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Animal Manures for Increasing Organic
Matter and Supplying Nutrients
The quickest way to rebuild a poor soil is to
practice dairy farming, growing forage crops,
buying . . . grain rich in protein, handling the manure properly,
and returning it to the soil promptly.
J. L. Hills, C. H. Jones, and C. Cutler, 1908
Once cheap fertilizers became widely available after
World War II, many farmers, extension agents, and scientists looked
down their noses at manure. People thought more about how to get
rid of manure than how to put it to good use. In fact, some scientists
tried to find out the absolute maximum amount of manure that could
be applied to an acre without reducing crop yields. Some farmers
who didn't want to spread manure actually piled it next to a stream
and hoped that next spring's flood waters would wash it away. We
now know that manure, like money, is better spread around than concentrated
in a few places. The economic contribution of farm manures can be
considerable. The value of the nutrients in manure from a 70-cow
dairy farm may exceed $7,000 per year; manure from a 50-sow farrow-to-finish
operation is worth about $4,000; and manure from a 20,000-bird broiler
operation is worth about $3,000. The other benefits to soil organic
matter build-up, such as enhanced soil structure and better diversity
and activity of soil organisms, may double the value of the manure.
If you're not getting the full fertility benefit from manures on
your farm, you may be wasting money.
Animal manures can have very different properties,
depending on the animal species, feed, bedding, and manure-storage
practices. The amounts of nutrients in the manure that become available
to crops also depend on what time of year the manure is applied
and how quickly it is worked into the soil. In addition, the influence
of manure on soil organic matter and plant growth is influenced
by soil type. In other words, it's impossible to give blanket manure
application recommendations. They need to be tailored for every
situation.
We'll deal mainly with dairy cow manure, because there's
more information about its use on cropland. We'll also offer general
information about the characteristics and uses of some other animal
manures.
Manure Handling Systems
Solid versus Liquid
The type of barn on the farmstead frequently determines how manure
is handled on a dairy farm. Dairy-cow manure containing a fair amount
of bedding, usually around 13 to 20 percent dry matter, is spread
as a solid. This is most common on farms where cows are kept in
individual stanchions. Liquid manure-handling systems are common
where animals are kept in a "free stall" barn with little
bedding. Liquid manure is usually in the range of from 2 to 10 percent
dry matter (90 percent or more water). Manures with characteristics
between solid and liquid are usually referred to as semi-solid or
slurry, depending on the method of handling.
Composting manures is becoming an increasingly popular
option for farmers. By composting manure you help stabilize nutrients,
have a smaller amount of material to spread, and have a more pleasant
material to spread (and if neighbors have complained about manure
odors, that might be a big plus). Although it's easier to compost
manure that has been handled as a solid, some farmers are separating
the solids from liquid manure and then irrigating with the liquid
and composting the solids. For a more detailed discussion of composting,
see chapter 12.
Storage of Manure
Researchers have been investigating how best to store manure to
reduce the problems that come with year-round manure spreading.
Storage allows the farmer to apply manure when it's best for the
crop and during appropriate weather conditions. This reduces nutrient
loss from the manure caused by water runoff from the field. However,
significant losses of nutrients from stored manure also may occur.
One study found that, during the year, dairy manure stored in uncovered
piles lost 3 percent of the solids, 10 percent of the nitrogen,
3 percent of the phosphorus, and 20 percent of the potassium. Covered
piles or well-contained liquid systems, which tend to form a crust
on the surface, do a better job of conserving the nutrients and
solids than unprotected piles. Poultry manure, with its high amount
of ammonium, may lose 50 percent of its nitrogen during storage
as ammonia gas volatilizes, unless precautions are taken to conserve
nitrogen.
Chemical Characteristics of Manures
A high percentage of the nutrients in feeds passes
right through animals and ends up in their manure. Over 70 percent
of the nitrogen, 60 percent of the phosphorus, and 80 percent of
the potassium in feeds may be available in manures for use on cropland.
