Ohio State University (OSU) experts say a number of parameters must be taken into account when putting a value on manure as a crop nutrient resource. In a recent Ohio State University Crop Observation and Recommendation Network (CORN) newsletter, Robert Mullen and Darlene C. Florence urge producers to consider the composition of the manure, source variability, and the need for crop nutrients based on soil test information.
The first step is to have the manure analyzed to determine which nutrients are present and in what quantities. This information, combined with a recent soil analysis, can help determine how much manure should be supplied to meet the nutritional needs of a crop.
According to the OSU experts, the value of total nitrogen (N) listed in the manure analysis is not particularly valuable because those values do not indicate 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 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 thought to be a fair assessment.
Organic-N, however, is a slower-release form of nitrogen because it requires a biological process to make it available to the plants. The environment-dependent nature of this biological process makes it difficult to ascertain its precise agronomic value. OSU Extension experts typically recommend using a lower cost N source (typically anhydrous ammonia) to calculate the value of organic-N based on the plant available estimate. Additional information is available in an online PDF version of the Ohio Livestock Manure Management Guide at ohioline.osu.edu/b604/pdf/b604.pdf.
Phosphorus and potassium availability in manure are considered to be about the same as their commercial counterparts, although they are actually slightly less available. Plant-available estimates for the different manure types do not currently exist, so establishing a value for phosphorus (P) and potassium (K) in manure can be calculated by a comparison to a commercially available form, they note.
Manure is considered a complete nutrient source because it contains everything a growing plant requires. An analysis will likely provide additional nutritional information, but it is not necessary to determine an economic value for all of the nutrients, such as micronutrients, on soils that typically do not exhibit deficiency symptoms. The organic matter in manure has some redeeming value, but it is difficult to assign an economic value and, therefore, is not recommended.
A challenge 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 and environmental outcomes. On the other hand, applying manure based on a sufficient P level usually results in an under-application of N, which can reduce crop yield.
The Ohio experts agree that soil testing and manure analysis will help determine how best to utilize manure nutrient resources while striving to maximize the economic benefit from their use in an environmentally responsible manner.
OSU has developed a spreadsheet to help determine manure application rates using soil and manure analysis. See the "Manure Allocation Spreadsheet" at agcrops.osu.edu/specialists/fertility. Read the CORN newsletter article online at corn.osu.edu/newsletters/2010/2010-28.