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Forage and Pasture
Forage Management Guides
Self-Study Guide Guide 8: Soil Fertility Management in Pastures Essential Nutrients for Plant Growth

Plants need certain elemental nutrients from the air and soil for proper growth and reproduction. Sixteen elements are essential for plant growth. Nutrients are considered to be essential if they are required for plants to grow and reproduce. Plants have different requirements for each essential nutrient. For example, about 96 percent of the weight of a plant is made up of only three elements - carbon (C), hydrogen (H), and oxygen (O). Plants take up these three elements mainly from air and water. The other 4 percent of the weight of a plant comes from the remaining 13 essential nutrients. These 13 elements are called mineral elements or nutrients. Plants get mineral nutrients from the soil.

Mineral nutrients in soils are found in rock, sand, silt, clay, and organic matter. Soils differ in total nutrient content and also in the amount of each nutrient available for plant growth. Upland soils such as those in the Ozark and Ouachita mountains are very old in geological terms and are highly weathered. Total mineral content may be high but the availability of those nutrients to plants is low. In contrast, river valley soils are geologically young and usually have a higher content of nutrients available for plant growth. The amount of nutrients available in a soil for plant growth can be determined by a soil test.

The nutrients needed from the soil in greatest amounts are nitrogen (N), phosphorus (P), and potassium (K). These are called primary nutrients. Secondary and micronutrients shown in Table 1 are needed in lesser amounts than the primary nutrients. Micronutrients are also called trace elements because they are only required in trace amounts by plants. Chemical fertilizers are most often used to supply additional mineral nutrients to plants although other sources including organic fertilizers and animal manure can be used, but the nutrient content of these materials should be determined.

Adequate soil fertility is required for good forage production. The native soil fertility level of many Arkansas soils is too low to support the production demands of most pasture and hay systems. Fertilizer is applied to increase nutrients available for forage establishment and production above the level supported by the existing soil fertility level. The amount of fertilizer needed is dependent on the forage species, production level, and the existing soil fertility levels.

Fertilizer applications for forages should be based on actual conditions for a particular farm instead of “rule of thumb” guidelines or popular opinion. Optimum soil fertility improves both forage vigor and persistence. While forages often show a growth response to a popular fertilizer rate, that rate likely won’t match the nutrient needs of a specific production system due to differences in soil nutrient levels and nutrient requirements of forage species. Using "rule of thumb" fertilizer rates often leads to too much or too little fertilizer being applied - either case being costly to the grower. Soil tests are excellent tools to use in matching the correct fertilizer rates to the forage production system.

Fertilizer Timing

Fertilizer should be applied to forages just before major periods of forage growth. For cool season grasses this can be in early spring or fall and for warm season grasses fertilizer is usually applied in late spring. Forage growth can be managed to a degree by proper timing of fertilizer applications. When cool-season grasses are heavily fertilized in the early spring, rapid growth often occurs. If too many acres of cool season grass are fertilized in spring the forage can become coarse and mature before the livestock can graze it. Forage growth can be better matched to livestock need if a portion of the cool-season grass acreage is fertilized in spring and the remainder of the acreage is fertilized in late summer or early fall for fall and winter grazing. Potential losses of N applied in fall are discussed in the section on nitrogen.

Warm-season grasses such as bermudagrass can be fertilized in late spring and again after each harvest if additional forage is needed. The first application should be made when night temperatures are 60? for one week. Legumes such as clover and alfalfa can be fertilized in spring before growth begins or after haying or grazing. With legumes it is important to build soil levels to medium or high levels to improve stand persistence

Indications of Low Fertility

Accurate fertilizer needs for a crop in a particular field cannot be determined by visual observation, however there are some visual indicators that often signal low soil fertility levels. Declining forage production, the loss of a forage species from the stand, or the appearance of weeds in a field can signal a soil fertility problem. Well fertilized forages are competitive with many weed species. Weed encroachment often becomes severe when fertility levels are too low for good forage vigor. The presence or appearance of broomsedge in a pasture or hayfield often suggests the need for limestone since this plant becomes competitive with forage plants at low soil pH levels. Note, however, that the presence of broomsedge may also be the result of low soil P and K levels or overgrazing. The increase of broomsedge or other weeds in a pasture indicates that the soil should be tested to determine the proper corrective action.

