Nutrient Management

Cover crops are an addition to cash cropping systems, and as such, they require management. Nutrient management will be different in a cash crop system with cover crops because of the nutrients and other resources that cover crops contribute and take up from the soil.

Managing Cover Crop Profitably (SARE Handbook Series Book 9) by Andy Clark (2007, 3rd ed.) is a good resource for getting started with cover crops.

General Nutrient Management

  • Crop production requires good nutrient availability
  • Cover crops are an excellent option for reducing nutrient loss
  • Cover crops will not change nutrient availability in the short term
  • Cover crops can be a good way to build soil health, biological activity, and nutrient cycling, but they do not add nutrients to the soil
  • A soil test is recommended to assess nutrient availability and crop production potential
  • Consult your local extension office or agronomist to determine if your soil fertility levels are adequate for crop production

Cover crop nutrient needs

In addition to considering the nutrient needs of the cash crops in a rotation, cover crops require adequate soil fertility to achieve desired biomass.

Brassicas and mustards need adequate nitrogen and sulfur fertility. Brassica sulfur (S) nutrition needs and S uptake capacity exceed those of many other plant species, because S is required for oil and glucosinolate production.

Cover Crops Roles

The addition of fresh crop residues stimulates growth of soil microbes and increases microbial demand for nutrients, particularly nitrogen. Microorganisms use carbon, nitrogen, and other nutrients as a food source in order to break down the residues. If the amount of nitrogen in the residues is too low, the microorganisms use soil nitrogen instead, which ultimately reduces nitrogen availability to the cash crop. This is called nitrogen immobilization.

If the amount of nitrogen in the residues is greater than demand from soil microbes, nitrogen is released from the residue, making that nitrogen available for plant growth. That process is called nitrogen mineralization.

The carbon to nitrogen ratio (C:N ratio) of cover crop residue is a good indicator of whether immobilization or mineralization will occur. Values exceeding 30 parts carbon to one part nitrogen (C:N ratio of 30:1) are generally expected to cause nitrogen immobilization during the early stages of the decomposition process, meaning that nitrogen is “tied up” until the residue more fully decomposes.

 

Over time, cover crop use increases biological activity, including mycorrhizal fungi. Mycorrhizal fungi form a symbiotic relationship with plant roots of many species of plants. The fungi colonize greater soil volume than plant roots and are able to remobilize P and other nutrients that would otherwise be out of reach of or in forms unavailable to crop roots. By planting cover crops and extending the amount of time soil has a living root, mycorrhizal fungi along with other species BUILD GREATER POPULATIONS? MORE ACTIVITY? Reduce soil disturbance to encourage fungi.

Cover crops in the Brassicaceae family do not form associations with mycorrhizal fungi. They do however contribute to increased nutrient cycling. Many brassicas have tap roots that bring nutrients like phosphorus from deep in the profile into the top few inches of soil. The availability of nutrients to subsequently planted crops varies depending on timing of termination. The biomass of brassicas that winter kill decomposes rapidly, releasing the nutrients in leachable/available forms including dissolved reactive phosphorus. When brassicas that winter kill like oilseed radish are seeded alone as a cover crop can cause phosphorus loss concerns.

Cover Crop Species Nutrient Interactions

When following a cover crop, it is important to consider the nutrient needs of the cash crop in it’s growing cycle with the potential nutrients held in the cover crop residue. Crops that require nitrogen early like corn, will benefit from nitrogen applied at or near planting following a grass cover crop.

While cover crops can reduce the need for nutrient applications with continued use, it is important to make sure that cash crops have the nutrients they need at the appropriate times. Following the 4 Rs of nutrient management (right place, right rate, right time, right source) is a good way to be successful in managing nutrients in a cover crop system.

Soil nitrate reduction
The timing of the release depends on the C:N ratio of the cover crop when it is terminated. Decomposition of high C:N cover crops often results in N immobilization. The temporary reduction in plant available N in the spring as high C:N cover crop biomass decomposes can cause stunting in cash crops with high N requirements early in the season, like corn. This early N tie-up has been shown to cause yield drags.

General recommendations to avoid yield drag in corn following high C:N cover crop:

  • Terminate overwintering grass cover crops early, 7-10 days ahead of corn planting
  • Apply starter fertilizer
    • "2x2” placement can avoid salt injury to crop seedlings
    • Early N should be counted towards total N fertilizer rates. Typically, total N rates do not need to be increased.
  • Check local resources for specific recommendations

Small grain and other grass cover crops usually result in an initial, if not persistent, immobilization of nitrogen during the cash crop season. Grasses aren’t considered to be nitrogen sources, but will take up available nitrogen that is susceptible to loss. As the grasses break down, the nitrogen, along with the other nutrients taken up by the cover crop, is released into the soil.

Legumes are generally considered to be a fair nitrogen source. Legumes can “fix” nitrogen from the atmosphere through beneficial bacteria on their roots. A legume cover crop that has time to establish can provide additional nitrogen to the following crop. The nitrogen provided by legume cover crops becomes available as soil microbes break down the legumes after termination. Due to a lower carbon to nitrogen ratio, legumes return nitrogen to the soil faster than grass cover crops. A good way to estimate the amount of nitrogen to credit a cover crop is to estimate the total aboveground biomass ahead of termination, then multiply by % nitrogen. Generally, legume aboveground biomass contains 3.5-4% nitrogen (on a dry matter basis).

Considerations for taking a N credit from your cover crop:

  • Typical corn and soybean rotations in the Midwest don’t allow enough time for a legume cover crop to produce sufficient biomass for N credit.
  • Maturity of cover crop and tillage will impact timing of N release
  • N release is dependent on climate, may not be available for crop, up to half may be available to this year’s crop
  • N accumulated in biomass is not the same as fertilizer replacement

Brassicas, mustards, and other broadleaf cover crops that tend to be killed over the winter and decompose quickly after termination (either winterkill, mechanical, or chemical). Brassicas that are expected to winterkill should not be seeded alone to avoid nutrient loss through the winter and early spring. Some cover crops like buckwheat may help scavenge phosphorus and release it in available forms as the plants break down.

ABOUT

The Midwest Cover Crop Council was first organized in 2006. Since its inception the organization has grown into a community that facilitates the sharing of cover crop knowledge within the Midwest and beyond.

CONTACT INFO

  • MCCC General Contact
    Comments or questions regarding the MCCC or our website can be directed to our Program Manager.
  • Anna Morrow
  • annamorrow@purdue.edu
  • 317-392-6417

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