Crop rotations cover crops and green manures as nutrient management components

Crop rotations and cover crops can add, deplete, or conserve soil nutrients, depending on how they are managed. They can also affect nutrient availability through a variety of processes. They use soil moisture, but they enhance water infiltration and aggregate formation. They also influence populations of soil organisms.(13)

Crop rotations and cover crops can add, deplete, or conserve soil nutrients, depending on how they are managed.

  • Nutrient additions. Legumes fix atmospheric nitrogen through a symbiotic relationship with nitrogen-fixing bacteria. Although some nitrogen is added to the soil while these plants are growing, the majority of fixed nitrogen becomes available when they die and decompose. The amount of nitrogen added by legumes depends on the type of legume, the soil conditions, and cropping practices. Alfalfa and clover fix around 175 pounds of nitrogen per acre, while soybeans fix only around 50 pounds. Most legumes fix more nitrogen when available nitrogen in the soil is low. Thus, growing legumes in nitrogen-depleted soil, underseeded with grasses or as companion crops with non-legumes, will stimulate them to fix more nitrogen.
  • Nutrient cycling. Deep-rooted plants can retrieve nutrients that have leached below the root depth of other plants. Other plants are hyperaccumulators—they take up higher-than-normal concentrations of specific nutrients. While these plants do not add nutrients to the soil, their ability to capture or accumulate nutrients can help build soil fertility. Nutrients absorbed by these deep-rooted or hyperaccumulating plants are not available until they are cut or incorporated and allowed to decompose.
  • Nutrient depletion and tie-up.

Crop rotations remove nutrients from the soil if the crops are harvested and portions are removed from the field. Even when they remain in the field, cover crops and crop residues can deplete nutrients temporarily, if they do not decompose before the nutrients they contain are needed for crop production. If the added plant materials are woody or dry, they may immobilize or tie up nutrients. Immobilization means that plant materials do not contain enough nitrogen for microorganisms to effectively decompose them. When this occurs, the microorganisms must take nitrogen from the soil as they proceed with decomposition and mineralization. As a result, nitrogen availability is temporarily depressed. While this process can hinder plant growth, it can also conserve nutrients for later use, since those same microorganisms release nutrients as they die and decompose.

♦ Nutrient conservation and management. Nutrient management in organic systems is not only the addition of plant nutrients but also their conservation. Good nutrient management uses crop rotations and cover crops to provide the main crop with nutrients when they are needed and to conserve nutrients in the soil between cropping seasons, when they otherwise might be lost through leaching, erosion, or volatilization. For example, organic matter decomposition and nutrient mineralization can continue to release nutrients into the soil following crop harvest. If not held in microbial biomass or taken up by other plants, these mineralized nutrients, particularly nitrate, can leach through the soil profile, beyond the root zone, or into the groundwater. In another example, you may apply manure that contains a ratio of nutrients different from that needed by your crops. This can cause excess levels of nutrients, particularly phosphorus, to build up in the soil.

Crop rotations can reduce or tie-up excess nutrients in the soil. Cover crops take up plant nutrients as they grow, then release these nutrients back to the soil when they are cut or incorporated and undergo decomposition.^, 14) Rye and other grasses are good scavengers of excess nitrogen; legumes are good for taking up excess phosphorus. Applying woody residues can help hold excess nutrients in the soil. Organisms involved in the decomposition of these carbon-rich and nutrient-poor residues need to use nutrients from the soil to make up for nutrients not available from the residues.

Planting rotations or mulches that include legumes, succulent crops, and woody crops favor a large and diverse population of soil organisms. These organisms retain nutrients in their biomass, which protects against nutrient loss through leaching, runoff, or volatilization.(11) Producers can also use cover crops to control weeds that otherwise would compete with crop plants for nutrients.

Related ATTRA Publications

For more information on methods for using crop rotations and cover crops to manage soil and plant nutrients, see the following ATTRA publications.

Organic Crop Production Overview

Intercropping Principles and Production Practices

Diversifying Cropping Systems Rye as a Cover Crop

Protecting Water Quality on Organic Farms

Also see the SAN handbook Managing Cover Crops Profitably (2nd edition).The handbook can be ordered at www.sare. org/publications/index.htm#books. The pdf version of this publication is also available at this Web address.

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