Information on Soil Test Reports

The routine soil test offered by the University of Arkansas Soil Testing Lab provides analyses for pH, phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), sulfate (SO4-S), nitrate (NO3-N), and soluble salts. Fertilizer recommendations are given as pounds of N - P2O5 - K 2O to apply per acre to reach a specific level of forage production. Limestone recommendations are given as the tons of limestone needed per acre to increase the soil pH to an optimum level for the forage to be grown on that soil. Notes giving specific information regarding fertilizer management are often are printed on the report.

Soil tests made on a regular basis can establish a baseline fertility level for a field and can show changes in soil fertility over time. A baseline fertility level is useful in determining if current fertilizer applications are keeping pace with the amount of nutrients being removed from the field by grazing or hay harvest. Pastures and hayfields should be tested every two to three years to monitor changes in soil fertility.

Getting a Good Soil Sample

Soil fertility levels will vary over every square inch of a field. Accurate soil test results begin with well-taken soil samples. Poorly taken soil samples result in poor analytical results and fertilizer recommendations that do not match the particular field and forage. There is no best time of year to take soil samples, but samples taken well in advance of the growing season allow adequate time for planning fertilizer and limestone applications.

Composite soil samples should be taken for each field to determine average fertility levels. Generally, composite samples are made up of 15-20 soil cores collected in a zig-zag pattern from all across the field. For pastures, each core should be made up of the soil from the surface down to four inches deep. For fields that will be plowed cores should be taken from the plow layer or from the surface down to six inches deep. Representative samples are difficult to collect from hard, dry soil. It is best to collect soil samples when the soil is moist enough to allow good penetration with a soil auger or probe.

Generally soil samples should represent 20 acres. Fields larger than 20 acres can be sampled according to the soil type or in units large enough to justify differential applications of fertilizer or lime. Avoid sampling odd areas such as burned bulldozer piles, old barn lots, manure and urine spots, etc.. Soil from these areas can cause biased soil test results and incorrect fertilizer recommendations.

Nitrogen (N)

Of the mineral elements, nitrogen is needed in the greatest quantity by plants. Plants take nitrogen up from the soil either as nitrate or ammonium. Nitrate is the primary form of N taken up in well aerated soils. Nitrogen is a critical component of protein and amino acids. Sources of N are soil organic matter, nitrogen fixation by legumes, fertilizer, and animal manure.

The most common nitrogen fertilizer sources are urea (46 percent N), ammonium nitrate (34 percent N), and ammonium sulfate (21 percent N and 24 percent S). Enough nitrogen becomes available from organic matter in good grassland soils during a year to produce about one ton of grass. Heavy applications of N fertilizer (>100 lbs. / acre / year) creates soil acidity and lowers soil pH. Low soil pH reduces fertilizer effectiveness. Soils receiving high N rates should be monitored with soil tests to help maintain adequate pH levels.

Minor nitrogen sources such as snow, rain, and atmospheric fixation by lightning provide negligible amounts of N and should not be considered in fertilizer planning. Soil test fertilizer recommendations are given as pounds of actual N to apply per acre.

Nitrogen is perhaps the most transient of the primary fertilizer nutrients. It does not build up in the soil to any appreciable extent like P and K. For good forage growth it must be applied just prior to rapid forage growth so that the plants can take it up before it moves out of the root zone or before it is used by other plants. Fertilizer N, depending on the N source, goes through certain chemical reactions in soil.

Urea fertilizer can undergo changes that allow N to volatilize into the air. During cool weather, urea and ammonium nitrate fertilizer produce similar amounts of forage per pound of N applied. During warm weather, however, N losses from urea can be substantial. When urea is surface applied, it can be converted to ammonia gas by an enzyme called urease so volatilization occurs. Urease is found in plant residue on the soil surface. Warm temperatures (80 degrees F or above), moist soil, wind, and high soil pH all increase N losses from surface applied urea. Nitrogen losses from surface applied urea can range from very slight to as high as 50 percent. If urea is incorporated into the soil soon after application, no N losses occur. Soil incorporation is not practical in pastures so urea is not a preferred fertilizer source for topdressing forages in warm weather.

