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  4. »eUpdate 854 May 13th, 2021»Crop residue decomposition and nutrient release rates

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Department of Agronomy

Kansas State University

1712 Claflin Rd.

2004 Throckmorton PSC

Manhatan, KS 66506



Extension Agronomy

Crop residue decomposition and nutrient release rates

Crop residue is often considered to be a valuable source of nutrients, especially when the residue is from a nitrogen-fixing legume. The nitrogen (N) and other nutrients released from plant residues can be available for use by subsequent crops. However, crop residue can also tie up nutrients – N, in particular – as it is decomposed by soil microorganisms.

What is the carbon-to-nitrogen ratio?

Before the N present in crop residue is available to a subsequent crop, the residue must be decomposed and the N mineralized, or converted to ammonium (an inorganic form of N that crops can use). How quickly crop residue decomposes depends on the residue’s ratio of carbon to nitrogen (C:N). The C:N ratio can vary greatly between different crop residues (Table 1). This ratio is really a measure of the %N in the residue since the proportion of C in crop residues averages around 40%.

The C:N ratio of crops can change depending on the growth stage due to differences in the %N in the plant tissue. Information collected by Tom Roth, agronomist with USDA-NRCS, showed a large difference the C:N ratio of cereal rye clipped in mid-March (~12:1) compared to termination in late April (~24:1). Decomposition will proceed more rapidly for crop residues that have smaller (more narrow) C:N ratios. Depending on the goal of your cover crop, a quicker rate of decomposition may not be desirable.


Table 1. Typical C:N Ratios for Different Crop Residues and Organic Materials

Crop Residue

C:N Ratio









Grain sorghum


Cover Crops


Cereal rye at flower


Cereal rye early vegetative




Crimson clover


Other Organic Materials




Soil organic matter


Barnyard manure




What determines how and when the N in crop residue, from either a cash crop or a cover crop will be released into the soil?

The C:N ratio of the residue is the key factor to look at when determining the timing of N tie-up and release from residue decomposition.

Scenario 1 - In residue a C:N ratio of less than 20:1, soil microorganisms have enough N available in the residue to do their work and residue decomposition proceeds quickly. In that case, organic N is mineralized or released, fairly quickly to the soil inorganic pool (plant available). Most residue with a C:N ratio of less than 20:1 is either a legume or young, lush vegetation, such as wheat prior to jointing.

Scenario 2 - With a C:N ratio above 25:1, N becomes a limiting factor for decomposition. The population of soil microorganisms needed to decompose the residue increases rapidly while consuming N from the soil in the process, if it is available. This uptake of available inorganic N to decompose the residue is called immobilization, or N tie-up. This is a temporary process, and some N may become available during the growing season depending on the residue. The higher the C:N ratio, the longer the N will be tied up. Corn or wheat residue will take longer to decompose as compared to soybean, alfalfa or other legume residues. When the available C or energy begins to run out, the population of soil organisms using the residue as energy will die back, releasing N back to the inorganic pool (mineralization).



Dorivar Ruiz Diaz, Nutrient Management Specialist

Deann Presley, Soil Management Extension Specialist