The following information is intended to help the
agricultural producer gain a better understanding of crop residue and its relationship to
the carbon/nitrogen ratio, residue decomposition, impact on the soil resource, and
subsequent crops. The information also can aid in making a more informed decision when
applying nitrogen fertilizers.
is the carbon/nitrogen ratio anyway?
The carbon/nitrogen ratio (C/N) is the amount of carbon in
a residue in relation to the amount of nitrogen. The rate of organic matter decomposition
and timing of nutrient availability are influenced by the C/N ratio. Everything organic
has a ratio of carbon to nitrogen in its tissues. The following table presents examples of
various materials and their C/N ratio.
ratio of selected materials and their corresponding percentages of N and crude protein
||% Crude Protein2
||8.0 - 4.0
||50 - 25
||5.0 - 3.3
||31 - 21
||17 - 18
||50 - 100/1
||0.8 - 0.4
||5 - 2
|Small grain straw
||80 - 150/1
||0.5 - 0.3
||3 - 2
||100 - 500/1
||0.4 - 0.08
||2 - 0.5
- 1. Percent N was calculated dividing 40 by the C/N ratio. Assuming percent
carbon of residues is equal to 40 percent.
- 2. Calculated by multiplying %N by 6.25.
- 3. Includes everything organic in the soil.
There is a close relationship between the C/N ratio and
percent N found in residues. The lower the C/N ratio the higher its N content and
consequently its crude protein (see table). Crop residues will decompose faster when the
C/N ratio is low or have a high N content. Nitrogen content of residues can be analyzed in
a soil analysis laboratory, or if the percent crude protein of a material is known, the
percent N can be calculated by dividing percent crude protein by a factor 6.25. Leaving
crop residues in the soil following harvest can increase organic matter and
contribute to the supply of nutrients. Understanding the chemical content and
decomposition process of crop residues provides important information beneficial to the
Role of C/N Ratio and N Content on Organic Matter
Decomposition and Soil Fertility
In general, organic materials (crop residues or animal
wastes) added to soils with C/N ratios greater than 30/1 or with 1.5% or less N (e.g.,
corn residues) will usually result in an initial nitrogen immobilization or "tie-up".
This means, that inorganic nitrogen such as ammonium (NH4+), and
nitrate (NO3-) from the soil solution will be "borrowed"
by the soil microorganisms to decompose the added material (microorganisms need nitrogen
for cell growth and function). Eventually the nitrogen will be returned to the soil as the
microorganisms die and decompose. The amount of available N in the soil solution will
depend upon crop uptake, volatilization, denitrification, immobilization,
and leaching. On the other hand, organic materials added to the soil with C/N
ratio of less than 20 or with 2% or more N(e.g., alfalfa hay) will result in an initial N mineralization.
This means that organic N will be transformed to inorganic N and be released to the soil
solution, making it readily available for crop uptake. Keep in mind that these are
generalizations, and many factors such as soil pH, soil temperature, moisture, etc.,
influence the decomposition rate of organic materials and the release or tie-up of
nitrogen. For instance, crop residues decompose very slowly in cold dry soils. These
generalizations will also apply if you are composting organic materials.
???Did you know that...???
75% of U.S. farmland were farmed with a conservation tillage system,
it would offset more than 1% of fossil fuel emissions. Applied on a global scale, the
estimate is 16%.
(Source: Conservation Technology Information Center)
can convert organic matter into available plant nutrients like nitrate-nitrogen,
ammonimum-nitrogen and soluble forms of phosphorus and potassium.
can hold the equivalent of 80-90% of its weight in water, making a soil more drought
Implications of residue decomposition
Additional N might be needed following high C/N
(>30/1) or low N materials such as wheat straw or corn stalks to compensate for the
temporary N tie-up, especially if residues are worked into the soil before planting. This
will be particularly true after a higher than average yield in which a high amount of
residue was produced and incorporated. To determine if N immobilization is occurring,
observe crop appearance for signs of N deficiency. Plants may be N deficient if they
become stunted and yellow. The yellowing usually appears first on the lower leaves while
the upper leaves remain green. This yellowing or chlorosis is uniform over the entire
leaf, with no spotting, stripping or discoloration at leaf edges (other plant nutrients
can also cause similar symptoms, so make sure you diagnose any plant abnormality correctly
before attempting to control it) . Apply N if needed or adjust your N application in
subsequent years based on observations. A legume crop can also be added to the rotation to
keep the soil N in balance.