When ammonium nitrate is applied to moist soil, it breaks down into ammonium and nitrate. The nitrate is immediately available to plants. The ammonium fraction is also available to plants, but since it is a positively charged ion, it often becomes attached to negatively charged soil particles and becomes unavailable. The ammonium bound to soil particles goes through a process called nitrification in which ammonium is converted to nitrate by soil bacteria. The nitrate produced by nitrification is then available for plant uptake.

Nitrogen losses from ammonium nitrate occur only by leaching which can occur in sandy or excessively drained soils during high rainfall periods or by denitrification, which occurs when soils become waterlogged. Ammonium nitrate fertilizer does not volatilize when applied to moist or dry soil so it is not lost into the air during hot or dry weather. For this reason, ammonium nitrate is a desirable material for topdressing forages in summer and early fall.

Phosphorus (P)

Phosphorus is very important for root development. It is also a key element in plant cell membranes and all energy functions within the plant. Arkansas soils are generally low in available phosphorus unless high amounts have been applied as fertilizer or manure. Phosphorus is most available to plants when the soil pH is between 6.0 and 7.0 (Figure 2) and rapidly becomes tied up with other soil elements such as aluminum or iron in very acid soils making it unavailable to plants. Phosphorus fertilization has also been shown to increase plant uptake of magnesium (Mg) in tall fescue. Maintaining adequate phosphorus levels in soil then could significantly reduce the risk of grass tetany in grazing animals. Grass tetany poisoning occurs when forages are too low in magnesium.

The phosphorus content of fertilizers is shown as the percent P2O5 instead of the amount of actual P. Soil test fertilizer recommendations are also made as pounds of P2O5 to apply per acre. Common phosphorus fertilizer sources for bulk blended fertilizer include diammonium phosphate (DAP) which contains 18 percent N and 46 percent P2O5, and triple superphosphate (TSP) which contains 46 percent P2O5, but no N or K2O.

Phosphorus is an immobile nutrient meaning that it does not leach from the soil. Phosphorus will accumulate in soils when application rates from fertilizer or manure exceed the removal rate in pasture or hay. Fertilizer P applications can be reduced without yield losses when soil P levels build up to high levels. Soil tests should be taken more frequently on high P soils to monitor soil levels when no fertilizer P is applied.

Potassium (K)

Potassium, often referred to as potash, is a critical nutrient for plant growth and persistence. Potassium is involved with transport of N and other nutrients within plants. All enzymes or proteins in plants are dependent on K. Low available soil K causes reduced forage yields because it is so essential in regulating many functions in the plant. Poor forage persistence is often observed in low K soils. A forage species may yield well for a short period under low K nutrition, but production tends to drop over time and the forage stand may begin to thin. Winter injury can be more severe in forages growing in low K soil.

Common fertilizer sources of potassium include potassium chloride (60-63 percent K2O) and potassium-magnesium-sulfate commonly called sul-po-mag. Sul-po-mag contains 22 percent K2O, 11 percent Mg, and 22 percent S. Soil test fertilizer recommendations are given as pounds of K2O to apply per acre.

Potassium is also a relatively immobile nutrient in soils, but it can leach a certain amount in sandy soils. Potassium levels can be built up in soils when application rates exceed removal in the forage. Soils that are high in K should be tested regularly if no K fertilizer is applied to monitor K levels. High rates of K are removed in hay so soil K levels can drop quickly in sandy soils and when fertilizer rates don’t match removal rates. High rates of K should be avoided on cool-season grass pastures because a nutrient imbalance could occur, resulting in a high potential for grass tetany in grazing livestock.

Legumes are very desirable forages to have in pastures because they are high in nutritive quality, they help offset the effects of fescue endophyte, and they reduce the need for nitrogen fertilizer because they can fix nitrogen from the air. Legumes, specifically the clovers and alfalfa, need higher soil fertility levels and careful management to maintain stands than do most grasses. For best establishment and stand persistence, soil pH and fertility levels should be medium to high before planting legumes. Planting clover into acid, low P soil will often result in a poor to marginal stand that dies during periods of stress. Soil pH should be 6.0 or higher and the soil P level should be at least in the medium range for best establishment. Clovers and especially alfalfa are heavier users of boron (B) and molybdenum (Mo) than are the grasses. These micronutrients should be applied at rates recommended by soil tests.

