Roadside Revegetation

IMPLEMENTATION GUIDES
Carbon-to-Nitrogen Ratio – The carbon-to-nitrogen ratio (C:N) is one of the most important
characteristics to consider when selecting a source of organic matter. It is an indicator of
whether nitrogen will be limiting or surplus (see Section 5.5.3.1, Carbon to Nitrogen Ratio
(C:N)). The higher the C:N, the greater the likelihood that nitrogen will be unavailable for plant
uptake. When an organic source with high C:N is incorporated into the soil, carbon becomes
available as an energy source for decomposing soil organisms. Soil microorganisms need
available nitrogen to utilize the carbon source. Not only do microorganisms compete with
plants for nitrogen, they store it in their cell walls, making it unavailable for plant growth for
long periods of time. As the carbon sources become depleted, the high populations of soil
microorganisms die and nitrogen is released for plant growth.
When C:N is greater than 15:1, available nitrogen is immobilized. As ratios dip below 15:1,
nitrogen becomes available for plant uptake. Most fresh and aged organic sources have C:N
greater than 15:1 (Table 10-8) and will immobilize nitrogen for some period of time when
incorporated into the soil. When these same materials are composted, C:N approaches or even
falls below 15:1. These materials will then provide a source of nitrogen to the soil. Co-composts,
for instance, can have ratios between 9:1 and 11:1, indicating they are a ready source of available
soil nitrogen. Since these materials provide nitrogen, they are often considered fertilizers
(Ratios below 10:1 are typically labeled as fertilizers).
The period of time that nitrogen remains immobilized in the soil is dependent on several factors:
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Climate. High moisture and warm temperatures are important for accelerating
decomposition rates. For example, organic matter will decompose faster in the coast
range of Oregon than in the mountains of Idaho.
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Quantity of incorporated organic matter. The more organic matter that is applied,
the longer the immobilization. A small amount of incorporated sawdust will immobilize
very little nitrogen as compared to several inches of the same material.
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C:N of organic amended soil. The combined C:N of soil and incorporated organic
matter gives an indication of how the type and rate of incorporated organic matter will
affect the soil C:N. An amended soil with a high C:N will have a longer immobilization
period than a soil with a lower C:N.
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Size and shape of organic matter. The more surface area of the organic source, the
faster decomposition will take place. A fine compost will decompose faster than a
coarse, screened compost.
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Nitrogen fertilization or fixation. Nitrogen supplied from fertilizers or nitrogen-fixing
plants will speed up decomposition rates.
It is not easy to predict how long nitrogen will be immobilized in a soil. The variety of available
organic sources, unique soil types, and range of climates of the western United States make
this difficult. For practical purposes, it should be assumed that without supplemental additions
of nitrogen (from fertilizers or nitrogen-fixing plants), the immobilization of nitrogen in soils
with high C:N will be in the order of months, if not years. To give some idea of decomposition
rates, Claassen and Carey (2004) found that partially composted yard waste with a C:N of 18:1
took over a year for nitrogen to become available under aerobic incubation testing conditions.
A high C:N might be beneficial to soil aeration and water movement because it does not
break down as fast as material with lower C:N. For example, the incorporation of alfalfa hay
(C:N =13:1) will decompose quickly, and the effects on soil structure might be short-lived.
On the other hand, wheat straw (C:N = 80:1) or pine needles (C:N = 110:1) can be effective for
several years. High C:N materials are also a longer-term energy source to soil organisms that
help create a stable soil structure.
Nitrogen based fertilizers can be applied to offset higher soil C:N and make soil nitrogen
available for plant growth. Section 10.1.1, Fertilizers, discusses fertilizer strategies for reducing
the effects of high C:N soils.
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