Step 2: Understanding a Manure Sample
Bulletin #2429
Timothy S. Griffin, Extension sustainable
agriculture specialist
Introduction
Taking both good soil and manure samples are
important early steps in using nutrients wisely. (See Cooperative Extension
bulletin #2428,
"Step One: Getting a Manure Sample" and #2286, "Testing Your Soil.") After
you take a manure sample and have it analyzed, you may be faced with simple
questions:
What does the analysis mean, and how do I use
this information?
The Importance of Manure Analysis
There are two very good reasons to get a manure
analysis. The first reason is obvious: you need to know what is in the
manure if your goal is to maximize the dollar value of this resource
(especially since you already 'bought" the nutrients in the manure in the
form of feed). The second reason is not quite so obvious: the nutrient
content of manure (N, P and K) changes drastically from one farm to the
next, depending on:
Table 1 shows this variation quite clearly.
Nearly 400 liquid samples and 400 solid samples are summarized. Note the
range in N, P and K content!
|
Table 1:
Variation in Manure Nutrient Content
|
|
|
Lowest Value |
Highest Value |
Average |
|
lb/1,000g (liquid) |
|
Nitrogen |
1 |
71 |
28 |
|
Phosphorus (P2O5) |
1 |
118 |
13 |
|
Potassium (K2O) |
1 |
171 |
29 |
|
lb/ton (solid) |
|
Nitrogen |
3 |
33 |
10 |
|
Phosphorus (P2O5) |
0.2 |
35 |
6 |
|
Potassium (K2O) |
0.2 |
24 |
11 |
|
From Combs and
Peters. Manure variability., a sampling and
laboratory analysis survey. Presented Sept.
23-24,1996, Raleigh NC. |
Many people ask if there is a good
"average" nutrient content that they can use for liquid or solid manures.
The information in Table 1 argues against taking this approach. Let's
quickly
compare the average values from Table 1 to a
couple of real values (one liquid and one solid) from Maine dairy farms.
d
What do these graphs mean? For the liquid
manure (Figure 1), which has a fairly high nutrient content, it means that
if you use an "average" value, you are applying more nutrients than you
thought. In fact, you have over-applied N by 50 percent, P2O5
by 38 percent, and K2O by 21 percent. Not only are the extra N
and P an environmental threat, they are also nutrients that won't be used by
the crop or on another field.
The picture is not very good for the solid
manure, either. Using an "average" instead of the actual analysis, you
applied only 70 percent of the N, 50 percent of the P2O5,
and 45 percent of the K2O that the crop needs. The point is,
there is too much variation between farms to use an average. If you
underestimate nutrient content, you lose valuable nutrients. If you
overestimate nutrient content, you short-change the crop. Take a sample and
get an analysis!
Reading a Manure Analysis
Before you use the information from an
analysis, you need to be able to read and understand the analysis sheet. An
example analysis (for a liquid manure) is included as Figure 3. Let's look
at the major components of the analysis.
Sample ID
The information at the top of the analysis
sheet simply tells 1) where the analysis was done (University of Maine, in
this case), and 2) who sent the sample to the lab. One suggestion: give the
sample a specific name and year, so the results won't be confused with
earlier or later samples.
Nutrient content on wet basis (as
received)
This section provides information on specific
nutrients in the manure, as a percentage of total manure weight. For
example, the analysis reads "% Nitrogen = 0.35%." This means that each 100
pounds of raw manure contains 0.35 pounds of total nitrogen. The analysis
also provides information on NH4-N (ammonium nitrogen), P and K.
The P and K are then converted to P2O5
and K2O, respectively, since these are the forms used on soil test
recommendations.
1,000 gallons of this manure (handled
as a liquid) contains
This section takes the information above and
converts it to units you can use on the farm. For example, there are 13
pounds P2O5 / 1,000 gallons. If you apply 5,000
gallons per acre, you have applied 65 pounds of P2O5
(5 X 13 lb./ 1,000 gallons).
Figure 3: Example analysis for a liquid manure
Dept. of Appl. Ecol. & Envir. Sci. Analytical Lab
University of Maine
5722 Deering Hall, Rm. 407
Orono, ME 04469-5722
(207) 581-2917
Eric Giberson
5722 Deering Hall
Campus |
05-17-1995 |
|
Sample name: Manure
#2 |
JOB #: 903 |
NUTRIENT CONTENT ON WET WEIGHT BASIS (AS RECEIVED)
| % Nitrogen = |
0.35 |
| % NH4-N = |
0.17 |
| % Phosphorus = |
0.07 x 2.29 = 0.17 %
P205 |
| % Potassium = |
0.36 x 1.21 = 0.43 % K20 |
| % H2O = |
91.1 |
1,000 gallons of this manure (handled as liquid material) contains:
28 pounds of total nitrogen (TKN)
14 pounds of ammonia-nitrogen (NH4-N)
13 pounds of phosphate (P2O5)
34 pounds of potash (K2O)
Sincerely,
William P. Cook
Assistant Chemist
|
Using the Information from a
Manure Analysis
The information on the manure analysis is only
one piece of information needed to balance nutrient application and removal.
The other important pieces of information are:
-
Crop nutrient needs (from a soil test)
-
Nutrient availability from the manure
-
Manure management, especially incorporation
of manure
The best way to learn these concepts is through
an example.
Crop Nutrient Needs
In this example, you are growing silage corn.
The soil test recommendations are shown in Figure 4.