Nitrogen should not be applied to grass-legume pastures when legumes are being established. Nitrogen fertilizer stimulates growth of the established grass enough so that it crowds out the developing legume seedling. Nitrogen fertilizer is not needed for legumes because in combination with symbiotic bacteria they can fix N from the air. Nitrogen fixation is done by Rhizobia bacteria that live in nodules on legume roots. A crop of alfalfa can fix 200-300 lbs. of N per acre per year. Good stands of red and ladino clover can fix between 100 and 200 lbs. of N per acre per year. When the percentage of legumes in a pasture is 25 percent or greater, enough N is generally available from the legume for adequate forage production without added N fertilizer. Adequate amounts of P and K must be applied to maintain good legume stands.

Currently, there is no distinction of fertilizer recommendations in Arkansas for hay or pasture production although the amounts of nutrients removed from a field differs greatly between these two uses. When the hay is harvested virtually all of the nutrients in the standing portion of the hay crop are removed from the field. Average nutrient removal per ton of hay by various forages is shown in Table 2. Nutrients removed in hay can be returned to a field through nutrient cycling only if hay is later fed to livestock in that field. Nutrient cycling often does not meet the fertility requirements of a hay system so additional fertilizer must be applied to maintain soil fertility levels.

In a pasture system, most of the nutrients in the standing portion of the pasture crop are returned to the soil through manure and urine. Only a small amount of the total nutrient content of the forage remains in the grazing animal’s body. Manure and urine tend to be concentrated around livestock loafing areas such as shade trees or watering sites so nutrients accumulate in these areas. With a good grazing system, the livestock can be managed to distribute many of the nutrients in manure and urine back on the field to enhance future forage production. Continued uniform distribution of manure and urine on the pasture will improve uniformity of soil fertility over time. As the uniformity of soil fertility improves across the field, less fertilizer is needed to maintain production. Fertilizer rates should be based on soil test levels regardless of the grazing system being used. Regular soil tests should be taken to monitor fertility levels and fertilizer requirements.

Occasionally forages show very little yield response to fertilizer even though the recommended amount was applied at the proper time. The lack of yield response to fertilizer may be caused by several factors. Poor growing conditions are a common cause for low response to fertilizer. Abnormally cold or dry spring weather can reduce growth of cool season forages and can delay the early growth of warm-season grasses. If summer-like temperatures arrive before the

plants recover from poor spring growing conditions yields are reduced even more. Drought conditions during the summer also reduce yield response of forages even with good fertility management.

Overgrazing reduces the vigor of forage plants. Plants that are overgrazed or grazed too frequently have shortened root systems. Weakened forage plants with short roots are slower to respond to good growing conditions and fertilizer than healthier plants. Low soil pH reduces the availability of many fertilizer nutrients compounding the effect of other stresses on the plants. Poorly taken soil samples may not reflect the true fertility level of the field. Fertilizer recommendations based on a poorly taken sample can also cause poor forage yields.

When forage yields are low the amount of nutrients removed from the soil is also low. If fertilizer is applied and the forage crop is a “failure” or very poor yielding, most of the P and K not removed by the crop will still be in the soil. Phosphorus and potassium tend to accumulate in soils when application rates exceed removal rates and can improve future forage production.

Fertilizer Grades

Chemical fertilizer is manufactured, mixed, and sold according to fertilizer grades. Fertilizer grade refers to the percentage of N, and the oxide forms of phosphorus (P₂O₅), and potassium (K₂O) in the fertilizer product. The practice of reporting phosphorus and potassium as the oxide forms rather than actual P and K in fertilizer grades dates back to early fertilizer research when P and K were believed to exist as these oxide forms. Later research allowed measurement of actual P and K in soil and fertilizer, but the practice of reporting these nutrients as the oxide forms in fertilizer grades is still used in the fertilizer industry. For consistency, fertilizer recommendations for P and K are also given as the oxide forms of P₂O₅ and K₂O.

A granulated fertilizer grade of 17-17-17 for example contains 17 pounds each of N, P₂O₅ and K₂O per 100 pounds of fertilizer. A potassium fertilizer material used in bulk blended fertilizers having a grade of 0 - 0 - 60 contains 60 pounds of K₂O, but no N or P₂O₅.

