Animal manure has long been recognized as a source of nutrients for crops. The effluent and solid waste generated from livestock operations can be used in agricultural fields to supply nutrients to crops and improve soil chemical and physical properties. Knowing the mineral contents of the manure enables the farmer to decide on the amount of manure to apply or to move off-site. By actively managing the nutrient balance on-farm and marketing or exporting manure when necessary, farms with confined animal operations can prevent future buildup of soil nutrients that may potentially move into streams and lakes.

There are three steps required in developing a nutrient budget:

1. Determine crop nutrient requirements.
This is accomplished by knowing the nutrient requirement of the crop and accounting for the residual nutrients in the field that will receive the manure. The field’s fertility is determined through soil testing. The fertilizer recommendation given in the soil test report is based on a calibration of the soil test results and the nutrient requirement of the crop to be grown.

2. Determine nutrients supplied by manure.
The animal manure is analyzed to determine the nutrient content of the manure. As with soil sampling, taking a representative sample is important to get an accurate estimate of the nutrient content of the manure. Using “book values” to estimate nutrient content of manure should be avoided whenever possible because of high variability between manures. Manure samples may be submitted to a reputable laboratory for analysis.

3. Balancing crop nutrient needs with nutrients supplied by manure.
The third and final step in calculating a nutrient budget for animal manures is to simply match the nutrient needs of the crop to be grown in step 1 (based on a soil test) with the nutrient content of the manure determined in step 2. For example, if 60 pounds of nitrogen per acre is recommended for a crop and the manure analysis indicates it contains 30 pounds available nitrogen per ton, then two tons of manure per acre would be recommended.

Nitrogen-based versus phosphorus-based manure application
Nitrogen (N) and phosphorus (P) are the major plant nutrients found in manures that can contaminate the environment if too much is applied. Therefore, application rates for manure or lagoon effluent must not exceed the field’s capacity to minimize nitrogen and phosphorus transport from the field to surface waters. It should also be noted that manures have a poor N:P ratio relative to plant needs; this is further explained below.

When applying manures to cropland, there are two strategies that can be followed:

1. N-based approach where manure is applied at rates that meet the crop’s need for nitrogen
2. P-based approach where manure is applied to satisfy the crop’s need for phosphorus

The N-based approach is the most common strategy for utilizing manure to meet the N requirement of crops. Manure application rates are calculated to provide N to crops because most crops require more N than P.

In many cases, N-based nutrient management plans are employed to supply just the right amount of N for optimum crop production, while avoiding excessive N applications that may cause nitrate leaching into groundwater. This strategy, however, usually results in the addition of P in excess of the nutrient needs of the crops. This leads to increases in available P in the soil.

The P-strategy, on the other hand, results in less P application, just enough to meet the needs of the crops. Additional N is needed to meet the needs of the crop, which is usually provided using commercial fertilizers. Legumes are an exception because they can manufacture their own nitrogen through N-fixation from the atmosphere. Thus, recent environmental concerns have focused more on excessive phosphorus applications from manures and its adverse effect on surface water quality.

Because of concern for P in runoff to sensitive water resources, many nutrient management plans now are based on P. The P-based plan, however, is not currently required but it will be considered in the future. Overall, P-based application for manures should be used where there is high risk for water contamination by excess P through runoff.

A tool to identify fields which have the potential for P pollution is the P index. The P index will help farmers identify areas and adjust manure application rates and other management practices to minimize P losses from agricultural areas and alleviate P pollution of water bodies.

Off-site use of manures
Aside from using the manures for crop production on-farm, farmers can sell their manures to other farmers to generate additional income. This would be a necessary option in situations where the supply of manures exceeds the needs of the crops on that farm. Sale of manure to other farmers is more likely when manure application is based on a P-strategy where a larger area is required to spread a given volume of manure compared to when the application is N-based.

Moving the manure to a site where it is needed will minimize the risk of water pollution. An analysis should be provided to the person taking the manure just like you would expect to get a fertilizer tag or analysis if purchasing chemical fertilizer.

By actively managing the nutrient balance on-farm and marketing or exporting manure when necessary, confined animal operations can achieve a relative level of nutrient sustainability that should prevent future buildup of soil nutrients like phosphorus.  ANM

References omitted but are available upon request at This e-mail address is being protected from spambots. You need JavaScript enabled to view it

—Excerpt from “Small Farm Nutrient Management Primer”