The application of animal manure to farmland is an economically and environmentally sound management practice for most feedlot operators and farmers. Land application returns nutrients from manure to the soil and helps build soil fertility. Manure provides nitrogen (N), phosphorus (P), potassium (K), calcium, magnesium, micronutrients and organic matter. Applying manure to soil has been shown to improve soil tilth, increase water-holding capacity, reduce wind and water erosion and improve soil aeration. Land application of manure should be managed to recycle plant nutrients rather than used as a disposal method.

The steps to properly using stockpiled manure are:

1. Determine crop nutrient requirements based on a realistic yield goal.
2. Determine the nutrient content of the manure.
3. Determine the fraction of manure nutrients available to the crop in the first year of application.
4. Calculate the application rate to supply crop nutrient needs.
5. Determine supplemental nutrients needed for optimum crop growth.

These steps will assure the proper amount (agronomic rate) of feedlot manure is applied, so the risk to soil and water quality is minimized.

Crop nutrient requirement
Feedlot manure should not be applied to fields beyond the limits of the growing crop’s nutrient needs. This ensures efficient use of manure nutrients and minimizes nutrient leaching or runoff into the water system. Applications of manure at agronomic rates generally will not create salinity problems. Any cropland scheduled to receive manure should first be soil tested to determine its present fertility level.

Periodic soil testing is recommended to monitor the nutrient- supplying capacity of the soil. Fertilizer recommendations, based on soil test results, are the only reliable way to obtain crop nutrient requirement. Crop nutrient needs are given in the interpretations and requirements section of the soil test report.

Manure nutrient content
The nutrient content of feedlot manure is highly variable. Fertilizer value depends on animal age, type of ration, manure handling methods and moisture content. Because of this variability in nutrient content, manure must be analyzed, and individual land application decisions should be based on the nutrient content of the manure to be applied. Samples collected for analysis should be representative of the lot or stockpile.

Feedlot manure should be analyzed for electrical conductivity (EC) and sodium content as well as major plant nutrients because EC is related to the amount of soluble salts you are going to add to the fields. Improper balance of sodium in relation to calcium and magnesium salts may cause the soil aggregates to disperse into individual particles and make the field difficult to manage.

Availability of manure nutrients to crops
Nutrients in manure cannot be substituted for those in commercial fertilizer on a pound-for-pound basis, since not all the nutrients listed on a manure analysis report are readily available to a crop in the year of application. Some elements are released when organic matter is decomposed by microorganisms. Some elements can combine with soil constituents and become unavailable. Other nutrients, such as nitrogen, may also be lost to the atmosphere through ammonia volatilization or denitrification or lost below the root zone through leaching.

Nitrogen available from feedlot manure during the year of application varies greatly and ranges from about 40 to 80 percent. Most of the nitrogen is present in the organic form. Organic N must be converted (mineralized) into inorganic forms such as ammonium and nitrate before it can be taken up by plant roots.

The amount of organic N converted to plant-available forms during the first cropping year after application varies depending on manure handling systems. In general, about 40 percent of the organic N may become available the year of application. Combining the inorganic N and the available N from organic matter, there is about 50 percent of total N available to crop uptake during the first year after application, 15 percent and 6 percent for the second and third year after application, respectively.

Soil test data should be used to follow the potential accumulation of N and P after repeated manure application. About 2 pounds of manure nitrogen are found to be equivalent to 1 pound of urea nitrogen. Phosphorus and potassium in feedlot manure are considered to be about 80 to 100 percent available when compared with commercial fertilizers for the first year after application. In general, 90 percent availability is assumed when determining an application rate based on P or K.

Application rate
Land application rates should be based on the nutrient requirements of the crop. Too little manure application will not provide sufficient nutrients for the desired crop production. Excess nutrients are a waste of resources, resulting in nutrient buildup in soils, which may cause water contamination. Soil testing, manure analysis and proper estimation of yield goal are necessary to calculate proper agronomic application rates of feedlot manure and additional fertilizers.

If manure is applied to the same field in the following years, nitrogen credits should be given to previous years’ application because a certain percentage of nitrogen is carried over to each year after first application for several years, as discussed in the nutrient availability section.

Application timing
Proper timing of feedlot manure application is essential to efficient use of nutrients and pollution prevention. It should be applied as near to the planting dates as possible. Applying manure outside of crop growth periods decreases nutrient availability and may increase the risk to environmental quality.

Manure applied during the spring provides the needed starter nutrients for summer crops and will release nutrients through mineralization during the growing season. Summer application is most suited to pasture. Grasses or pasture also offer the flexibility when crop fields are not available to receive manure or too wet to conduct spreading operations.

Generally, applying manure in the fall results in greater nutrient loss than spring application, especially if the manure is not incorporated. If the manure is incorporated and the soil temperatures are below 50°F, most of the available nutrients will be retained by soil until the following spring. Sometimes fall application is needed to free up manure storage space. In this case, it is best to apply manure to fields where winter grains or cover crops will be planted.

Winter application is the least desirable because manure must remain on the soil surface for three to four months ahead of the crop’s active growing period.

Proper manure application
Feedlot manure normally is applied to land by surface broadcasting using a manure spreader followed by plowing or disking or broadcasting without incorporation. Research has shown maximum nutrient benefit is realized when manure is incorporated into the soil immediately after application. Immediate incorporation of solid manure minimizes nutrient loss in runoff and nitrogen loss to the air and allows soil microorganisms to start decomposing the organic fraction of the manure quickly. This increases the amount of available nitrogen to the crop.

Manure should be applied uniformly to prevent localized high concentrations of ammonium and other inorganic salts which can reduce germination and affect yields. The application equipment needs to be calibrated to achieve accurate application rates.

Prevention of salt buildup in soils from manure application
Salt buildup also is possible if application rate is excessive and rainfall is low. The amount of salt a soil can handle depends on soil texture, amount of irrigation and natural rainfall. It is also important to know what kinds of salts and how much of each kind is being added to the soil. Improper amount of sodium in relation to calcium and magnesium salts in soil may cause the aggregates to disperse into individual particles. This will result in poor soil physical properties.

Tests including soil salinity and manure salt content may also be needed to monitor soil salt levels in dry climate regions. It is recommended to include EC (electrical conductivity) and ESP (exchangeable sodium percentage) in your annual soil testing program to monitor the effects of manure salts. ANM

References omitted due to space but are available upon request.

—From Oklahoma State University Extension website