook. The Fertilizer Institute, Washington D.C.
- Ohio State University Extension. Ohio Agronomy Guide. Bulletin 472.
- Ohio State University Extension. “Selecting Forms of Nitrogen Fertilizer.” Agronomy Fact Sheet 205.
The authors would like to thank the following individuals for reviewing the manuscript: Mark Sulc, OSU Extension Forage Specialist; Hank Bartholomew, Southern Ohio Grazing Coordinator; Ed Vollborn, Grazing Leader; Jay Johnson, OSU Extension Fertility Specialist; Maurice Watson, OARDC; Steve Hibinger, District Conservationist, NRCS-USDA. Also, thank you to Carla Wickham, Office Assistant (OSU Extension Noble County) and Joy Bodner, Office Assistant (OSU Extension Guernsey County); Dennis Young, Roger Davis, Ralph Bauserman, Kevin Sweeny, Don Potts, Perry and Guernsey County Grazing Council members.