Fertilizer grade should not be confused with fertilizer rate. Fertilizer grade refers to the percent or pounds of plant food per 100 pounds of a fertilizer material. Fertilizer rate refers to the total amount of plant food nutrients (N, P₂O₅ and K₂O ) recommended per acre for a particular yield level of forage. For example, a fertilizer rate of 60-90-90 is made up using certain amounts of a fertilizer grade or by bulk blending high analysis fertilizer materials together. The actual spreading rate refers to the total number of pounds of all the fertilizer materials needed to apply a fertilizer rate per acre.

Bulk Blending of Fertilizer

Bulk blending is the mechanical mixing of dry, solid fertilizer materials. Bulk blended fertilizers have advantages over granulated fertilizers such as 13-13-13. Bulk blended fertilizers can be mixed using high analysis fertilizer materials, which allows the same amount of plant food nutrients to be applied to a field with less weight and transportation costs. For example, 462 pounds of a granulated 13-13-13 fertilizer would be needed to apply a recommended fertilizer rate of 60-60-60 per acre, but only 339 lbs per acre of a bulk blended fertilizer would be needed to apply the same amount of nutrients. The savings in time and transportation costs can be substantial when fertilizer is applied to large acreages.

Bulk blended fertilizers also allow more accuracy in nutrient application rates which reduces costly under- or over-application of specific nutrients. For example, a soil low in P may need an establishment fertilizer rate of 25-100-50, but N and K₂O will both be over-applied if the P requirement is met by using 13-13-13 or even 17-17-17 fertilizer. Bulk blended fertilizer can be mixed to apply precise amounts of N, P₂O₅ and K₂O thus lowering fertilizer cost by reducing application of unnecessary nutrients.

One of the most important soil factors measured is pH. The pH is a measure of the acidity or alkalinity of the soil. Values for pH are based on a scale that ranges from 0 to 14. A pH of 7.0 is neutral. Values above pH 7.0 are alkaline and values below pH 7.0 are acidic. The pH range for most mineral soils across the U.S. is 3.5 to 10.5 (Figure 1). Most Arkansas soils are acidic and generally range in pH from 4.0 to 7.0. The acidity of a soil increases ten-fold for each whole number decrease below pH 7.0. For example, a pH of 6.0 is 10 times more acidic than a pH of 7.0, a pH of 5.0 is 100 times more acidic than pH 7.0, and a pH of 4.0 is 1000 time s more acidic than pH 7.0. This means that a greater amount of lime is needed to increase the pH of a soil from 4.0 to 5.0 than from 5.0 to 6.0.

Liming Soils

Limestone is applied to soil to neutralize soil acidity and raise the soil pH. Low soil pH can have a dramatic negative effect on forage production. Soil acidity can reduce fertilizer efficiency, nitrogen fixation by legumes, and can lead to micronutrient toxicities (Figure 2). Limestone is made up primarily of calcium carbonate. Limestone sources that contain appreciable amounts of magnesium are called dolomite or dolomitic limestone. The carbonate fraction of the limestone material is actually the material that neutralizes soil acidity. The calcium or magnesium fraction does little to change the soil pH. Soil acidity is caused by hydrogen ions (H+) in the soil. Calcium and magnesium ions from the limestone replace acidic H+ ions on the negatively charged soil particle. The carbonate fraction of the limestone then combines with the H+ thus neutralizing the acidity and raising the soil pH.

Liming is most effective if the lime is incorporated into the soil by plowing or disking. Incorporated lime has greater contact with the soil and reacts more quickly to increase the soil pH. In pastures, incorporation of lime is not practical except during pasture renovation so lime must be surface applied. Surface applied lime requires six months or more to react with enough soil acidity to appreciably increase the soil pH. Lime can be applied to pastures during any time of year, but the lag time between lime application and pH change must be considered when planting forages requiring high soil pH levels. Generally, lime should be applied to established pastures in the fall, especially if legumes will be overseeded during the winter. Most forages, especially legumes, establish poorly and do not produce well if the soil pH is lower than the optimum range.

Some forages are more tolerant of soil acidity than are others. The optimum pH range varies for different forage crops. The recommended soil pH ranges for forages grown in Arkansas are shown in Table 3.

