importance for routine manure use. For example, while
forMs of nItroGen In MAnures
the average liquid dairy manure is around 25 pounds of
Nitrogen in manure occurs in three main forms: am-
N per 1,000 gallons, there are manures that might be
monium (NH +), urea (a soluble organic form, easily
10 pounds N or less OR 40 pounds N or more per 1,000
4
converted to ammonium), and solid, organic N. Ammo-
gallons. Recent research efforts have focused on more
nium is readily available to plants, and urea is quickly
efficient use of nutrients in dairy cows, and N and P
converted to ammonium in soils. However, while
intake can often be reduced by up to 25% without losses
readily available when incorporated in soil, both am-
in productivity. This helps reduce nutrient surpluses on
monium and urea are subject to loss as ammonia gas
farms using only needed P.
when left on the surface under drying conditions—
with significant losses occurring within hours of apply-
EFFEcTS OF MANURINg ON SOIlS
ing to the soil surface. Some manures may have half
Effects on Organic Matter
When considering the influence of any residue or organic
or three-quarters of their N in readily available forms,
material on soil organic matter, the key question is how
while others may have 20% or less in these forms. Ma-
much solids are returned to the soil. Equal amounts of
nure analysis reports usual y contain both ammonium
different types of manures will have different effects on
and total N (the difference is mainly organic N), thus
soil organic matter levels. Dairy and beef manures con-
indicating how much of the N is readily available—but
tain undigested parts of forages and may have significant
also subject to loss if not handled carefully.
quantities of bedding. They therefore have a high amount
of complex substances, such as lignin, that do not decom-
systems make manure analyses quite variable. Yet as
pose readily in soils. Using this type of manure results
long as feeding, bedding, and storage practices remain
in a much greater long-term influence on soil organic
relatively stable on a given farm, manure nutrient
matter than does a poultry or swine manure without
characteristics will tend to be similar from year to year.
bedding. More solids are commonly applied to soil with
However, year-to-year differences in rainfall can affect
solid-manure-handling systems than with liquid systems,
stored manure through more or less dilution.
because greater amounts of bedding are usually included.
The major difference among all the manures is that
A number of trends in dairy farming mean that manures
poultry manure is significantly higher in nitrogen and
may have less organic material than in the past. One is
phosphorus than the other manure types. This is partly
the use of sand as bedding material in free-stall barns,
due to the difference in feeds given poultry versus other
much of which is recovered and reused. The other is the
farm animals. The relatively high percentage of dry mat-
separation of solids and liquids with the sale of solids or
ter in poultry manure is also partly responsible for the
the use of digested solids as bedding. Under both situa-
higher analyses of certain nutrients when expressed on a
tions much less organic solids are returned to fields. On
wet ton basis.
the other hand, the bedded pack (or compost barn) does
It is possible to take the guesswork out of estimat-
produce a manure that is high in organic solid content.
ing manure characteristics; most soil-testing laborato-
When conventional tillage is used to grow a crop
ries will also analyze manure. Manure analysis should
such as corn silage, whose entire aboveground portion
become a routine part of the soil fertility management
is harvested, research indicates that an annual applica-
program on animal-based farms. This is of critical
tion of 20 to 30 tons of the solid type of dairy manure
132
Building SoilS for Better CropS: SuStainaBle Soil ManageMent