Cover crops are becoming an important part of many Kansas cropping systems. They play many potential roles, including: 

* Trapping residual nitrogen (N) that may remain after a crop

* Fixing nitrogen for use by subsequent cereal or forage crops 

* Fixing carbon as a source of additional crop residue and soil organic matter

* Providing competition to limit weed growth

* Serving as “water pumps” to remove excess moisture on poorly drained soils

The first two of those roles are perhaps the least understood and most variable. To understand what to expect in the way of N cycling or additions from cover crops, it is important to briefly review the principles of N reactions in the soil.

Nitrogen can undergo a series of important transformations in soils. Before the N present in a cover crop can become available to a subsequent crop, the vegetation must be decomposed, and the N mineralized -- or converted to ammonium and released into the soil inorganic pool. 

How quickly residue decomposes is a function of the carbon (C) and N contents of the residue and soil. As a cover crop dies or is killed and returns to the soil surface, it begins to decay. In residue with a C:N ratio of less than 20:1, adequate N is present to support a build-up in population of residue-decomposing soil organisms, and decomposition proceeds quickly. In that process, excess N (amounts above the needs for the decomposing organisms) is mineralized or released fairly quickly to the soil inorganic pool. Most residues with C:N ratios of less than 20:1 are young, lush vegetation such as wheat prior to jointing or legumes. The table below gives the C:N ratio of many common crops.

Typical Carbon and Nitrogen Content of SOM, Crop Residues or Cover Crop Materials.

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            Source                         % Carbon       % Nitrogen    C:N Ratio      

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            Microorganisms                   50                       6.2               8:1

            Soil OM                              52                       5.0             10:1

            Young clover                        -                           -               12:1

            Alfalfa  , bloom        40                       3.0             13:1

            Soybean residue                  40                       2.6             15:1

            Brome hay, 15% protein   40                          2.4             17:1

            Well composted manure     -                             -             <20:1

            Green Rye                            -                           -               36:1

            Grass cover mix                  40                        1.0             40:1

            Corn stalks                         40                        0.7             60:1

            Wheat straw                       40                        0.5             80:1

            Sorghum stalks       40                        0.5             80:1

            Sawdust                             40                        0.1           400:1

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With a C:N ratio above 20:1, free N from the inorganic pool will be used by microorganisms to grow and multiply, resulting in temporary immobilization of that nitrogen and a decrease in N availability to crop plants. Only when the excess C has been used, or decomposed, will the microbial population die back and the N within the microbial community be released, or mineralized, for use by the crop.

Nutrient trap crops

For nutrient trap crops, the use of fast growing, N-demanding crops are ideal. Summer crops such as millet or forage sorghum planted in the summer after wheat make ideal trap crops. So do cereal rye, wheat, triticale, or canola, turnips, or radishes planted in the fall after summer crops. Trap crops will use soil N to support growth. In most cases, cereal trap crops will have a wide C:N ratio, so the release of the N to subsequent crops may be slow. Canola, turnips, radishes, or legumes will produce less dry matter, but have a narrower C:N ratio, and have a quicker decomposition and N release. 

When using cereals, corn, sorghum, or millet it may be the second crop grown that actually benefits from the trapped N. But during that interval from the time the trap crop dies until a subsequent crop can utilize the N, the trapped N will not move through the soil to contaminate groundwater. In addition to N, nutrient traps will also help slow the loss of S, K, and Cl on sandy soils prone to leaching of mobile nutrients.

Most trap crops also are also well suited as residue cover sources. Remember, the rate of residue decomposition can be slowed to some extent by selecting a cover crop with high C, low N residue, such as forage sorghum, millet or cereal rye, and letting it become fairly mature, in an N-deficient environment. 

N-fixing cover crops

If wanting to grow cover crops to provide supplemental N to future cereal or forages, legumes are preferred. But cereals can also be useful, especially due to lower seed costs, if killed at an early growth stage when the residue has a low C:N ratio so that it will decompose fairly quickly.