In addition to the nitrogen, phosphorus, and potassium contributions
given in table 9.1, manures also contain significant amounts of
other nutrients, such as calcium, magnesium, and sulfur. In regions
where the micronutrient zinc tends to be deficient, there is rarely
any deficiency on soils receiving regular manure applications.
The values given in table 9.1 must be viewed with
some caution, because the characteristics of manures from even the
same type of animal may vary considerably from one farm to another.
Differences in feeds, mineral supplements, bedding materials, and
storage systems make manure analysis quite variable. Yet, as long
as feeding, bedding, and storage practices remain unchanged on a
given farm, manure characteristics will be similar from year to
year.
The major difference among all the manures is that
poultry manure is significantly higher in nitrogen and phosphorus
than the other manure types. This is partially due to the difference
in feeds given poultry versus other farm animals. The relatively
high percentage of dry matter in poultry manure is also partially
responsible for the higher analyses of certain nutrients, when expressed
on a wet ton basis.
It is possible to take the guesswork out of estimating
manure characteristics; most soil-testing laboratories will now
analyze manure. Manure analysis should become a routine part of
the soil fertility management program on animal-based farms.
Effects of Manuring on Soils
Effects on Organic Matter
When considering the influence of any residue or organic material
on soil organic matter, the key question is the amount of solids
returned to the soil. Equal amounts of different types of manures
will have different effects on soil organic matter levels. Dairy
and beef manure contain undigested parts of forages, as well as
bedding. They, therefore, have a high amount of complex substances,
such as lignin, that do not decompose readily in soils. Using this
type of manure results in a much greater long-term influence on
soil organic matter than does a poultry manure without bedding.
More solids are commonly applied to soil with solid manure-handling
systems than with liquid systems, because greater amounts of bedding
are usually included.
When conventional tillage is used to grow a crop (such
as corn silage), where the entire above-ground portion is harvested,
research indicates that an annual application of 20 to 30 tons of
the solid type of dairy manure per acre is needed to maintain soil
organic matter (table 9.2). As discussed
above, a nitrogen-demanding crop, such as corn, may be able to use
all of the nitrogen in 20 to 30 tons of manure. If more residues
are returned to the soil by just harvesting grain, lower rates of
manure application will be sufficient to maintain or build up soil
organic matter.
Manure Influences Many Soil Properties
Application of manures causes many soil changes biological,
chemical, and physical. A few of these types of changes are indicated
in table 9.2, which contains the results of a long-term experiment
in Vermont with continuous corn silage on a clay soil. Manure counteracted
many of the negative effects of a monoculture cropping system in
which few residues are returned to the soil. Soil receiving 20 tons
of dairy manure (wet weight, including bedding) maintained organic
matter and CEC levels and close to the original pH (although acid-forming
nitrogen fertilizers also were used). Manures, such as dairy and
poultry, have liming effects and actually counteract acidification.
High rates of manure addition caused a build-up of
both phosphorus and potassium to high levels. Soil in plots receiving
manures were better aggregated and less dense and, therefore, had
greater amounts of pore space than fields receiving no manure.
An example of how manure addition might balance annual
loss is given in figure 9.1. One large Holstein "cow year"
worth of manure is about 20 tons. Although 20 tons of anything is
a lot, when considering dairy manure, it translates into a much
smaller amount of solids. If the approximately 5,200 pounds of solid
material in the 20 tons is applied over the surface of one acre
and mixed with the 2 million pounds of soil present to a 6-inch
depth, it would raise the soil organic matter by about 0.3 percent.
