Sampling and Testing Manure
for Nutrient Utilization

 

Charles Linderman

NDSU Carrington Research Extension Center

 

 

Introduction

New regulations adopted by the U.S. Environmental Protection Agency classify certain animal feeding operations as Concentrated Animal Feeding Operations, or CAFOs.  Those operations will be required to adopt comprehensive nutrient management plans or CNMPs to assure that nutrients in the manure do not become pollutants of surface or ground water.  Most CNMPs will involve using the manure nutrients for crop production.

 

Complying with this part of environmental regulation can result in some economic advantage, particularly if a significant amount of purchased fertilizer can be replaced by manure.  For that reason, those animal feeding operations that are not CAFOs may also find that utilizing manure nutrients for crop production may be a practice that is both environmentally and economically sound.  Two-thirds of producers filling out surveys at recent livestock waste workshops indicated that they have used manure nutrients to reduce fertilizer purchases.

 

The manure nutrients of most concern in the environment are nitrogen and phosphorus.  As it happens, those are also the two nutrients most often added to the soil as fertilizer for crop production.  Excess amounts of nitrogen and phosphorus can impact surface and ground water quality.

 

Planning the utilization of manure nutrients for crop production obviously requires some knowledge about the crops to be grown and yield goals, the nutrient concentrations in the soil, the nutrient concentrations necessary to reach those yield goals, and the amount of fertilizer material which must be added to the soil to reach those goals.  Most crop producers in the state are familiar with NDSU Extension bulletins that cover choosing realistic yield goals, sampling and testing soil, and calculating appropriate fertilizer nutrient application rates.  Many are using those practices in their crop production programs.  However, when using manure to supply the fertilizer nutrients, it becomes much more complicated.

 

Why Test Manure?

Manure is a highly variable product.  Its composition depends on the type of animals, what the animals are fed, and the conditions under which they are fed.  In addition, after the manure is produced, it can change greatly due to the conditions under which it is treated and stored.  University of Minnesota bulletins list average nutrient contents for dairy, beef, and swine manure.  However, their research results found that analyses of manure samples ranged from 25 to 300 percent of those average values.  That emphasizes the value and importance of sampling and testing the actual manure being land applied for nutrient utilization.

 

Sampling

The most important and most difficult step in getting a meaningful manure nutrient analysis is always to get a representative sample.  This is accomplished by taking a number of sub samples and combining them to get a composite sample.  A clean plastic five-gallon bucket makes a good, cheap, and readily available container for collecting sub samples and mixing a composite sample.

The most variable manure is probably solid manure which has been piled or stacked from an open lot or barn.  Therefore, it requires the most sub samples.  Using a clean tiling spade or similar device, collect small amounts of solid manure from various locations and depths in the pile.  Avoid the dry outside crust and any obviously non representative spots.  Twenty or more sub samples should produce a composite sample within an acceptable range of accuracy.  Be sure to thoroughly mix the sub samples before taking a composite sample.

 

Liquid manure stored in pits, tanks, or holding ponds tends to stratify as the solids settle.  It needs to be mixed by agitating with a pump when the manure is to be removed from storage.  After the liquid is thoroughly agitated, a good composite sample can be obtained from about five sub samples collected in a plastic bucket from five points in the storage or from five separate tank loads while hauling.  Liquid in a runoff holding pond can likewise be sampled from five different spots in the pond or five sub samples can be taken at intervals during pump out.  Again, thoroughly mix the sub samples before taking a composite sample for analysis.

 

Manure samples can be analyzed by most soil testing labs.  NDSU Soil Testing Lab can handle solid manure samples of about two quart size in a sealable plastic bag.  Some private labs have plastic bottles and mailers they can furnish for submitting samples.  Air should be expelled from plastic bags before sealing.  Some space should be left at the top of plastic sample bottles when they are filled to accommodate any gases produced.  It is best to contact the lab before doing manure sampling to be sure the composite sample is sized and packaged as needed by that particular lab.  Sample containing bags or bottles should be put into a second sealable bag to help prevent accidental leakage in transit.

 

Because manure is an organic material it is constantly undergoing mineralization.  Samples should be refrigerated and transmitted to a lab as soon as possible.

 

Timing

Ideally, the time of sampling should be as close as possible to the time the manure is land applied.  Practically, it needs to be 10 days or so before hauling to allow time for transmitting and processing the sample.  In the case of liquid manure, an acceptable sample requires that the manure be mixed by agitation which is usually done just before hauling begins.  That means the manure analysis probably will not be available until the manure has been hauled and applied.  However, it is still valuable information.

 

Even though manure is a highly variable product, it tends toward a pattern on a particular operation if the feed and manure handling conditions remain constant.  Therefore, analyses of manure samples from the previous couple years will be a good indication of manure values if the feeding operation has not changed.

 

What information do we need from a manure sample analysis?

Obviously, we need to know the total nitrogen and phosphorus contents.  Also, we might want to know how much of the nitrogen is in the inorganic form versus the organic form.  That will help us know how quickly the nitrogen will be available to the crop.  Plants can take up only the inorganic form, which is essentially the ammonium-nitrogen and nitrate-nitrogen content of the manure.  That portion of the total nitrogen will be available immediately.  The organic nitrogen will not be available until mineralization takes place, so it will become available over about a three-year time.

 

About 80 percent of the phosphorus will be available to the first crop after manure application. 

It may be useful to know the moisture content.  Also, if we are concerned about total salts, an electrical conductivity value might be useful.

 

Why not test for potassium or K? 

Most labs will do that as part of their standard test and potassium percentage will usually be significant.  However, potassium is already high in most North Dakota soils and is not considered a water quality risk.  Potassium could be part of any problem of excess total salts.  You can calculate total manure potassium application if you wish.

 

How should information be reported?

The lab you choose should report the nutrient content of the manure on an as-is or as-received basis.  Then the nutrient content numbers will reflect the material just as it is when you are land applying it.  If it is reported on a dry matter basis, you will need to do an extra calculation using the moisture content to convert the analysis to an as-is basis before you can calculate application rates.  The percent of a nutrient on a dry matter basis multiplied by the percent dry matter and divided by 100 will equal the as-is nutrient percent.

 

Also, the lab will probably report the manure nutrients as a percentage.  To make manure application calculations easy, you will want to know the nitrogen and phosphorus contents in pounds per ton of solid manure or pounds per 1000 gallons of liquid manure.  Most labs will do this calculation for you in their report.  If they do not, simply multiply percent by 20 to get pounds per ton or by 83.5 to get pounds per 1000 gallons. 

 

For water contained in a feedlot runoff holding pond which is to be applied to the land by an irrigation system, the percent of nutrient content can be multiplied by 2266 to get the pounds per acre-inch of water.

 

Fertilizer applications of phosphorus are given as phosphate or P2O5.  Most manure testing labs will report the phosphorus content of the manure in the P2O5 form.  If they report elemental phosphorus or P, that number must be multiplied by 2.29 to obtain the corresponding P2O5 value.  Elemental potassium, K, can be multiplied by 1.20 to get potash or K2O.

 

 

 

Summary

With a crop rotation plan, yield goals, soil tests, and manure tests in hand, the livestock producer or crop producer planning to utilize available manure has the basic tools needed to develop a manure application plan. 

 

For further information or assistance, contact the NDSU Extension Livestock Waste Management Specialist or watch for future workshops.