Limestone Quality

Limestone effectiveness is determined by the CCE (calcium carbonate equivalent) and the sieve size percentage or the degree of finely ground particles in the material. The CCE of a liming material is determined by reacting the liming material with acid. The amount of acid neutralized by the limestone in this reaction tells the level of CCE in the lime material. Pure limestone with no impurities would be 100 percent CCE. The standard CCE level for liming materials in Arkansas is 80 percent. Some liming materials may have higher CCE percentages and others may be lower.

Agricultural limestone must be finely ground to effectively reduce soil acidity. Ag lime is produced by grinding or pulverizing limestone fine enough so that the resulting material will pass through specific sized screens or sieves. A good quality ag lime is typically ground fine enough so that 90 percent of the material passes through a 10 mesh sieve, 40 percent passes through a 60 mesh sieve, and 25 percent passes through a 100 mesh sieve. The fineness of the pulverized material does not affect the total solubility of a liming material, but it directly affects the rate of reaction with soil acidity. Course limestone particles have essentially no effect on soil pH. Finer materials have more surface area and react with soil acidity rapidly, increasing the soil pH. Lime particles coarser than 10 mesh in size have essentially no neutralizing value, those between 10 and 60 mesh have 40 percent efficiency, and particles smaller than 60 mesh have a 100 percent efficiency factor.

Arkansas does not have a law requiring a minimum quality level for agricultural limestone, but the lime dealers are required to state the CCE and sieve size percentages of any liming material sold. Values for both CCE and the sieve size percentages should be used when comparing different liming materials. A finely ground liming material with a relatively low CCE can be just as effective in neutralizing soil acidity as a more coarsely ground material having a high CCE.

Animal manure can be an inexpensive source of fertilizer nutrients for pasture and hay production. Animal manures vary widely in nutrient content due to the ration fed to the animal, nutrient losses during storage, and moisture content of the manure at application. Laboratory analysis of the manure is recommended to accurately determine the application rate per acre that will supply the necessary amount of nutrients to the forage. If the analytical results are reported by the laboratory on a dry weight basis, they must be converted to an “as is” basis for calculating application rates. If no laboratory analysis is available, application rates can be based on the average nutrient values of similar manures as shown Table 4.

Generally about half of the nitrogen in manure is lost to the atmosphere through volatilization when it is surface applied and not incorporated. Little loss of N occurs when manure is incorporated in the soil immediately after application. All of the P and K in manure is considered to be available to plants over time.

Animal manures can be substituted for commercial fertilizer according to the following guidelines:

• For surface applied animal manure substitute two pounds of N in animal manures for each pound of recommended commercial fertilizer N. If manure is immediately incorporated into the soil, substitute N from manure on a pound-per-pound basis with recommendations for commercial fertilizer N. Substitute P and K on a pound-per-pound basis with recommendations for fertilizer P and K for both surface applied and soil-incorporated manure. Apply at the beginning of the growing season for the particular crop.

• Limit recommended annual applications of poultry manure and litter depending on soil test P and K values shown in Table 5.

• On cool-season grasses and small grains grown on low fertility soils, limit poultry litter rates to no more than 4 tons per acre per year, regardless of higher soil test fertilizer recommendations, to reduce chances of grass tetany or nitrate poisoning in cattle.
   
• Where total annual rates are to exceed 4 tons per acre, make split applications of no more than 2.5 tons per acre. Split applications at least two to three months apart reduce the chances of excessive N and P getting into streams or groundwater. Follow best management practices developed for Arkansas or your farm.
   
• To check for nutrient balance in soil, test the soil at least every two years where high manure application rates are used. Do not apply P from any source when the soil test P exceeds 300 pounds per acre.
• There is a high risk of grass tetany in cattle grazing cool-season forages when soil test K levels are above 800 pounds per acre and Ca levels are below 2000 pounds per acre.
   
• When animal manures high in moisture are applied to growing forages, foliage burn often occurs especially in summer when soil moisture is limited.

Information for this guide was taken from: 1998 Arkansas Soil Testing Guide, Brady, N. C. 1974. The nature and properties of soils. 8th ed. MacMillan Publ. Co. New York.; and Tisdale, S.L. and W. L. Nelxon. 1975. Soil Fertility and Fertilizer, 3rd ed. MacMillan Publ. Co. New York.

For more information about forage management, contact your county Extension office or refer to one of our publications.

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