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0209: Know your manure’s available nutrients for crop production

Dorivar Ruiz Diaz Published on 03 March 2009

Manure can contain significant amounts of nitrogen (N), phosphorus (P), potassium (K) and other essential plant nutrients.

Concentration of these nutrients in livestock manure can also be highly variable and depends mainly upon the animal species, age, diet and how the manure has been stored and handled. Therefore, laboratory analysis is the best way to determine the level of nutrients in the material to be applied.

Nitrogen is a nutrient needed in relatively large quantities by most grain crops

, and manure can be an excellent source of N. Farmers usually apply manure at rates that would supply the N needs for grain crops. However, applying N-based manure would typically lead to over-application of P, which in turn, can lead to a large build-up of extractable P in the soil.

Also, large applications of nutrients can increase nutrient loss from fields, especially N and P, that can be detrimental to aquatic ecosystems. These factors result in a challenge for livestock producers and manure users for marketing, hauling and utilizing manure as a crop nutrient resource instead of a waste product.

A commonly asked question is: What are nitrogen and P crop availability shortly after manure application? In the case of N, it is important to consider that this nutrient is primarily in organic form, especially in dry manure that normally has higher levels of organic N when compared to liquid manure.

Organic N needs to mineralize before becoming available to crops. A fraction of this organic N may become part of the soil organic matter pool and unavailable to crops in the short-term.

Field and laboratory studies suggest that the fraction of total nitrogen that becomes plant-available the first year of application varies depending upon species, components in the manure and the method of handling and application. For example, manure that contains a large fraction of bedding material (source of carbon) will tend to have lower N availability during the year of application.

Reduction in N availability may also occur when manure is aged and has undergone some level of composting. Nitrogen lost from the ammonium fraction at the moment of application to the soil surface can also reduce plant-available N.

Ammonium volatilization is typically higher if applied during windy and warm days. Incorporation of manure immediately after application will reduce volatilization and potential loss by water runoff in the case of a rainfall event, which in turn will increase crop nutrient use efficiency from manure sources.

Application timing of manure is especially important for efficient crop N utilization and to reduce N loss. If the producer is applying manure with high levels of organic N, application should be made with sufficient time to allow conversion of organic N to crop-available inorganic N. However, if the manure has significant levels of inorganic ammonium N, application should be made as close to crop planting as possible to avoid N losses by leaching or denitrification.

The use of emerging technologies, such as aerial photos, hand-held chlorophyll meters and canopy sensing can help to improve the efficacy of manure N. Availability of manure N can be highly variable and affected by manure source, soil type, and climatic conditions. Using in-season assessment of nitrogen supply from manure N allows farmers to adjust N rates during the growing season, avoiding possible shortage during periods of high crop demand.

With high prices and uncertainty in the price of P fertilizer, many farmers are looking for manure sources to cover not just N, but also P crop demand.

Estimated values of P availability are typically from 50 to 100 percent for most Midwestern states. This range aims to account for variation in sampling and analysis and for P requirements with different soil test levels. The lower end of the range of P availability is usually recommended for soils with very low levels of P.

In these situations, large yield loss could occur if insufficient P is applied. On the other hand a value of 100 percent availability is typically suggested when manure is applied to maintain soil test P in an optimum soil test category and when the probability of a crop yield response is small.

Several studies have shown that manure P is a valuable resource, comparable to inorganic fertilizer P for crop production. These two P sources are similarly effective when the manure P concentration is known and the manure is applied properly.

Therefore, supplemental P fertilization after land application of manure is unnecessary when manure application rates provide sufficient amounts of P. However, it is important to indicate that supplemental N may be required when application rates are based on manure P content. Another important crop nutrient that can be supplied by manure is K.

Potassium is present in inorganic form and therefore plant-available and equally effective as fertilizer, if applied properly.

For maximum efficiency of manure use, it is essential to know the nutrient content of the manure. Using a manure lab analysis will help in determining the actual nutrient rates applied.

Producers should think in terms of actual nutrient application rates and not just gallons or tons per acre of manure being applied. Uniform application of manure nutrients at precise rates can be difficult.

Careful calibration of manure applicators is needed. If these aspects are considered, the efficiency of manure compared with inorganic fertilizer can be very good, and many farmers are recognizing that careful management pays off. The use of livestock manure can contribute to decrease the overall cost of fertilizer inputs for many crop producers, depending on the price and transportation cost of the manure. For many farmers, the use of livestock manure may represent significant savings.  ANM

**This article originally ran in an issue of Ag Nutrient Management.

Dorivar Ruiz Diaz
Department of Agronomy, Kansas State University