Sampling the gold

by Sally Colby

Farm animal manure is one of the most valuable and economical sources of nutrients for crops. Knowing the nutrient content of manure, along with soil tests and proper manure spreader calibration, helps make the best use of this resource.

Fertilizer products for field application have an exact analysis, but nutrient components of manure can vary greatly. Manure sampling helps determine accurate values for nutrients, which results economical and environmentally safe application.

Aaron Nygren, Extension educator in Nebraska, has some tips for sampling manure. “Nutrient concentrations will vary a lot,” he said. “Within an operation, [concentrations] won’t vary as much as between two operations.” Nygren added that variability is due to feeding programs and practices as well as variations from season to season and year to year.

There are two ways to plan manure use according to nutrient content of manure: book value and manure sampling. Book values are determined by averaging the results of many manure tests, while sampling involves testing the manure from a particular farm. Nygren reminded producers that published book values for manure are an average, and that individual producers’ numbers may be quite different from those figures. For those who choose to test manure, consistent sampling technique will offset variability. Some states have specific requirements for sampling, so it’s important to be aware of and follow all state regulations for manure testing.

Manure sampling can be done either before application or during application. “If we sample before application, one of the main advantages is that those results help determine application rates,” he said. “We can send samples to the lab, see what the test shows and base the application on that.”

There are several disadvantages of pre-application sampling, including the potential difficulty of obtaining a good, representative sample. It’s also possible that a sample may not represent the final nutrient content. Also, if samples are taken and submitted, then there’s a delay prior to manure application, will the manure content be the same as the sample results?

Sampling during loading and application is an option that overcomes issues of pre-application sampling. Nygren said the biggest advantage of sampling at this time is the ease of pulling several samples over time for a representative sample while manure is being agitated and easy to access. “We can get a very good nutrient content because we know exactly that is what it was when we applied it,” he said. “We don’t have to go out of our way to sample because we’re already in the process of working with the manure.”

Because results are not available immediately, the results of that sample probably can’t be used to adjust the rate of application. In this case, the farmer has to rely on a long-term average figure, and for many situations, that’s a reasonable approach. However, if a long-term average figure is used and the samples come back showing a considerably different actual nutrient content, the farmer may have either over-applied or should have added fertilizer to correct the deficiencies.

For producers who haven’t sampled manure, Nygren suggested sampling frequently at first to obtain baseline values and averages. Sampling should be repeated after any changes to the ration or feeding system. For example, if a ration change results in a higher rate of distiller’s grains, or if there are changes in how or where manure is scraped, or if there are changes in stockpiling, manure should be sampled again. Seasonal changes can also affect manure nutrients, so testing should be done during more than one season.

Nygren described the method for sampling solid manure. “Take a lot of sub-samples and mix them well,” he said. “The more samples, the better. The key to the number of sub-samples needed is to consider how variable is the manure. Another key is to write down the protocol so the method and sampling is consistent from year to year.”

The optimum number of sub-samples depends on how well the manure is mixed because mixing reduces variability. Nygren said 10 sub-samples is a usually a good number. “If it’s from one spreader load, get 10 samples from that spreader load,” he said. “If you’re sampling for a whole field, take a sample from each load that goes out. Again, check state requirements.”

When samples are taken from a spreader, manure should be collected on a tarp or in a pan rather than collected directly from the spreader to eliminate bias. If sampling from a pile or a windrow, there will be more variability, so at least 15 samples should be collected. “Always discard a ‘crust’ layer and sample from beneath it,” said Nygren, describing the collection method from a windrow or compost pile. “The recommendation is at least six inches beneath. You might need an auger or a soil probe to get to the sample.” Samples collected directly from an open lot will vary the most, so Nygren suggested collecting at least 20 samples. Be careful to pick up only manure and not soil from the lot.

Manure from confinement barns varies among operations, but Nygren said an Iowa State University study showed that location (pack, bedded pen, apron, stockpile) didn’t make a difference. “They did find that nutrient content varied a lot by operation,” he said. “Some of the reasons for that were type of housing, type of bedding, pen density, time of year, type of feed, housekeeping (how often the area is scraped) and manure storage.”

For operations with liquid manure in storage, Nygren said it’s easiest to obtain samples during pumping. After collection, liquid samples should be double bagged, then refrigerated or frozen and mailed to the lab early in the week.

What will lab results show? “Total nitrogen content and ammonium nitrogen content,” said Nygren. “We don’t typically test for organic nitrogen because we can subtract ammonium from total and we know what organic is. Nitrate-N is usually very low and we wouldn’t test for that. Phosphorus is going to be a big one because it’s a major component of manure.” Other results include moisture content, pH, potassium and micronutrients such zinc and sulfur. Nygren said it’s important to know the level of soluble salts if manure will be applied to a growing crop. For manure that will be pumped through a center pivot to a growing crop, make sure the EC (electrical conductivity) isn’t too high.

“The key to testing is having a good representative sample,” said Nygren. “It’s only as good as the time you put into getting those samples. Sample enough to have confidence in the results. If you take one [sample] and it’s vastly different from the next, take more. Lastly, write down your sampling protocol and use it consistently over time.”

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