Since manure is not a typical “traded commodity” and no formal marketing system is in place, determining its actual fertilizer value requires certain considerations. The value of manure fertilizer nutrients (primarily nitrogen, phosphorus, and potassium) is often estimated using comparative current costs of commercial fertilizer nutrients. Although this may seem like a simple comparison, many considerations must be taken into account in evaluating the worth of manure nutrients in a livestock operation. Some of these considerations are as follows:
* Nutrient losses in collection and storage. Nitrogen in manure is subject to bacterial degradation and volatilization of ammonia to the atmosphere. This process begins as soon as manure is voided by the animal. Volatilization is greatest in manure storage facilities with a high “exposure factor,” i.e., a large surface area exposed to the atmosphere. Studies indicate that 60% or more of the manure nitrogen entering a lagoon may be lost to the atmosphere through volatilization.
* Some nutrients may not be “recovered” from storage. While manure contained in manure tanks can usually be agitated and completely removed, the nutrients in settled solids and sludge in earthen impoundments may be difficult to agitate and remove, and these nutrients will not be available for land application.
* Field losses or “plant availability.” All of the manure nutrients applied to a crop may not be available to the plant during the growing season. Nitrogen in surface-applied manure is subject to volatilization losses as noted above. Injecting or incorporating manure into the soil reduces losses of manure nitrogen. Phosphorus and potassium are generally considered to be nearly 100% available to plants whether surface spread or injected into the soil.
* Manure nutrient displacement of actual fertilizer costs. Manure nutrients may have worth only if they displace commercial fertilizer costs. For example, fertilizer recommendations for legumes typically include little or no nitrogen. Hence, if applied to a legume, nitrogen in manure may have little or no value. Similarly, the N/P/K fertilizer ratio for the crop in question may be significantly different from the N/P/K ratio of the manure being applied. In these cases, the manure application rate must be adjusted to supply the “least required” nutrient if excess application of the other nutrients is to be avoided.
* Positive effects of organic matter in manure. It is generally accepted that the organic matter in manure contributes to soil-building properties and hence has some value. However, there is no general agreement on how to evaluate the value of this effect. Obviously, the soil-building properties of manure would be more valuable on poorer soils and perhaps less so on highly fertile soils. Suggestions range from zero to 50% of the manure nutrient value as added value of organic matter.

Livestock species, diet, feed intake, animal and manure management, storage conditions, and climate are factors that contribute to variability in manure nutrient content. Therefore, an attempt to assign a value to manure nutrients should begin with a laboratory analysis of a representative sample of the manure in question. After these data are in hand, the considerations outlined above, along with current commercial fertilizer prices, can be used to arrive at an estimate of the value of manure nutrients.

Your local university Extension Service, the NRCS, or a qualified professional can provide assistance in estimating the value of manure nutrients.