Washington State University Extension

Irrigated Agriculture

Concentrate Organic Matter at Surface to Improve Soils

By Andrew McGuire

Researchers are finding, in the US and in many other countries, that concentrating soil organic matter in the top 2″ promotes several aspects of soil health including nutrient cycling, resistance to erosion, and water infiltration and storage. They find that maintaining a high proportion of organic matter at the soil surface, relative to deeper layers, is more important than the total level of organic matter in a soil.

One key measurement of soil health is organic matter content. The % organic matter of the top 6, 8, or 12″ of soil is often used to evaluate whether a soil is improving or degrading. The total amount of organic matter in a soil is important, but research is showing, so is the location of the organic matter, especially when it is concentrated in the top 2″.

The soil surface is where wind and water erosion start, or are prevented, where gases are exchanged, where most soil microorganisms live, where most plant roots proliferate (especially when covered with crop residues), and perhaps most important in our region, where water enters the soil. So concentrating organic matter in this surface layer provides multiple benefits, controlling erosion, promoting gas exchange, microorganism activity, and root growth, and increasing water infiltration and movement into the soil profile. These, in turn, reduce runoff and nutrient loss and ultimately benefit the crop. This layer of concentrated organic matter gives you improved soil health.

The benefits from concentrating organic matter at the surface have been found to be greater than those from increased total organic matter, distributed in the soil profile. In one study, increased total organic matter improved water infiltration by 27%, but concentrating organic matter at the surface improved water infiltration by nearly 3x.

Minimizing tillage is the key to getting these benefits. Tillage both reduces the total amount of organic matter by increasing decomposition and reduces surface organic matter by distributing it more evenly throughout the tillage zone.

Practices which result in surface-concentrated organic matter include various forms of high residue farming, but also systems which use shallow surface tillage (top 3″) combined with deep tillage (>8″, when the soil is dry) that minimize surface disturbance. Reducing tillage also maintains a cover of residue on the soil. This cover provides a source of organic matter to the soil’s surface layer and enhances the benefits of surface organic matter by protecting it from wind and water droplets, and by moderating soil temperatures.

Moldboard plowing decreases surface organic matter by inverting the soil profile. The resulting low organic matter at the surface is then prone to erosion, crusting, poor water infiltration. If too much residue is the problem, removal of part of it is often better for the soil than moldboard plowing.

To measure the effects of surface-concentrated organic matter, researchers use a “stratification ratio”, SR:

SR = (organic matter in the top 2″)/(organic matter in the 6-12″ zone).

Moldboard plowing often has a SR near 1 while less intensive tillage, like disking or chisel plowing, results in SR values from 1 to 2. Low disturbance no-till methods have the highest SR values, ranging from 1.8 for short-term no-till to near 4 for long-term no-till fields. Negative processes such as runoff, nutrient leaching, and erosion tend to decrease as SR increases, while water infiltration rates, gas exchange, and biological activity increase with increasing SR values. Because of these relationships, researchers suggest that SR may be a good indicator of soil health.

Concentrating organic matter at the surface offers another advantage to regions like the Columbia Basin. In the summer, our warm temperatures and wet soils promote rapid decomposition of organic matter, limiting how high soil organic matter levels can be increased. However, by taking the organic matter we have available and concentrating it at the surface, we can get equal or more benefits than those produced by an overall increase in organic matter levels. This matters because building organic matter can be slow and expensive.

Franzluebbers, A. J. (2002). Soil organic matter stratification ratio as an indicator of soil quality. Soil and Tillage Research, 66(2), 95–106.
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