However, much of the manure will decompose during the year, so the
net effect on soil organic matter will be even less. Let's assume
that 75 percent of the solid matter decomposes during the first
year and the carbon ends up as atmospheric CO2. At the
beginning of the following year, only 25 percent of the original
5,200 pounds, or 1,300 pounds of organic matter is added to the
soil. The net effect is an increase in soil organic matter of 0.065
percent (the calculation is [1,300/2,000,000] x 100). Although this
does not seem like much added organic matter, if a soil had 2.17
percent organic matter and 3 percent of this was decomposed annually
during cropping, then the loss would be 0.065 percent per year and
the manure addition would just balance this loss.
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Figure 9.1 Example of dairy manure addition
just balancing soil organic matter losses. |
Using Manures
Manures, like other organic residues that decompose
easily and rapidly release nutrients, are usually applied to soils
in quantities judged to supply sufficient nitrogen for the crop
being grown in the current year. It might be better for building
and maintaining soil organic matter to apply manure at higher rates,
but doing so may cause undesirable nitrate accumulation in leafy
crops and excess nitrate leaching to groundwater. High nitrate levels
in leafy-vegetable crops are undesirable in terms of human health,
and the leaves of many plants seem more attractive to insects. In
addition, salt damage to crop plants can occur from high manure
application rates, especially when there is insufficient leaching
by rainfall or irrigation. Very high amounts of added manures, over
a period of years, also lead to high soil phosphorus levels (table
9.2). It is a waste of money and resources to add unneeded nutrients
to the soil, nutrients which will only be lost by leaching or runoff,
instead of contributing to crop nutrition.
Application Rates A common per-acre rate of
dairy-manure application is 10 to 30 tons fresh weight of solid,
or 4,000 to 11,000 gallons of liquid manure. These rates will supply
approximately 50 to 150 pounds of available nitrogen (not total)
per acre. If you are growing crops that don't need that much nitrogen,
such as small grains, 10 to 15 tons of solid manure should supply
sufficient nitrogen per acre. For a crop that needs a lot of nitrogen,
such as corn, 20 to 30 tons per acre may be necessary to supply
its nitrogen needs. Low rates of about 10 tons per acre are also
suggested for each of the multiple applications used on a grass
hay crop. In total, grass hay crops need at least as much total
nitrogen applied as does a corn crop. There has been some discussion
about applying manures to legumes. This practice has been discouraged
because the legume uses the nitrogen from the manure, and much less
nitrogen is fixed from the atmosphere. However, the practice makes
sense on animal farms where there is excess nitrogen.
For the most nitrogen benefit to crops, manures should
be incorporated into the soil immediately after spreading on the
surface. About half of the total nitrogen in dairy manure comes
from the ammonium (NH4+) in urine. This ammonium
represents almost all of the readily available nitrogen present
in dairy manure. As materials containing urea or ammonium dry on
the soil surface, the ammonium is converted to ammonia gas (NH3)
and lost to the atmosphere. If dairy manure stays on the soil surface,
about 25 percent of the nitrogen is lost after one day and 45 percent
is lost after four days but that 45 percent of the total represents
around 70 percent of the readily available nitrogen! This problem
is significantly lessened if about ½ inch of rainfall occurs
shortly after manure application, leaching ammonium from manure
into the soil. Leaving manure on the soil surface is also a problem
because runoff waters may carry significant amounts of nutrients
from the field. When this happens, crops don't benefit as much from
the manure application and surface waters become polluted. Some
liquid manures those with low solids contents penetrate the soil
more deeply. When applied at normal rates, these manures will not
be as prone to lose ammonia by surface drying.
Other nutrients contained in manures, in addition
to nitrogen, make important contributions to soil fertility. The
availability of phosphorus and potassium in manures should be similar
to that in commercial fertilizers. (However, some recommendation
systems assume that only around 50 percent of the phosphorus and
90 percent of the potassium is available.) The phosphorus and potassium
contributions of 20 tons of dairy manure is approximately equivalent
to about 30 to 50 lbs. of phosphate and 180 to 200 lbs. of potash
from fertilizers. The sulfur content as well as trace elements in
manure, such as the zinc previously mentioned, also add to the fertility
value of this resource.
