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Central Great Plains Research Station 
USDA-ARS / NRCS / CSU cooperating in Akron, Colorado 
1907 - 2004

 

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Conservation Tillage Fact Sheet

Feeding the Soil - Carbon/Nitrogen Ratio


Manual Rosales, Josh Saunders, Mike Sucik
Soil Quality Team, Natural Resources Conservation Service


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.

What 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.

C/N ratio of selected materials and their corresponding percentages of N and crude protein

Material C/N Ratio % N1 % Crude Protein2
Microbes 5-10/1 8.0 - 4.0 50 - 25
Soils3 8-12/1 5.0 - 3.3 31 - 21
Pine needles 10/1 4.0 25
Sewage sludge 12/1 3.3 21
Alfalfa hay 15/1 2.7 17
Legumes 15-30/1 2.7-1.3 17 - 18
Manures 20/1 2.0 12
Grass clippings 20/1 2.0 12
Corn stalks 50 - 100/1 0.8 - 0.4 5 - 2
Small grain straw 80 - 150/1 0.5 - 0.3 3 - 2
Wood chips 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 land manager.

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...???

  • If 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)

  • Earthworms can convert organic matter into available plant nutrients like nitrate-nitrogen, ammonimum-nitrogen and soluble forms of phosphorus and potassium.

  • Humus can hold the equivalent of 80-90% of its weight in water, making a soil more drought resistant.

Practical 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 Key Words

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.

???Did you know that...???

  • For each percent of organic matter in the upper six inches of soil, 30 pounds of N per acre can be released each year.

References Consulted

Barbarick, K. A. 1996. Nitrogen sources and transformations. p 1-3. Colorado State University Cooperative Extension. Circular # 0.550.

Glossary of soil science terms. 1997. Soil Science Society of America, Inc. 677 South Segoe Road, Madison, Wisconsin. 138 pp.

Hartmann, H. T., W. J. Floker, and A. M. Kofraneck. 1981. Plant Science: Growth, development, and utilization of cultivated plants. Prentice-Hall, Inc., Englewood Cliffs, NJ. 676 pp.

Hill, P. R. 1996. The economic value of crop residues. National Conservation Tillage Digest. November issue. p. 14-15.

Krasny, M. E., and N. M. Trautmann. 1997. Compost chemistry. 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.

Tisdale, S. L., and W. L. Nelson. 1975. Soil fertility and 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 pp.

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.

Waskom, R. M.. 1994. Best management practices for nitrogen fertilization. p. 1-9. Colorado State University Cooperative Extension Bulletin #XCM-172. 


By: Manual Rosales, Josh Saunders, Mike Sucik - USDA-NRCS Soil Quality Team, 40335 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, Colorado.

 

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Last edited:
Wednesday April 14, 2004