Subsurface application of N is frequently more effective
than surface application in reduced tillage systems and results in higher yields. Nitrogen
applied below the crop residue avoids the potential for temporary N immobilization and
volatilization, increasing the efficiency of the applied fertilizer.
Annual soil sampling of each field is recommended to
determine the appropriate amount of fertilizer to apply.
Legume residues provide a higher N content than corn or
wheat. A minimum of 30 pounds N per acre should be credited to your fertilizer program in
the first year after any legume.
Burning crop residues will reduce the amount of organic
matter returned to the soil. Crop residues are about 50 percent carbon, and carbon is
volatile under most fire conditions, causing the loss of carbon to the air. Nearly all of
the nitrogen and about half of the sulfur and phosphorus is also lost.
Set realistic yield goals. It is recommended that yield
expectations should be no more than 5% above historical yield averages.
High residue crops such as small grains will contribute
more residues to the soil, which eventually lead to more N mineralized (see
discussion in first bullet item) and a build up of humus. Wheat, a high residue
crop, will produce about 80-100 pounds of residue per bushel of grain produced.
Soils that are under continued cultivation will
experience a decline in humus content if crop residues and sufficient N are not returned
to the soil. Most of the applied N is removed from the soil in the harvested grain.
Maintaining humus content of a soil requires a balance
between additions of organic materials and losses through decomposition. Reducing the rate
of decomposition will aid in maintaining humus. An effective way to reduce decomposition
is to avoid or reduce tillage.
- Glossary of
Decomposition The chemical degradation or
breakdown of mineral or organic matter into simpler compounds; rotting or decaying.
Denitrification The process where certain bacteria
change nitrate back into nitrogen gas.
Composting A controlled biological process which
converts organic residues, usually wastes, into humus-like materials suitable for use as a
soil amendment or organic fertilizer.
Humus The stable fraction of the soil organic
matter remaining after the major portion of added plant and animal residues have
decomposed. Usually it is dark-colored, porous, nutrient-rich, with an earthy fragrance.
Leaching The removal of soluble materials from one
zone in soil to another via water movement in the profile.
Legume Plant member of the family Leguminosae,
with the characteristic of forming nitrogen-fixing nodules on its roots, thus making use
of atmospheric N possible. Examples of legumes are: Alfalfa, beans, clovers, lupines,
peas, peanut, and soybean.
N-immobilization The conversion of inorganic or
mineral nitrogen (available form) to the organic form (unavailable).
N-mineralization The conversion of organic
nitrogen to a mineral form.
(ammonium- NH4+, nitrite- NO2-, nitrate- NO3-).
Organic matter The remains, residues or waste
products of any living organism.
Volatilization The escape of chemical elements
into the atmosphere after being transformed into a gaseous state.
you know that...???
Barbarick, K. A. 1996. Nitrogen sources and
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Prentice-Hall, Inc., Englewood Cliffs, NJ. 676 pp.
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Cornell Center for the Environment. http://www.cfe.cornell...compost/chemistry.html
Magdoff, F. 1992. Building soils for better crops. Organic matter
management. University of Nebraska Press, 901 North 17Th Street, Lincoln, NE
68588. 176 pp.
Power, J. F., and J. W. Doran. 1988. Role of crop residue
management in nitrogen cycling and use. p. 101-113. In cropping strategies for efficient
use of water and nitrogen. ASA- CSSA- SSSA, special publication # 51.
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fertilizers. Third edition. Macmillan Publishing Co., Inc., NY. 694 pp.
Thorup, R. M. 1984. Ortho Agronomy handbook. A practical guide to
soil fertility and fertilizer use. The Fertilizer Division, Chevron Chemical Company. 454
Vigil, M. F., and D. Sparks. 1995. Factors affecting the rate of
decomposition of crop residue decomposition under field conditions. Conservation tillage
Fact Sheet 33-95. Published by USDA-ARS and USDA- NRCS, Akron, Colorado.
Vigil, M. F., and D.E. Kissel. 1991. Equations for estimating the
amount of nitrogen mineralized from crop residues. Soil Sci. Am. J. 55:757-761.
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nitrogen fertilization. p. 1-9. Colorado State University Cooperative Extension Bulletin
Rosales, Josh Saunders, Mike Sucik - USDA-NRCS Soil Quality Team,
County Road GG, Akron, CO 80720, (970-345-2259)
The Soil Quality Team would like to thank the
following for reviewing and editing; NRCS-Northern Plains Soil Quality Working Group;
Agricultural Research Service Scientists; and Ken Remington, farmer, Washington County,