Ohio State University Extension
Department of Horticulture and Crop Science
2021 Coffey Road, Columbus, Ohio 43210-1044

Best Management Practices: Land Application of Animal Manure

AGF-208-95

Land application of animal manure is an efficient utilization alternative because of usually lower costs compared to treatment and the nutrient benefits derived by crops from the manure. Manure nutrients help build and maintain soil fertility. Manure can also improve soil tilth, increase water-holding capacity, lessen wind and water erosion, improve aeration, and promote beneficial organisms. There are two principal objectives in applying animal manure to land: 1) ensuring maximum utilization of the manure nutrients by crops and 2) minimizing water pollution hazard.

Surface spreading and subsurface injection are two of the most common land-application methods. Several guidelines should be followed to achieve maximum nutrient use with minimal environmental hazard:

• Test soil to establish existing soil-fertility levels.

• Test manure and wastewater to determine nutrient content.

• Select an application rate that does not exceed crop nutrient requirements and avoids soil contamination, crop damage, and runoff and contaminated tile flow. A computer program available from Ohio State University Extension can assist in selecting appropriate application rates. ECP 102, "Crop Nutrient Management," is available from local county Extension offices.

• Check soil moisture before applying liquid wastes, and adjust application rates to avoid runoff. Estimate soil moisture based on soil feel and appearance. Refer to AEX 704, "Land Application of Manure and Waste water - Part 1." Limit the volume of water applied to an amount that brings the soil to field moisture capacity. Available moisture capacity for different soils can be found in the "Ohio Irrigation Guide," available from the state Soil Conservation Service or the Department of Agricultural Engineering, The Ohio State University.

• To avoid runoff, do not apply manure to frozen or saturated soils.

• Calibrate application equipment to obtain the desired application rate. For more information on spreader calibration, refer to AEX 707, "Land Application of Waste ... Spreading and Injection," available from local county Extension offices.

• Incorporate raw or untreated manure to reduce odors and nitrogen losses.

Available land for manure application is an important consideration for existing livestock operations as well as new or expanding operations. Typically, if enough land is available in an operation to produce feedstuffs for the animals, there is enough land to apply manure nutrients to minimize environmental effects.

When planning a new operation or expanding an existing operation, enough land area for manure application must be included in the plan. A conservative approach to determining the amount of land required is to consider the crop nutrient removal of the harvested crop. This will ensure that enough land area is available in future years to prevent nutrient buildup in the soil beyond suggested agronomic and environmental levels. For information on determining required land area for manure application based on crop nutrient removal, refer to "Land Required for Manure Application Based on Crop Removal."

Land application of plant nutrients, including manure, must take into account both crop needs and the potential for environmental degradation. Nutrients should not be applied in quantities that exceed the amount needed for adequate plant nutrition. All nutrient sources from commercial fertilizer, manure, and sludge application must be considered. Excess application may induce nutrient deficiencies in the soil and increase the potential for excess nutrients to enter waterways. High nutrient concentrations can accelerate eutrophication, a condition that reduces dissolved oxygen in the water, increasing plant growth and limiting animal life.

Manure contains plant nutrients and should be managed as a mixed fertilizer applied to satisfy crop nutrient needs. Alternate strategies are available to animal producers who have more manure than can be effectively land applied based on nutrient needs. In many situations, relatively deep incorporation and intensive control of runoff at the application site can minimize environmental impacts.

In animal manure management, phosphorus (P) is the nutrient of major concern on soils with high phosphorus fertility levels. Phosphorus applied to fields as manure or commercial fertilizer can move into bodies of water during erosion and runoff events, and is largely responsible for the accelerated eutrophication of many bodies of water in Ohio. It accumulates in soils if applied in quantities greater than those removed by crops. Accumulation of phosphorus in the soil can be measured by accepted soil test procedures. Agronomic crops grown in Ohio rarely respond to applications of additional phosphorus when soil test levels exceed 30 ppm (60 lb/A) of phosphorus, and crops grown in soils with very high phosphorus levels may actually produce lower yields due to nutrient imbalances. Concentrations of algal-available phosphorus (the phosphorus responsible for eutrophication) increase as soil-test phosphorus levels increase, meaning that as soil concentrations of phosphorus rise, the potential increases for soils to degrade the environment through runoff and erosion. Slowing the eutrophication of Ohio's surface waters involves reducing the amount of phosphorus delivered to these waters. Therefore, recommended strategies for manure applications to land are based on the following principles:

• There is no agronomic justification for raising soil-test phosphorus levels above those that provide adequate nutrition to the crop.

• Increasing soil test phosphorus levels at the soil surface increases a field's pollutant potential.

