fertility. The availability of phosphorus and potassium in
considerable nitrogen loss, even if manure is incorpo-
manures should be similar to that in commercial fertil-
rated. Losses of nitrogen from fall-applied manure in
izers. (However, some recommendation systems assume
humid climates may be as much as 25% to 50%—result-
that only around 50% of the phosphorus and 90% of the
ing from conversion of ammonium to nitrate and then
potassium is available.) The phosphorus and potassium
leaching and denitrification before nitrogen is available
contributions contained in 20 tons of dairy manure are
to next year’s crop. It was determined in modeling
approximately equivalent to about 30 to 50 pounds of
studies that fall applications of liquid manure posed the
phosphate and 180 to 200 pounds of potash from fertil-
greatest risk for nitrate leaching in a dairy system in
izers. The sulfur content as well as trace elements in
New York.
manure, such as the zinc previously mentioned, also add
Without any added nitrogen, perennial grass hay
to the fertility value of this resource.
crops are constantly nitrogen deficient. Application of a
Because one-half of the nitrogen and almost all of
moderate rate of manure—about 50–75 pounds worth
the phosphorus is in the solids, a higher proportion of
of available nitrogen—in early spring and following each
these nutrients remain in sediments at the bottom when
harvest is the best way to apply manure. Spring applica-
a liquid system is emptied without properly agitating the
tions may be at higher rates, but wet soils in early spring
manure. Uniform agitation is recommended if the goal
may not allow manure application without causing
is to apply similar levels of solids and nutrients across
significant compaction.
target fields. A manure system that allows significant
Although the best use of manure is to apply it near
amounts of surface water penetration and then drain-
the time when the crop needs the nutrients, sometimes
age, such as a manure stack of well-bedded dairy or
time and labor management or insufficient storage
beef cow manure, may lose a lot of potassium because
it is so soluble. The 20% leaching loss of potassium
from stacked dairy manure mentioned above occurred
because potassium was mostly found in the liquid por-
tion 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 the discussion of
C:N ratios in chapter 9). This allows for rapid incorpora-
tion by plow, chisel, harrow, disk, or aerator. 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
Figure 12.2. Injection of liquid manure into shal ow frozen soils, which as a side-dressing to row crops. Fall manure applica-eliminates compaction concerns and reduces spring application volumes.
tions on annual row crops, such as corn, may result in
Photo by Eleanor Jacobs.
135
Building SoilS for Better CropS: SuStainaBle Soil ManageMent