BU-07403     2000

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SOIL MANAGER

Aim for diversity

Soil performance generally improves when the complexity, or diversity, of the soil biological community increases. (See p. 11 for an explanation of complexity.) There is a limit to the amount of diversity that is economically and agronomically beneficial, but trying to increase diversity may be a useful way to manage soil biology.

How do I encourage a diverse soil community?

The next few pages will describe common ways to encourage healthy soil biology, and explain why some farm practices affect soil organisms.

If you ask others for suggestions, keep in mind that what works in one place might not work in another. To manage the soil biological community, be a careful observer of the changes on your farm.

How to encourage healthy soil biology

Supply organic matter

• maximize crop residue
• apply compost or manure
• plant cover crops

Increase variety

• create a diverse landscape
• rotate crops

Protect the habitat of soil organisms

• reduce tillage
• minimize compaction
• minimize fallow periods
• minimize the use of pesticides
• improve water drainage

Managing for Soil Biology

Supply organic matter

Most soil organisms rely on organic matter for food. Each source of organic matter favors a different mix of organisms, so a variety of sources generally supports a variety of organisms. The location of the organic matter - whether at the surface or mixed in the soil - also makes a difference.

Maximize crop residue

Crop residue is a convenient and valuable source of organic matter. Corn harvested for grain will grow 3 to 4 tons of surface residue per acre and 1 to 2 tons of root biomass. Dense, sod-type crops produce generous amounts of root biomass. Soybeans generate notoriously little surface residue.

Surface residue encourages the decomposers - especially fungi - and increases food web complexity. Fungi increase because they have some advantage over bacteria in digesting surface residue, though greener and younger plant residues are easy for bacteria to use. Residue provides food and habitat for surface feeders (such as some earthworms) and surface dwellers (such as some arthropods). It also changes the moisture and temperature of the soil surface. Some pathogens will be increased by the residue, others will be decreased.

Apply compost or manure

• Compost inoculates the soil with a wide variety of organisms and provides them with a high quality food source. Some species thrive in both compost and in the soil, but many do not. For example, the redworms (Eisenia fetida) that thrive in worm compost do poorly in soil.
• Animal manure: Manure patties provide food and habitat for larger soil organisms, and manure in any form is a significant source of nutrients. Manure and plant matter each support different mixes of soil organisms.
• Sludge: Like manure, sludge can be an excellent food source for organisms. However, high levels of metals in some sludge will kill some organisms.

Plant cover crops

Cover crops extend the growing season and increase the amount of roots and above-ground growth that becomes part of soil. As with other crops, the rhizosphere (the area immediately surrounding roots) of a cover crop provides food for bacteria when food sources would otherwise be scarce. Because of each crop’s unique physiology, populations of certain pathogens will decline under specific cover crops.

Increase variety

Create a diverse landscape

Diverse habitats support complex mixes of soil organisms. Diversity can be achieved with crop rotations, vegetated fence rows, buffer strips, strip cropping, and small fields.

Rotate crops

Crop rotation puts a different food source into the soil each year. This encourages a wider variety of organisms and prevents the build-up of a single pest species. Cover crops increase the variety of plants in a field each year.

Protect the habitat of soil organisms

Large and small soil organisms need air, moisture, a constant food supply, and room to move in a protected place. Reduced tillage, lack of compaction, constant ground cover, and minimum disruption by chemicals protect the environment of soil organisms.

Reduce tillage

Tillage enhances bacterial growth in the short-term by aerating the soil and by thoroughly mixing the organic matter with bacteria and soil. The bacterial activity increases the loss of carbon as CO2, and triggers explosions of bacterial predators such as protozoa. A single tillage event is generally inconsequential to microorganisms, but repeated tillage eventually reduces the amount of soil organic matter that fuels the soil food web.

The mechanical action of tillage can kill individual organisms and tends to temporarily reduce populations of fungi, earthworms, nematodes, and arthropods. Over the long term with repeated tillage, these populations may decline as a result of lack of food (i.e., surface residue), rather than because of the mechanical action of tillage.

No-till: The environment for soil organisms can differ significantly in no-till compared to conventionally tilled soils. No-till soils are more likely to have anaerobic environments, soil may be cooler in spring because of surface cover, there can be more macropores, and organic matter is not evenly mixed throughout the top-soil.

The result is a lower rate of organic matter decomposition. In addition, the lack of disturbance and the presence of surface residue encourages fungi and relatively large organisms such as arthropods and earthworms. No-till soils generally have a higher ratio of fungi-to-bacteria.

Minimize compaction

Compaction reduces the space available for larger organisms to move through the soil. This favors bacteria and small predators over fungi and the larger predators. Arthropods are severely affected. Among nematodes, the predatory species are most sensitive to compaction, followed by fungal-feeders and bacterial-feeders. Root-feeding nematodes are the least sensitive to compaction - perhaps because they do not need to move through soil in search of food.

Compaction changes the movement of air and water through soil, and may cause a switch from aerobic to more anaerobic organisms.

Minimize fallow periods

During long fallow periods, most arthropods will emigrate or die of starvation. Mycorrhizal fungi (fungi that need to form associations with plant roots and are critical to the growth of most crops) also "starve" during a fallow period and recover slowly after the fallow period ends. Cover crops help maintain or build arthropod populations and diversity by reducing the length of fallow periods at the beginning and end of growing seasons. Cover crops also affect the biological habitat by changing temperature and moisture levels.

Minimize the use of pesticides

All pesticides will impact some non-target organisms. Pesticides feed some organisms and harm others. Labels generally do not list the non-target organisms affected by a product, and few pesticides have been studied for their effect on a wide range of soil organisms, so the net effect of moderate pesticide use is not well understood. Heavy pesticide use probably reduces soil biological complexity.

Herbicides may not affect many organisms directly, but the weed loss changes the food sources and habitats available to organisms.

Improve water drainage

Good water drainage improves microbial habitats by increasing oxygen availability.

Inorganic fertilizers

Fertilizers provide some of the nutrients needed by organisms and will favor those species that can best use these forms of nutrients. The pH and salt effect of some fertilizers (e.g. ammonium nitrate, ammonium sulfate, and urea formaldehyde) reduces populations of fungi, nematodes, and probably protozoa, at least temporarily. It is not clear how long this effect lasts in different situations.

Because fertilizer use increases plant growth, and therefore organic inputs into the soil, biological activity may be higher in fertilized soil than in soil with low levels of plant nutrients.

Soil Inoculants

So far, this publication has described how soil management practices change the environment that supports soil organisms. Another approach to managing soil biology is to inoculate the soil with desired species or reduce the activity of undesired species. For example, products are available that allow farmers to:

• inoculate soil or seeds with nitrogen-fixing bacteria,
• introduce bacteria, nematodes, or insects that are predators of pest organisms,
• add nitrification inhibitors to reduce the activity of specific bacteria that convert ammonium to nitrate.

Many of these products are effective and valuable, but there are limits to what can be accomplished with a management approach that targets specific organisms. Inoculants will have little effect or only a temporary effect if the organisms cannot compete in their new environment. Furthermore, some benefits of soil organisms come from a mix of organisms, not from a few specific species.

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