• Site characteristics, application methods and management, cropping system, and erosion/runoff abatement practices all affect the quantity of manure that can be safely applied to a given field.

Factors Controlling Application Rate

The factors that most often limit the amount of manure that should be applied to cropland are existing soil-fertility levels, manure nutrient content, crop nutrient needs, site limitations, slope, runoff potential, and leaching potential. Nitrogen and/or phosphorus is usually the limiting nutrient for manure application. All manure contains measurable amounts of both. Applying levels that exceed crop nutrient requirements may lead to nutrients entering surface waters or leaching into ground water.

The amount of nutrients available in raw manure varies with the type and size of animal. Table 1 provide average nutrient values. The nutrient composition of waste is affected by housing and the waste-handling system. Bedding and additional water can dilute manure, resulting in less nutrient value per pound. Nutrient losses from storage and handling reduce the amount of nutrient available for land application. Phosphorus and potassium losses are usually negligible, but nitrogen losses can be significant. Table 2 provides a comparison of nitrogen losses due to storage and handling. Land application methods also affect the amount of nutrients available for crop uptake. Most losses occur within 24 hours of application. Manure should be incorporated into the soil as soon as possible after application. Injecting, chiseling, or knifing liquid manure into the soil minimizes odors and nutrient losses to the air or as surface runoff. Tables 3 and 4 present approximate nutrient values for land-applied solid and liquid manure, taking into account handling and storage losses. The amount of nitrogen available in the soil depends on the method of application and days to incorporation. Table 5 can be used to estimate the availability of ammonia and organic nitrogen in the soil. The phosphorus and potassium applied to the soil will be available unless removed by surface runoff and soil erosion. Nearly 100 percent of total phosphorus and potassium from manure application are considered available the first growing season.

TABLE 1. Annual Raw Manure Production per 1,000 lb Animal Weight.

TABLE 2. Nitrogen Losses During Storage and Handling (a)

TABLE 3. Approximate Fertilizer Nutrient Values of Animal Manure at Time Applied to Land - Solid Handling Systems (a)

TABLE 4. Approximate Fertilizer Nutrient Value of Animal Manure at Time Applied to Land - Liquid Handling Systems (a)

TABLE 5. Method of Calculating N Availability of Manures (a)

Only about one-third of the organic nitrogen in animal manure is available to crops during the year it is applied, and the remaining two-thirds, residual organic nitrogen, becomes part of the soil organic matter. It is mineralized or becomes available at the rate of about 5 percent a year. To determine how much nitrogen will be available to crops from manure applications, growers must take into account the mineralized nitrogen that will become available from previous manure applications (Table 6). Manure is also a good source of phosphorus and potassium. Tables 1, 3, or 4 can be used to calculate the amount of phosphorus and potassium that will be available from the manure. The phosphorus and potassium in manure will be as available to the crop during the year it is applied as would the equivalent amount of fertilizer-grade phosphorus and potassium.

TABLE 6. Percent of Residual Organic Nitrogen Made Available From Manure Applied Previous Years.

If the growers have applied phosphorus and potassium over the years, soil-test levels may be in the adequate to high range. To ensure high nutrient efficiency from added manure, growers should apply manure at rates that would satisfy only the crop's phosphorus and/or potassium needs. Use the method presented in "Determining Recommended Manure-Application Rates" when calculating application rates for phosphorus and potassium. It is important to note that manure contains much more potassium than magnesium or calcium, and after many years of continued manure application, the ratio of potassium to magnesium and calcium may be too high for optimum crop growth. To adjust the ratio, additional magnesium and/or calcium may have to be added as dolomitic or calcitic limestone. The soil should be tested regularly to determine these nutrient levels. If a mineral imbalance is suspected, plant-tissue analysis should be made to determine the extent of the problem.

TABLE 7. Recommended Maximum Manure Application Rates at Different Soil Test Levels (1).

Fields receiving manure should be tested for available nutrients before application. The manure should also be tested. Application rates should be determined using the results of these tests. Table 7 provides recommended maximum manure application rates for different soil test levels and site conditions.

Adding high levels of manure may increase salts in soils and reduce plant stands. When manure is applied to a field, annual soil tests are strongly recommended to more accurately monitor soil nutrient levels and determine appropriate manure application rates.

Manure applications should not provide more available nitrogen (N) than what is needed by the succeeding crop. The determination of total available nitrogen should include credits for any contributions of the present or preceding crop, any nitrogen fertilizer added, and available nitrogen provided by previous manure applications.