Legumes fix N, if nodulated, but the amount produced will vary widely. Also, remember the C:N ratio still applies when determining how quickly the fixed N within a legume will be available to subsequent crops. Fine-textured, low C:N plants such as alfalfa, clover, soybeans, or peas will decompose much more quickly, releasing N much more rapidly, than coarse-textured, wide C:N plants such as mature sunn hemp.

Nitrogen fixation is a very energy intensive process, whether done by legumes or in a Bosch-Haber fertilizer plant. That means you must have conditions favorable to photosynthesis and high yield. In many cases, some combination of dry soils, short daylength, and cool temperatures limit N fixation by many legumes planted after wheat, or planted in the fall after summer crops. Also, high soil nitrates will feed back and “poison” N fixation. So the potential N fixation from many legumes is not guaranteed, and can be limited. In the best-case scenario, the total N present in legume cover crops can be impressive. But the net difference between the amount of N trapped by a cereal and the amount of N present in a legume, considering both the residual in the plant plus the amount produced through N fixation, can be small.

How much N can be expected to be trapped or produced by cover crops?

Research in Kansas has shown that the amount of N that can be trapped or produced through fixation can vary widely. 

It is common for 20 to 60 pounds of N to be trapped by crops such as millet, forage sorghum, or sudan grass planted after wheat. In fields with a history of manure applications, or when planted after a failed or drought-damaged wheat or corn crop, values could be higher. When planting after a failed crop, however, the stage of growth of the crop when it failed will determine how much N is left in the soil. The vast majority of the N used by the crop will have already been taken up when wheat heads or corn tassels. Winter cereals planted after corn will generally trap less N due to limited fall growth. In many cases this can be as low as 20 to 40 pounds N per acre. Cereals after sorghum will typically trap even less than that, because N fertilizer rates on sorghum are lower, and sorghum residue has a wider C:N ratio.

As for the amount of N produced through N fixation, remember that legumes are basically lazy, and will utilize available N present in the soil before spending energy to fix N. So the additional N produced, above the amount a trap crop would take up, may be limited, particularly if the legume is terminated early to minimize the amount of growth (such as is often the case with sunn hemp). Generally, sunn hemp and long-season soybeans that stay vegetative will produce/trap the largest amounts of N, perhaps as much as 100 pounds N per acre. But the portion that will become available for future crop use can be a question, especially with very aggressive species such as sunn hemp, which produces lots of biomass with a wide C:N ratio. Cowpeas would be expected to produce significantly less N.

The amount of N produced by winter legumes such as vetch and winter peas planted after corn and other summer crops is primarily determined by the timing of termination in the spring. Generally the equivalent of 30 to 50 pounds of fertilizer N can be produced if these species are allowed to grow until mid to late May. Killed earlier, N production is considerably less. Unfortunately, the cost of seed and seeding, and the yield reduction possible from delayed planting of a crop such as corn, can exceed the value of N produced from many legume cover crops.

Other considerations: What about water management?

Good, actively growing cover crops utilize water much like any other growing crop. This may be important to legumes in their production of N. But overall, water use by cover crops can be a blessing or a curse, depending on the soil and rainfall situation, and can determine when a cover crop should be terminated. 

In sandy soils, or areas where rainfall may be limited, an aggressive cover crop can utilize the available soil moisture and make germination and stand establishment of the following cash crop difficult. In these situations, terminating the cover crop several weeks prior to planting the cash crop may be important to ensure adequate moisture at planting beneath the cover crop residue.

However, on poorly drained soils, especially in areas with potentially high spring rainfall, actively growing cover crops can serve as water pumps to help dry the soil to facilitate trafficability and early planting. Waiting to kill the cover crop until just prior to or immediately after planting can be an advantage in these situations. Killing a cover crop early on a heavy soil, then getting subsequent heavy rain, can make it very difficult to get the soil under the cover crop residue to dry adequately to avoid soil compaction at planting.

-- Dave Mengel, Soil Fertility Specialist

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