legumes. This practice has been dis-
couraged because the legume uses the
20 tons fresh-weight dairy manure
lost by decomposition =
nitrogen from the manure, and much
at 13% dry matter =
5,200 lbs of solids
1,300 lbs
less nitrogen is fixed from the atmo-
sphere. However, the practice makes
x 0.25
x 0.03
sense on intensive animal farms where
(75% decomposes
(3% decomposes
in first year)
per year)
there can be excess nitrogen—although
grasses may then be a better choice for
soil organic matter
gain from manure = 1,300 lbs
2,000,000 lbs in surface 6 inches x 0.0217 =
manure application.
43,400 lbs of organic matter
For the most nitrogen benefit to
crops, manures should be incorporated
Figure 12.1. Example of dairy manure addition just balancing soil organic matter losses.
into the soil in the spring immedi-
application rates, especially when there is insufficient
ately after spreading on the surface. About half of the
leaching by rainfall or irrigation. Very high amounts
total nitrogen in dairy manure comes from the urea in
of added manures, over a period of years, also lead to
urine that quickly converts to ammonium (NH +
4 ). This
high soil phosphorus levels (table 12.2). It is a waste of
ammonium represents almost all of the readily avail-
money and resources to add unneeded nutrients to the
able nitrogen present in dairy manure. As materials
soil, nutrients that will only be lost by leaching or runoff
containing urea or ammonium dry on the soil surface,
instead of contributing to crop nutrition.
the ammonium is converted to ammonia gas (NH3) and
lost to the atmosphere. If dairy manure stays on the soil
Application Rates
surface, about 25% of the nitrogen is lost after one day,
A common per-acre rate of dairy-manure applica-
and 45% is lost after four days—but that 45% of the total
tion is 10 to 30 tons fresh weight of solid, or 4,000 to
represents around 70% of the readily available nitrogen.
11,000 gallons of liquid, manure. These rates will supply
This problem is significantly lessened if about half an
approximately 50 to 150 pounds of available nitrogen
inch of rainfall occurs shortly after manure application,
(not total) per acre, assuming that the solid manure is
leaching ammonium from the manure into the soil.
not too high in straw or sawdust and actually ties up
Leaving manure on the soil surface is also a problem,
soil nitrogen for a while. If you are growing crops that
because runoff waters may carry significant amounts
don’t need that much nitrogen, such as small grains,
of nutrients from the field. When this happens, crops
10 to 15 tons (around 4,000 to 6,000 gallons) of solid
don’t benefit as much from the manure application, and
manure should supply sufficient nitrogen per acre. For
surface waters become polluted. Some liquid manures—
a crop that needs a lot of nitrogen, such as corn, 20 to
those with low solids content—penetrate the soil more
30 tons (around 8,000 to 12,000 gallons) per acre may
deeply. When applied at normal rates, these manures
be necessary to supply its nitrogen needs. Low rates of
will not be as prone to lose ammonia by surface drying.
about 10 tons (around 4,000 gallons) per acre are also
However, in humid regions, much of the ammonia-N
suggested for each of the multiple applications used on a
from manure may be lost if it is incorporated in the fall
grass hay crop. In total, grass hay crops need at least as
when no crops are growing.
much total nitrogen applied as does a corn crop. There
Other nutrients contained in manures, in addi-
has been some discussion about applying manures to
tion to nitrogen, make important contributions to soil
134
Building SoilS for Better CropS: SuStainaBle Soil ManageMent