N, P, and K in Hog, Dairy,
and Beef Cattle Manures |
|
Feces |
Urine |
N |
½ |
½ |
P |
most |
- |
K |
- |
most |
Because one-half of the nitrogen and almost all of
the phosphorus is in the solids, much of these nutrients remain
in sediments at the bottom when a liquid system is emptied without
properly agitating the manure. On the other hand, almost all of
the potassium will be applied with the liquid portion, even if it's
applied without the solids. A manure system that allows significant
amounts of surface water penetration and then drainage, such as
a manure stack of well-bedded dairy or beef cow manure, may lose
a lot of potassium. The 20 percent leaching loss of potassium from
stacked dairy manure mentioned above occurred because potassium
was mostly found in the liquid portion of the manure.
Timing of Applications
Manures are best applied to annual crops, such as corn, small grains,
and vegetables, in one dose just before soil tillage (unless a high
amount of bedding is used, which might tie-up nitrogen for a while
see discussion of C:N in chapter 8). This
allows for rapid incorporation by plow, chisel, harrow, or disk.
Even with reduced tillage systems, application close to planting
time is best, because the possibility of loss by runoff and erosion
is reduced. It also is possible to inject liquid manures either
just before the growing season starts or as a sidedress to row crops.
Fall manure applications on annual row crops, such as corn, may
result in considerable nitrogen loss, even if manure is incorporated.
Losses of nitrogen from fall-applied manure in humid climates may
be around 50 percent resulting from leaching and denitrification
before nitrogen is available to next year's crop.
Without any added nitrogen, perennial grass hay crops
are constantly nitrogen deficient. Application of a moderate rate
of manure about 50 lbs. worth of available nitrogen in early spring
and following each harvest is the best way to apply manure. However,
wet soils in early spring may not allow manure application without
causing significant compaction.
Although the best use of manure is to apply it near
the time when the crop needs the nutrients, sometimes insufficient
storage capacity causes farmers to apply it at other times. In the
fall, manure can be applied to grasslands that don't flood or to
tilled fields that will either be fall plowed or planted to a winter
cover crop. Although legal in most states, it is not a good practice
to apply manures when the ground is frozen or covered with snow.
The nutrient losses that can occur with runoff from winter-applied
manure are both an economic loss to the farm as well as an environmental
hazard. Winter spreading should be done only on an emergency basis.
However, new research on frost tillage has shown that there are
windows of opportunity for incorporating winter-applied manure during
periods when the soil has a shallow frozen layer, 2 to 4 inches
thick (see chapter 15). Farmers may use
this time window to inject manure during the winter.
Potential Problems
As we all know, too much of a good thing is not necessarily
good.
Excessive manure applications may cause plant-growth
problems. It is especially important not to apply excess poultry
manure, because the high soluble-salt content can harm plants.
Plant growth is sometimes retarded when high rates
of fresh manure are applied to soil immediately before planting.
This problem usually doesn't occur if the fresh manure decomposes
for a few weeks in the soil and can be avoided by using a solid
manure that has been stored for a year or more. Injection of liquid
manure sometimes causes problems when used on poorly drained soils
in wet years. The extra water applied and the extra use of oxygen
by microorganisms may mean less aeration for plant roots.
Ever hear of E. Coli 0157:H7?
A bacteria strain known as E. Coli (an abbreviation
that is pronounced e-COLE-eye) 0157: H7 has caused numerous
outbreaks of severe illness in people who ate contaminated
meat. It also has caused one known outbreak when water used
to wash lettuce was contaminated with animal manure.
This particular bacteria is a resident of cows'
digestive systems. It does no harm to the cow, but probably
because of the customary practice of feeding low levels of
antibiotics when raising cattle it is resistant to a number
of commonly used antibiotics. This problem only reinforces
the common sense approach to manure use. When using manure
that has not been thoroughly composted to grow crops for direct
human consumption especially leafy crops like lettuce that
grow low to the ground and root crops such as carrots and
potatoes special care should be taken. Before planting your
crop, avoid problems by planning a three-month period between
incorporation and harvest. For short season crops, this means
that the manure should be incorporated long before planting.
