than compost. (However, adding compost does improve
young, and very green plants, decompose rapidly in the
soils in many ways, including increasing the water-
soil and, in the process, may readily release plant nutri-
holding capacity.)
ents. This could be compared to the effect of sugar eaten
Although it’s important to have adequate amounts
by humans, which results in a quick burst of energy.
of organic matter in soil, that isn’t enough. A variety of
Some of the substances in older plants and in the woody
residues are needed to provide food to a diverse popula-
portion of trees, such as lignin, decompose very slowly
tion of organisms, provide nutrients to plants, and fur-
in soils. Materials such as sawdust and straw, mentioned
nish materials that promote aggregation. Residues low
above, contain little nitrogen. Well-composted organic
in hemicellulose and lignin usually have very high levels
residues also decompose slowly in the soil because they
of plant nutrients. On the other hand, straw or saw-
are fairly stable, having already undergone a significant
dust (containing a lot of lignin) can be used to build up
amount of decomposition.
organic matter, but a severe nitrogen deficiency and an
Mature plant stalks and sawdust that have C:N over
imbalance in soil microbial populations will occur unless
40:1 (table 9.3) may cause temporary problems for
a readily available source of nitrogen is added at the
plants. Microorganisms using materials that contain 1%
same time (see discussion of C:N ratios below). In addi-
nitrogen (or less) need extra nitrogen for their growth
tion, when insufficient N is present, less of the organic
and reproduction. They will take the needed nitrogen
material added to soils actually ends up as humus.
from the surrounding soil, diminishing the amount
C:N ratio of organic materials and nitrogen
of nitrate and ammonium available for crop use. This
availability. The ratio of the amount of a residue’s
reduction of soil nitrate and ammonium by microorgan-
carbon to the amount of its nitrogen influences nutri-
isms decomposing high C:N residues is called immobili-
ent availability and the rate of decomposition. The
zation of nitrogen.
ratio, usually referred to as the C:N ratio, may vary
When microorganisms and plants compete for scarce
from around 15:1 for young plants, to between 50:1 and
nutrients, the microorganisms usually win, because
80:1 for the old straw of crop plants, to over 100:1 for
sawdust. For comparison, the C:N ratio of soil organic
matter is usually in the range of about 10:1 to 12:1, and
the C:N of soil microorganisms is around 7:1.
80
The C:N ratio of residues is really just another way of
short-term reduction in nitrogen availability
(immobilization)
looking at the percentage of nitrogen (figure 9.3). A high
60
C:N residue has a low percentage of nitrogen. Low C:N
residues have relatively high percentages of nitrogen.
40
nitrogen release
C:N ratio
Crop residues usually average 40% carbon, and this
figure doesn’t change much from plant to plant. On the
20
other hand, nitrogen content varies greatly depending
on the type of plant and its stage of growth.
0 0
1
2
3
4
5
6
7
If you want crops to grow immediately following the
% N
application of organic materials, care must be taken to
make nitrogen available. Nitrogen availability from resi-
Figure 9.3. Nitrogen release and immobilization with changing nitrogen
dues varies considerably. Some residues, such as fresh,
content. Based on data of Vigil and Kissel (1991).
93
Building SoilS for Better CropS: SuStainaBle Soil ManageMent