A fundamental question often asked by agricultural producers is how they should value manure as a nutrient resource. This essential question should be asked by those who have access to manure because it allows a way to quantify the economic value of that material. If this question were directed at commercially produced materials, the answer would be straightforward. With manure, however, a number of parameters need to be considered including the composition of manure, the source variability and the need for the nutrients based upon soil test information.
The first step in valuing manure as a nutrient supplement is to have the material analyzed to determine which nutrients are present and in what amounts. This information, combined with a recent soil analysis, can tell you how much manure should be supplied to meet the nutritional needs of a crop.
Lets examine the manure analysis first. Values of total N are not particularly valuable to the producer because they do not inform of the amount that is available to the crop. Of greater importance is determining the ammonia-N (or ammonium-N) and organic-N content. Ammonia-N contained in manure is similar to any form of commercial fertilizer; it is readily available the day of application. Therefore, valuing ammonia-N similar to a commercially available form is certainly a fair assessment.
Organic-N, however, is a slower release form of N because it requires a biological process to make it plant available. The environment-dependent nature of this biological process makes it difficult to ascertain its precise agronomic value. What we typically recommend is to use a lower-cost N source (typically anhydrous ammonia) to calculate the value of organic-N based on the plant available estimate (for more information on estimating plant availability from animal wastes see Bulletin 604).
Phosphorus and potassium contained in manure is considered approximately as available as their commercial counterparts are (actually, they are slightly less available, but currently we do not have plant available estimates for the different manure types). Therefore, valuing P and K from manure could be calculated by a comparison to a commercially available form.
Manure is considered a complete nutrient source because it contains everything a growing plant requires, and the analysis will likely provide you with additional nutritional information. However, it is not necessary to determine an economic value for all of the nutrients. This is especially true for those nutrients that do not necessarily require supplementation to ensure an adequate plant supply (i.e. micronutrients on soils that typically do not exhibit deficiency symptoms). Organic matter contained in the manure does have some redeeming value, but it would be extremely difficult to assign an economic value to it, and it is not something we currently recommend.
One of the challenges in using manure as a fertilizer source is the unbalanced nature of the nutrients. Applying enough manure to reach sufficient N and K levels usually results in the over application of P, which can have negative economic (from a sense that it would be more beneficial on other fields) and environmental outcomes. However, applying manure based on a sufficient P level usually results in an under-application of N, which can lead to a reduction in yield.
This brings us to soil analysis. In a system where P is rarely limiting (especially a field that has a history of receiving manure applications), balancing P, N and K requirements can certainly be a challenge if relying solely on a manure source. If a field has a soil test level above the maintenance range for producing a crop, does it make sense to calculate the economic value for that nutrient? For example, assume a field has a soil test P level well above the established critical level, we would not recommend calculating the economic value of the P contained in the manure when attempting to determine its economic value or recommend applying the manure to this field.
Conducting soil testing and manure analysis will help you determine how best to utilize your manure nutrient resources and get the maximum economic benefit from their use. Additionally, you will be able to do so in an environmental responsible manner.
A spreadsheet has been developed to assist in the determination of manure application rates based on the manure analysis and field information. The “Manure Allocation Spreadsheet” is available at http://agcrops.osu.edu/specialists/fertility.