Manure Analysis Provides Accurate Account Of Plant Available Nutrients

By Sarah Fronczak

With the current economic challenges faced by Michigan farmers, taking full advantage of the plant nutrients found in livestock manure is an easy choice. But if you aren’t testing manure for its specific nutrient content, you need to depend on published “book values” to estimate nutrients applied from manure application. This approach can result in significant miscalculation of nutrients applied to fields.

Fertilizer is too valuable to not take credit for manure nutrients in your nutrient management plan. Book values of manure are just averages and don’t consider factors such as varying storage and application losses, temperature, diet or other factors. The first step to figuring the value of your manure is to take a representative sample and send it in for analysis. Several certified laboratories are available, with a typical cost of around $32 per sample for a basic analysis. It is important to use proper collection technique when the sample is collected. The lab you select will have sampling instructions on their website.

Collecting a liquid manure sample

Liquid storage structure stratifies over time. If you have a large liquid storage, it is recommended that you take at least three representative samples as you empty it. Each layer will have a differing manure analysis that will affect the application rate and dollar savings on fertilizer. Since these layers vary so widely, and the manure has such a high value, it just makes good economic sense to take multiple samples.

Over time, solids tend to settle to the bottom of the manure storage. According to Fertilizer Nutrients in Livestock and Poultry Manure by Fulhage et al., up to 80% of the phosphorus can remain in the sludge layer of a liquid storage structure if not completely agitated.

A good guideline when hauling solids from the bottom of a storage structure is to utilize it on your lowest phosphorus testing fields, even if that means driving some distance. Depending on the analysis, you may also have to apply it much thinner compared to the manure in the upper portion of the storage. 

If manure is a significant source of nutrients for your crops, Michigan State University Extension recommends you have it tested annually following recommended sampling procedures. With the current high nutrient prices, the cost of manure sampling in time and cash, along with good manure storage, handling, application and incorporation practices, can be a very good investment.

What should you test for on the analysis?

A basic 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.

Taking good samples and keeping the results year after year builds 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 between the season of application, incorporation methods, field-to-field variations, and other variables. 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 take credit for it in your nutrient management plan for the fertilizer value it contains and reduce purchased fertilizer accordingly.