frequently contain contaminants from industry and
Table 9.3
C:N Ratios of Selected Organic Materials
from various products used around the home. Although
Material
c:N
many of these metal contaminants naturally occur at
Soil
10–12
low levels in soils and plants, their high concentrations
Poultry manure
10
in some sludges create a potential hazard. The U.S.
Clover and alfalfa (early)
13
standards for toxic materials in sludges are much more
Compost
15
lenient than those in some other industrialized countries
Dairy manure (low bedding)
17
Alfalfa hay
20
and permit higher loading of potentially toxic metals.
Green rye
36
So, although you are allowed to use many sludges, you
Corn stover
60
should carefully examine a sludge’s contents before
Wheat, oat, or rye straw
80
applying it to your land.
Oak leaves
90
Another issue is that sludges are produced by varied
Fresh sawdust
400
processes and, therefore, have different properties.
Newspaper
600
Most sludges are around neutral pH, but, when added
Note: Nitrogen is always 1 in the ratios.
to soils, cause some degree of acidification, as do most
they are so well distributed in the soil. Plant roots are
nitrogen fertilizers. Because many of the problem met-
in contact with only 1–2% of the soil volume, whereas
als are more soluble under acidic conditions, the pH
microorganisms populate almost the entire soil. The
of soils receiving these materials should be monitored
length of time during which the nitrogen nutrition of
and maintained at around 6.8 or above. On the other
plants is adversely affected by immobilization depends
hand, lime (calcium hydroxide and ground limestone
on the quantity of residues applied, their C:N ratio,
used together) is added to some sludges to raise the pH
and other factors influencing microorganisms, such as
and kill disease bacteria. The resulting “lime-stabilized”
fertilization practices, soil temperature, and moisture
sludge has extremely high levels of calcium, relative to
conditions. If the C:N ratio of residues is in the teens or
potassium and magnesium. This type of sludge should
low 20s, corresponding to greater than 2% nitrogen, there
be used primarily as a liming source, and levels of
is more nitrogen present than the microorganisms need
magnesium and potassium in the soil carefully moni-
for residue decomposition. When this happens, extra
tored to be sure they are present in reasonable amounts,
nitrogen becomes available to plants fairly quickly. Green
compared with the high levels of added calcium.
manure crops and animal manures are in this group.
The use of “clean” sludges—those containing low lev-
Residues with C:N in the mid 20s to low 30s, correspond-
els of metal and organic contaminants—for agronomic
ing to about 1–2% nitrogen, will not have much effect on
crops is certainly an acceptable practice. Sludges should
short-term nitrogen immobilization or release.
not be applied to soils when growing crops for direct
Sewage sludge on your fields? In theory, using
human consumption unless it can be demonstrated that,
sewage sludge—commonly called biosolids—on agri-
in addition to low levels of potentially toxic materials,
cultural land makes sense as a way to resolve problems
organisms dangerous to humans are absent.
related to people living in cities, far removed from the
Application rates for organic materials. The
land that grows their food. However, there are some
amount of residue added to a soil is often determined by
troublesome issues associated with agricultural use of
the cropping system. The crop residues can be left on the
sludges. By far, the most important problem is that they
surface or incorporated by tillage. Different amounts of
94
Building SoilS for Better CropS: SuStainaBle Soil ManageMent