Determining the fertilizer value of manure

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Spreading manure
Manure being spread on a field.

What is the value of organic fertilizers, such as manure? While it can be difficult to quantify the true value of manure, the value of nitrogen (N), phosphorus (P2O5) and potassium (K2O), the three major components can be calculated relatively easily. 

Rely on laboratory analysis when utilizing manure as a crop nutrient. This analysis provides us with the best means of determining the product’s true value. 

Local Ohio State University extension offices can help you find laboratories capable of providing manure analysis.

Lab analysis

grazing chart

Consider (see chart) the hypothetical broiler litter analysis (analysis is on an as-is basis).

The actual analysis of manure will vary based on storage, digestion method, bedding and animal diet, as well as other factors. 

Therefore, it is a good idea to have the analysis performed near the time of application. Laboratories providing this service also provide sampling instructions. 

Price of nutrients

The established price of nutrients in manure is based on commercial fertilizer values. Check with local fertilizer dealers for prices as these can vary based on region, fertilizer type, and quantity. 

For purposes of illustrating one method of value determination, (see chart) we will use the following prices for Nitrogen (N), Phosphate (P2O5) and Potash (K2O). 

Calculating the nitrogen value for manure first, we must determine plant-available nitrogen in this manure sample. The amount of plant-available nitrogen in a manure sample varies with temperature, method of application, soil pH, and method of treatment or processing. 

Manure may contain up to three types of nitrogen, each varying in their plant availability. Microorganisms (mineralization) break down the organic nitrogen found in manure over time. 

Mineralization rates are adjusted according to digestion process, time of application and days until incorporation. Typically, 30 to 33% of the organic nitrogen applied is available to the plant in the year of application. Ammonium nitrogen found in manure may vary in availability from 15 to 75%. 

The table utilized to estimate nitrogen availability is found in Ohio State University Extension Bulletin 604 and available for purchase from local OSU Extension offices. 

Utilizing the resources mentioned above, we calculated that in this hypothetical example there is 18.03 pounds/ton of Plant Available Nitrogen. 

We have 18.03 pounds/ton of nitrogen available for growing plants in the initial year of application. The remaining organic nitrogen will be broken down and some N will be available next year (see chart). 

Manure can be a substantial source of crop nutrients if utilized correctly. There is also a value in the organic matter and mineral content of manure if the soil test calls for these nutrients.

Consider manure nutrients when planning a fertility program. The calculations have shown that the use of manure can be profitable when a current soil test recommends these nutrients.

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