Figure 4:
Silage corn soil test recommendations
|
Nutrient |
Pounds Needed/Acre |
|
N |
150 |
|
P2O5 |
60 |
|
K2O |
140 |
The soil test actually estimates
plant-available P and K. The estimate for N is based on a standard N
response curve. There is no pre-season soil test for N requirement. If
recommendations for P2O5 and K2O are zero
(or near zero), it is probably the result of long-term applications of
manure at rates higher than crop requirements.
Nutrient Availability from
Manure
Soil factors like soil pH will affect the
availability of nutrients (especially P) from any nutrient source. The P2O5
and K2O in manure are essentially as available as the same
nutrients supplied by fertilizer. Applying 100 pounds K2O/acre
from manure is the same as applying it as muriate of potash (0-0-60)
fertilizer.
The big differences in nutrient availability is
for N. A manure analysis (for a solid manure) is shown in Figure 5, with
nutrient content in "pounds per ton."
Figure 5:
Example manure analysis
|
Nutrient |
Lbs/Ton of Manure |
|
Total N (TKN) |
13 |
|
Ammonium nitrogen (NH4-N) |
5 |
|
Phosphate (P2O5) |
4 |
|
Postash (K2O) |
10 |
The ammonium nitrogen (NH4-N) is the
same as the N in ammonium-based fertilizers. It is quickly converted to
nitrate (NO3-N) available to the plant, although it may be lost
in other ways (as we will see later). As a starting point, we can say that
it is "100 percent available.”
The difference between ammonium nitrogen (NH4-N)
and total N (TKN) is organic N. For the plants to use this N, it must first
be broken down or mineralized by soil microbes. Over the course of a growing
season, 30 to 40 percent of this organic N will be converted to
plant-available forms (first NH4-N, then NO3-N). An
additional 10 to l5 percent will become available the year following
application, and three to five percent the following year. Using this
information, we can move from total N content to available N content (Figure
6).
Figure 6: Total and available nitrogen
|
N Form |
Pound per Ton |
Percent Available |
Pounds Available |
|
Ammonium nitrogen (NH4-N) |
5 |
100 |
5 lb./ton |
|
Organic N (total N minus NH4-N) |
8 |
35 |
3 lb./ton |
|
Total N available |
|
|
8 lb./ton |
Calculating Application Rates
We
have estimated the amount of available nutrients per ton of manure. Now we
can calculate the amount of manure needed to meet crop nutrient needs. To
find the tons of manure needed per acre, we simply divide the nutrient
requirement by the nutrient content of the manure. As a starting point, we
can do this for all three major nutrients
(Figure 7).
Figure 7: Rate
|
Nutrient Need |
Divided
by |
Nutrient Content |
Rate to Apply |
|
150 lb. N/acre |
/ |
8 lb. N/ton |
18.8 ton/acre |
|
60 lb. P2O5/acre |
/ |
4 lb. P2O5/acre |
15.0 ton/acre |
|
140 lb. K2O/acre |
/ |
10 lb. K2O/acre |
14.0 ton/acre |
In many
cases, the rate that is actually applied is determined by the N requirement,
because N is easily lost and is commonly the most limiting nutri10ent for
crops like corn. In the situation described above, this means that we will
over-apply P and K (by 15 lb. P2O5/a and 48 lb. K2O/a).
In the short term, this probably would not concern us, since these nutrients
can be held in the soil and used by following crops. In the long term,
however, this nutrient loading can lead to situations where the manure
supplies only N for the growing crop. This reduces its real value as a
nutrient source because you waste the P and K that could be used on other
fields. Such fields are easy to spot using soils tests; the recommendations
for P and K are zero.
Manure Management
In the example above, it looks like manure
nutrients can be closely matched to crop needs. If this is true, why are
excessive P and K levels commonly found on corn fields with a history of
manure application? There are two possibilities:
- Over-Application of Manure
Rather than applying the 19
ton/acre in the example above, say that applications are 30 tons/acre. If
this is done, N, P2O5 and K2O are
over-applied by 88, 60 and 160 lb./ acre every year. This scenario is
actually fairly common as dairy herds expand faster than the land base
available for manure application.
-
Application Management
The available N in the example includes a very
important assumption: that the manure is incorporated immediately after it
is applied. This prevents ammonia volatilization, where NH4-N is
converted to NH3 (ammonia gas) and is quickly lost to the
atmosphere. This volatile loss of surface-applied N occurs quickly, and is
accelerated by high temperatures and wind. An example of just how quickly it
occurs is shown in Figure 8.
Figure 8: Incorporation of nitrogen
d
If the same manure was applied but not
incorporated for two days, we could expect approximately 50 percent of the
NH4-N to be lost. The impacts of this?
-
Instead of 8 lb. N/ton of manure, we get 5.5
lb. N/ton.
-
To supply the same amount of available N, we
must apply almost 50 percent more manure.
It is easy to see why rapid incorporation
is a Best Management Practice: it saves N from being lost and reduces P and
K build-up in the soil.
Conclusion: Three Easy Steps
A manure analysis provides
farm-specific information that helps match manure nutrient applications with
crop demands. There are three things to remember:
- Have manure analyzed.
If nutrition, bedding or manure storage change, take a new sample. If
you have different systems for different groups of animals, sample them
separately.
- Calculate application rates, based on
crop needs.
If calculated rates are similar to meet N, P and K needs, then use that
rate. If they are not similar, past management has wasted nutrients and
additional applications will do likewise. Look for other ways to use manure
to increase its value.
- Pay attention to application management.
Nutrient loss in the first several days after application can be high,
especially if manure is not tilled in. Make this incorporation part of your
management plan.
©
2002
Last
Modified:
08/19/09
|