c:n rAtIo of ActIve orGAnIc MAtter
24
As residues are decomposed by soil organisms, carbon
22
is lost as CO2, while nitrogen is mostly conserved.
This causes the C:N ratio of decomposing residues
20
to decrease. Although the ratio for most agricultural
C:N 18
soils is in the range of 10:1 to 12:1, the different types of
16
organic matter within a soil have different ratios. The
14
larger particles of soil organic matter have higher C:N
12
ratios, indicating that they are less decomposed than
smal er fractions (see figure 9.4, right). Microscopic
10 a) greater b) size of c) size d) smaller than
evidence also indicates that the larger fractions are less
than sand
coarse
of fine sand size, probably
size
sand
sand
not active
decomposed than the smal er particles.
Figure 9.4. C:N ratio of different-size fractions of organic matter.
From F. Magdoff, unpublished data, average for three soils.
residue will remain under different crops, rotations, or
recreation. In locations where this occurs, farmers
harvest practices. For example, 3 or more tons per acre of
must be careful to avoid loading the soil with too
leaf, stalk, and cob residues remain in the field when corn
much phosphorus, from either commercial fertilizers
is harvested for grain depending on yield. If the entire
or organic sources.
plant is harvested to make silage, there is little left except
Effects of residue and manure accumula-
the roots.
tions. When any organic material is added to soil,
When “imported” organic materials are brought
it decomposes relatively rapidly at first. Later, when
to the field, you need to decide how much and when
only resistant parts (for example, straw stems high in
to apply them. In general, application rates of these
lignin) are left, the rate of decomposition decreases
residues will be based on their probable contribution to
greatly. This means that although nutrient availability
the nitrogen nutrition of plants. We don’t want to apply
diminishes each year after adding a residue to the soil,
too much available nitrogen because it will be wasted.
there are still long-term benefits from adding organic
Nitrate from excessive applications of organic sources
materials. This can be expressed by using a “decay
of fertility may leach into groundwater just as easily as
series.” For example, 50, 15, 5, and 2% of the amount of
nitrate originating from purchased synthetic fertilizers.
nitrogen added in manure may be released in the first,
In addition, excess nitrate in plants may cause health
second, third, and fourth years following addition to
problems for humans and animals.
soils. In other words, crops in a regularly manured field
Sometimes the fertility contribution of phosphorus
get some nitrogen from manure that was applied in past
may be the main factor governing application rates of
years. So, if you are starting to manure a field, somewhat
organic material. Excess phosphorus entering lakes
more manure will be needed in the first year than will
can cause an increase in the growth of algae and other
be needed in years 2, 3, and 4 to supply the same total
aquatic weeds, decreasing water quality for drinking and
amount of nitrogen to a crop. After some years, you may
95
Building SoilS for Better CropS: SuStainaBle Soil ManageMent