SOIL MANAGER

How do I build up organic matter?

1) Add organic material.
2) Reduce losses.
3) Manage the new dynamics of the system.

1) Add organic matter

• Grow more organic matter, grow healthy and productive crops, and plan a high residue rotation that includes:
  
  
  • sod crops that leave lots of roots in the soil (small grains or forages),
    
    
  • crops that leave a lot of surface residue (e.g., grain corn),
    
    
  • cover crops that supply both.
• Apply livestock manure. Manure is an excellent way to build organic matter. See Manure Management (BU-7401 in this series).
• Locate off-farm sources of organic matter, such as food processing wastes or manure from neighbors.

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.

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.

Research suggests that nutrient credits are lower if you reduce tillage. As you reduce tillage, some of the nutrients in manure or legumes will go into building soil organic matter levels and not into your crops. The University of Minnesota Extension Service recommends that you determine the nitrogen fertilizer value of your manure or legume using standard tables. Then, decrease the nitrogen credit for legumes and manure by 20% if you do not use primary tillage (e.g., no-till, ridge-till, light spring discing).

Why is the carbon-to-nitrogen ratio important?

Each organic amendment has a characteristic amount of carbon in proportion to nitrogen. (See table below). A low carbon-to-nitrogen ratio means the material is high in nitrogen. Materials with a high C:N ratio (low nitrogen) decompose slowly and may trigger nitrogen deficiency in plants as they decompose.

How can adding organic matter trigger nitrogen deficiency?

Plants depend on microbes to break down organic matter and make the nutrients available to them. Most microbes get energy from carbon compounds such as sugars, carbohydrates, fats, and other substances. Mixing organic material into the soil triggers a feeding frenzy and a burst in microbial growth. To grow, microbes need carbon for energy and nitrogen to build proteins. For every twenty to thirty carbon atoms they consume, they use about one nitrogen atom. If that nitrogen is not available from the newly-added organic material, microbes will take it from the soil, and deprive growing plants of nitrogen.

As a rule-of-thumb, materials with C:N ratios less than 30:1 will not trigger temporary nitrogen deficiency.

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.

What about disease and other problems?

As with any new management practice, challenges can emerge with the benefits. Before switching your whole farm to a new practice, try out new organic materials or tillage practices on a small plot of land. Watch how the new system works and anticipate problems. The following are some problems you might look for and ask about.

Weeds

Some fresh manures may promote weed growth and may contain weed seeds. Test new manure sources on a small area before spreading on large acreage. Worse than manure with a lot of weed seeds is manure with new weeds that have never before contaminated a particular field or farm. For this reason, ask where the feed for the animals came from before buying manure from another farm.

Diseases

Some residues harbor disease that may affect the next crop. For example, white mold on sunflower hulls will affect beans. If you have a specific disease problem, and are considering adding a new crop to your rotation, ask whether the crop can host the disease organism.

Growth inhibitors

"Allelopathic" chemicals are chemicals produced as a plant grows (or as its residue decays) that can inhibit the growth of other plants. Allelopathy can be a benefit or a problem. For example, rye residue reduces weed germination and is sometimes used for weed control. On the other hand, replanted alfalfa may not grow well in alfalfa residue. Allelopathic effects are complex and depend on plant genetics, weather, and plant stresses. Unfortunately, there is not much practical guidance available for farmers at this time.

Contaminants

Municipal solid wastes and sewage may contain heavy metals and other contaminants. Test the material before application and monitor soil levels when using these products.

A more common contamination problem relates to phosphorus. Manure can lead to phosphorus build-up in the soil if used over an extended period of time. Apply manure at rates based on crop nutrient needs and soil nutrient levels.