Although there has never been a confirmed instance of contamination
of vegetables by E. Coli 0157: H7 or other disease organisms
from manure incorporated into the soil as a fertility amendment,
being cautious and erring on the side of safety is well justified.
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When manures are applied regularly to a field to provide enough
nitrogen for a crop like corn, phosphorus and potassium may build
up to levels way in excess of crop needs (see table
9.2). Erosion of phosphorus-rich topsoils contributes sediments
and phosphorus to streams and lakes, polluting surface waters. When
very high phosphorus build-up occurs from the continual application
of manure applied at rates to satisfy crop nitrogen needs, it may
be wise to switch the application to other fields or to use strict
soil-conservation practices to trap sediments before they enter
a stream. Including rotation crops, such as alfalfa that do not
need manure allows a "draw-down" of phosphorus that accumulates
from manure application to grains. (However, this may mean finding
another location to apply manure. For a more detailed discussion
of nitrogen and phosphorus management, see chapter
17.)
Farms that purchase much of their animal feed may
have too much manure to safely use on their own land. Although they
don't usually realize it, they are importing large quantities of
nutrients in the feed that remain on the farm as manures. If they
apply all these nutrients on a small area of soil, nitrogen and
phosphorus pollution of groundwater and surface water will occur.
It is a good idea to make arrangements with neighbors for use of
the excess manure. Another option, if local outlets are available,
is to compost the manure (see chapter 14)
and sell the product to vegetable farmers, garden centers, landscapers,
and directly to home gardeners.
Sources
Elliott, L. F., and F. J. Stevenson, eds. 1977. Soils for Management
of Organic Wastes and Wastewaters. Soil Science Society of
America.
Madison, WI. Madison, F., K. Kelling, J. Peterson, T. Daniel,
G. Jackson, and L. Massie. 1986. Guidelines for Applying Manure
to Pasture and Cropland in Wisconsin. Agricultural Bulletin
A3392. Madison, WI.
Magdoff, F. R., and J. F. Amadon. 1980. Yield trends and soil
chemical changes resulting from N and manure application to continuous
corn. Agronomy Journal 72:161164. See this reference
for dairy manure needed to maintain or increase organic matter
and soil chemical changes under continuous cropping for silage
corn.
Magdoff, F. R., J. F. Amadon, S. P. Goldberg, and G. D. Wells.
1977. Runoff from a Low-cost Manure Storage Facility. Transactions
of the American Society of Agricultural Engineers 20:658660,
665. This is the reference for the nutrient loss that can occur
from uncovered manure stacks.
Magdoff, F. R. and R. J. Villamil, Jr. 1977. The Potential
of Champlain Valley Clay Soils for Waste Disposal. Proceedings
of the Lake Champlain Environmental Conference, Chazy, NY. July
15, 1976.
Maryland State Soil Conservation Committee. Undated. Manure
Management HandbookA Producer's Guide. College Park, MD.
Ontario Ministry of Agriculture and Food. 1994. Livestock
and Poultry Waste Management. Best Management Practices
Series. Available from the Ontario Federation of Agriculture,
Toronto, Ontario (Canada).
Ontario Ministry of Agriculture and Food. 1997. Nutrient Management.
Best Management Practices Series. Available from the Ontario Federation
of Agriculture, Toronto, Ontario (Canada).
Soil Conservation Society of America. 1976. Land Application
of Waste Materials. Soil Conservation Society of America.
Ankeny, IA.
van Es, H.M., A.T. DeGaetano, and D.S. Wilks. 1998. Space-time
upscaling of plot-based research information: frost tillage. Nutrient
Cycling in Agroecosystems 50:8590.
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