2) Reduce organic matter losses

• Reduce tillage. Merely maintaining soil organic matter levels is difficult if soil is intensively tilled (such as with annual use of a moldboard plow.) Reducing tillage means leaving more residue, and tilling less often and less intensively than conventional tillage. No-till is the most extreme version of reduced tillage, but is not desirable on some soils and is not the only way to conserve soil organic matter.
• Control erosion. The soil that erodes from the surface of your land is the soil with the highest concentration of organic matter. Erosion is especially detrimental where topsoil organic matter is shallow.

3) Manage the changes in your new system

• Plan how to manage changes in weed or pest problems associated with increased surface residue. Weeds and pests are not necessarily greater problems with increased residue, but may require different management strategies.
• Monitor soil and keep records so you know what effects your practices are having. Include an organic matter test in your regular soil testing. Labs are beginning to offer tests (such as the particulate organic matter test) that measure the active fraction. Monitor changes in patterns of weed and pest problems.

Questions and Answers

What are some sources of organic matter?

Crop residue. For most farms, roots and above-ground plant growth are the most important sources of organic matter. When choosing varieties and crop rotations, consider how much residue will be generated, and how many months each year plants will be growing and creating organic matter.

Green manure. Finding appropriate cover crops is difficult in regions with a short growing season, but there are options and the research and development of appropriate crops is expanding.

Livestock manure. Spreading manure over your land is an excellent way to enhance organic matter, supply nutrients, and prevent manure from becoming a pollutant. If animals are not part of your operation, neighboring farmers may be a source. See Manure Management (BU-7401 in this series) for more information.

Sewage sludge. Human wastes are also nutrient-rich. Spreading sludge on land is cheaper for communities than some other methods of treatment and disposal. If your local community will make it available to farmers, ask about the source and content of the sludge. Contaminants such as heavy metals are an important concern to investigate and test for.

Processing wastes. Vegetable processing plants, wood processing plants, breweries, and other industries generate organic wastes that can be useful soil amendments.

Don't forget the roots Surface residue is only part of what plants contribute to soil organic matter. Roots can add half again as much material. One quarter of the organic matter produced by corn or soybeans is produced by the roots. In prairies, half of plant production is underground. How does that translate into weight? Corn may produce 1/2 to 2 tons of root organic matter in each acre. Soybeans may produce 1/3 of a ton, and the prairie makes over 2 tons. In no-till situations where, surface residue is not regularly tilled into soil, roots become especially critical as a source of soil organic matter.

How do I choose a source of organic matter?

For most people, the choice is determined by convenience and economics. Here are some other questions to ask when assessing a source of organic matter. These issues are addressed in the following pages.

• What is the nutrient content? How much should I apply and will it allow me to reduce fertilizer applications?
• What is the C:N ratio? Will crops get the nitrogen they need if I use this material?
• In what form is the material? For example, is it a liquid that will decompose quickly, or coarse material that can help control runoff?
• Does it contain harmful components such as weed seeds, heavy metals, or chemicals that may inhibit crop growth?
• Will growth inhibitors produced as it decomposes affect the following crop?
• What is required for transportation and application? Consider equipment required, timing (relative to other farm activities), and whether incorporation into the soil is desirable.

Is it possible to add too much organic matter?

Adding too much is not a concern if the source of organic matter is green manures or crop residues from the same field where they are grown. However, it is possible to apply too much manure, sludge, or other rich materials. The consequences of over-application are

• inefficient use of your nutrient resources,
• excess soil nitrogen that harms plants or leaches into water,
• excess phosphorus or other nutrients that become an environmental hazard.

Normally, application rates of manure and other organic amendments should be based on the nutrient needs of the coming year’s crops. This requires testing the nutrient content of the material and your soil. See Manure Management (BU-7401 in this series) for more information about this process.

Can I reduce fertilizer applications when I add organic materials?

Yes, nitrogen credits have been developed for some crop residues and organic amendments. (See references in margin.) Subtract the nitrogen credit from the fertilizer recommendation you receive from a soil testing lab.

Recommended nutrient credits account for the fact that not all of the nutrients in organic material will be available to plants in the first year. The organic compounds must be broken down by microorganisms and transformed into inorganic forms that plants can use. Generally, about half of fresh residue or manure will decompose in the first year, making half its nutrients available to plants. Smaller amounts are available in subsequent years.

The nitrogen is not lost from the soil - it is still present in the cells of microbes - but plants cannot use it. During this initial decay process, microbes are giving off large amounts of CO₂ to the atmosphere and the carbon-to-nitrogen ratio of the remaining organic material declines. Microbial activity slows because the remaining compounds are more recalcitrant (difficult to decompose). At this point, nitrogen from the dying microbes becomes available to plants.