Soil Quality in Vegetable and Small Fruit Production

Bulletin 898

Managing Soil Quality

The Role of Organic Matter, Rotation, and Tillage

Many factors affect soil quality, but soil organic matter is arguably the most critical. Soil organic matter is an excellent example of a soil component having physical, chemical, and biological properties. Maintaining levels of stable organic matter in the soil is important, but the quantity of organic matter is only part of the story.

Organic matter is a central component of the soil food web, so soil organisms naturally decompose and deplete it as they grow. More important, the types and activities of soil organisms and the biological processes in which they are involved change with time, climate, rotation, and other factors. This results in different types of organic matter being present in the soil at any one time. Why is this imporant? We are learning that the type of organic matter present is at least as important as the total amount.

Types of organic matter are very difficult to describe objectively. But, for convenience, soil organic matter is often described as belonging to one of three types or "pools:"

• The diverse array of living organisms, including plant roots.
• An "active" pool that is readily turned over or decomposed under favorable conditions (in periods often measured in months).
• Humus, which is relatively stable and resistant to further decomposition (often lasting hundreds of years).

  The breakdown of "active" organic residues produces long polysaccharides (sugars) that are gummy and bind soil particles into stable aggregates that resist compaction and erosion.

Aggregation is also promoted by networks of filaments (hyphae) from fungi growing through the soil, the binding action of plant roots, and root exudates. Aggregation and the activity of earthworms, burrowing insects, and plant roots create channels that aid water infiltration, aeration, and drainage.

In quick-draining sandy soils, organic matter increases soil water-holding capacity. Assimilation by living plants, microbes, and other soil organisms retains nutrients, preventing them from leaching. Later, breakdown of "active" and stable types of organic matter releases nutrients essential for the growth of plants and soil organisms. Stable organic matter (humus) also buffers soil pH (making soil more resistant to changes in pH) and retains nutrients through its contribution to CEC. A plentiful amount and diverse range of organic matter provides food and energy for soil organisms, and the resulting biological diversity and competition can significantly suppress some plant pests.

Nutrient and water uptake and air exchange occur by means of roots, especially small root hairs. The size and arrangement of soil particles must allow for easy movement of nutrients, water, and air to and from roots. Loose, friable soils, depicted at left, permit free exchange and promote root growth. Compacted soils restrict exchange and prevent root growth and penetration. As a result, crops grown in compacted soils are weaker, less stress tolerant, and require greater inputs.

Organic matter levels have declined 30% to 50% in many areas since the introduction of agriculture, primarily based on short rotations and aggressive tillage and cultivation. However, many soil- and crop-management practices can increase SOM, stimulate biological activity, improve soil structure, and reduce erosion. These practices include:

• Reducing tillage. This slows decomposition of crop residue and limits erosion of organic-matter-rich topsoil.
• Avoiding the urge to work soil when it is wet, common in springtime planting preparation. Working wet soil compacts it. Working wet soil leads to a cascade of events, resulting in severely degraded soil structure and weak, input-demanding, low-yielding crops.
• Diversifying and lengthening rotations. Include legumes and deep-rooted and high-residue crops to add nitrogen, recycle nutrients from the subsoil, disrupt plow pans, and stimulate soil biological diversity.
• Adopting a "no bare soil" philosophy. Plant cover crops to increase organic matter inputs, recycle nutrients, reduce runoff and erosion, suppress weeds (which may limit the need for cultivation), and add nitrogen (if they include legumes). Practices such as intercropping, double-cropping, and using living or plant-residue mulches increase the time the soil is covered and provide many of the same bene-fits as cover crops.
• Applying organic soil amendments, such as manures, composts, biosolids (sewage sludge), food waste and processing effluent, leaves, wood chips, sawdust, grass clippings, and other high-carbon residues and wastes, to stimulate biological activity and add organic matter and nutrients. Be careful to avoid crop nitrogen deficiency when applying material with a high carbon:nitrogen ratio (above 30:1). Time applications to permit significant decomposition before planting and adjust fertilizer nitrogen to maximize the benefits of organic amendments.