Injection or incorporation by primary tillage promptly following application should minimize the potential for direct runoff. Manure should not be applied to the surface of wet, sloping, or frozen fields if normally anticipated rainfall would cause overland flow from the point of application. Liquid manure should not be applied at rates that exceed the volume needed to bring the soil to field moisture capacity and cause surface runoff or direct entry of manure into subsurface drainage system.

The manure utilization and cropping systems used on a particular field should maintain Bray P1 soil test phosphorus levels no greater than 30 ppm (60 lb/A) of phosphorus in the top 8 inches of soil. Special precautions should be taken if manure is applied where Bray P1 levels already exceed this level.

If manure must be applied to fields with levels greater than 30 ppm (60 lb/A) of phosphorus, note the following recommendations:

• No additional phosphorus fertilizer should be used on such fields.

• Crops should be monitored for nutrient deficiencies using plant-tissue analysis. Increasing soil test phosphorus and potassium levels above recommended levels increases the probability of yield-reducing nutrient imbalances.

• Manure should be applied in quantities such that the long term phosphorus level at the soil surface does not increase appreciably. This can be accomplished by observing the following guidelines:

  (a) Ensure that applications supply no more nitrogen or phosphorus (whichever is lower) than will be removed by the next crop (one season) if the manure is surface-applied or incorporated to a shallow depth (within the tillage depth).

  (b) Incorporate manure below the depth of tillage, generally deeper than 8 inches, using rates great enough to satisfy nitrogen requirements for a succeeding grass crop. Note: Based on soil testing, shallow tillage may be necessary to reduce surface phosphorus levels that have increased due to crop residue deposition. Producers should be aware that this system may promote development of a soil zone with extreme nutrient concentration that may increase phosphorus runoff if brought to the surface by subsequent tillage. Fields under such management should be sampled deep enough to include the zone of incorporation. On soils with high nutrient levels, manure application should be skipped for one or more seasons to allow depletion of accumulated nutrients.

• Adequate soil and water conservation should be practiced to control erosion and minimize runoff.

• Manure application is not recommended for crop production where the Bray P1 level in the top 8 inches of soil exceeds 150 ppm (300 lb/A)of phosphorus.

• If deep application and incorporation of manure are required to maintain low runoff and more than 250 pounds of P2O5 per acre are applied, soil at the depth of application should not be brought to the surface for three years unless low-runoff conditions can be maintained. Moldboard plows should be set to two-thirds the depth of incorporation during this three-year period, and non-inversion tillage tools such as chisel plows should be set no deeper than the depth of application. Deep incorporation of less than 250 pounds of P2O5 per acre may be conventionally tilled the following year.

Factors Affecting Runoff Potential

Runoff potential is affected by numerous factors, some of which are fixed by the nature and location of the field and others that can be altered through management. Quantitative evaluation of these factors can be difficult because the factors either have not been quantified or they interact with each other in field conditions. Therefore, runoff potential must be determined on a site-by-site basis.

Naturally occurring factors that affect runoff potential include:

• Location of receiving stream. Runoff to streams is much more likely when the field selected for manure application is bordered by a stream or other surface water rather than separated from surface water by another field, pasture, wooded area, or other suitable buffer strip.

• Slope steepness and complexity. Runoff is more likely from fields sloping steeply and evenly toward a stream than fields with a gentle or no slope. Fields with depressional areas between the area of manure application and the stream have a lower potential for stream contamination.

• Soil and weather conditions. Runoff is more likely when applied on frozen, saturated, or compacted soils, especially during months when rainfall exceeds evapotranspiration.

• Soil type. Soils with low infiltration rates and/or soils with limited water-holding capacity are more likely to promote runoff than soil types that absorb and retain large quantities of water.

Management factors that can alter the potential for manure runoff into a stream include:

• Buffer strips. Properly designed buffer strips along stream banks adjacent to sites with high-runoff potential can reduce the amount of manure entering a stream.

• Soil surface condition. A rough or covered soil surface reduces runoff compared with soil surfaces that are smooth or have very little residue cover.

• Manure characteristics, application rate, and application method. Liquid manure applied at rates greater than the soil infiltration rate or water-holding capacity can promote runoff. Injection or incorporation of applied manure reduces chances of runoff.

• Pre-existing soil nutrient status. More nutrients are likely to move off fields when soils have a high crop-nutrient or soil test level rather than lower crop-nutrient test levels.

• Surface and subsurface drainage. Proper installation and routine maintenance of surface and subsurface drainage systems can reduce the potential for runoff or direct discharge of manure from land application.

Prepared by:
Dr. Jay Johnson
Extension Agronomist
Dr. Don Eckert
Professor, Natural Resources