Sampling and Testing Manure
for Nutrient Utilization
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.
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
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.