Book values of manure are just averages and don’t take into account factors such as varying storage and application losses, temperature, diet, etc. To first step to figuring the value of your manure effectively is to take a representative sample and send it in for analysis. Fertilizer is too valuable to not take every step in crediting manure nutrients.
In liquid and semi-solid systems, solids can settle out to the bottom of a storage structure. About 90% of the phosphorus will be in this settled solid portion, so complete agitation is a must. When agitating a storage pit below a building, be sure to provide adequate ventilation. Next, prepare your plastic sample bottle. Write the date, type of manure, and any other description. After adequate agitation, load your spreader or tanker. lmmediately use a clean plastic pail to collect manure from the loading or unloading port. Stir the sample in the bucket to further suspend solids. Fill the sample bottle only three-quarters full. You must leave space for freezing expansion prior to mailing.
To collect samples of irrigated waste, you can use inexpensive aluminum roaster pans scattered around the field and catch representative samples.
If complete agitation is a problem, especially if the storage area being emptied is large, you may want to take separate samples at the beginning, middle and end of the emptying process. Make note on the bottles and on which fields each type of manure was applied.
What should you test for on the analysis? A useful analysis includes: moisture, total N (Nitrogen), NH4 (Ammonium- N), P (Phosphorus) and K (Potassium). Both analyses of the nitrogen are vital, and will tell you not only the total N content, but also the portion that is in the ammonium form. Under some conditions, ammonium (NH4) is readily transformed to ammonia (NH3) and is lost as a gas. When this happens, the total N available for the plant is reduced. Knowing the amount in the ammonia form will also indicate how important it is for that manure source to be incorporated the same day as application to retain the greatest fertilizer value.
Take good samples, and keep the results year after year, to build confidence in the value of your manure. You can compare these tests with corresponding soil tests where manure was applied. You will see and learn the differences in season of application, incorporation methods, field-to-field variations, etc. This year-to-year tracking will lead to more confidence in crediting manure nutrients adequately, optimizing use and fine tuning purchased fertilizers. Remember, using manure effectively is an important economic management tool and promotes good environmental stewardship. The true value of manure is only realized when you credit it for the fertilizer value it contains, you retain of much of that as possible, and reduce purchased fertilizer accordingly.
Source: Roberta Osborne, Michigan State University Extension