Concentrate Organic Matter at Surface to Improve Soils

posted in: Science | 5
tilling the field
Intensive tillage with moldboard plow

Organic matter is the key to soil quality, but building soil organic matter levels can be slow and expensive. There is an alternative. Research shows (Franzluebbers, 2002) many soil functions improve when organic matter is concentrated in the top 2-3″ of the soil, and that, for many soils and environments, this may be the most effective way to improve soil quality.

Soil organic matter (SOM) is crucial for many soil functions, and so has been a primary indicator of soil quality. The % organic matter of the top 6, 8, or 12″ of soil is often used to evaluate whether a soil is improving or degrading. In gardens and small fields, it is relatively easy to increase the quantity of SOM. Not so in large fields where crop residues are often the only source of organic matter, and where increasing SOM by even 0.5% may require a decade of costly effort. In these fields, shifting focus from the total amount of organic matter in a soil to the organic matter concentration at the soil surface is most beneficial.

While the total amount of soil organic matter is important to storage of nutrients and water, it is the surface organic matter that is important for many other soil functions. A functioning soil surface prevents wind and water erosion, supports free exchange of air, enhances biological activity, allows seedlings to emerge unhindered and plant roots to proliferate. Perhaps most importantly it promotes quick water infiltration and movement deep into the soil. Where the surface soil does not function properly the resulting runoff, water and wind erosion, nutrient loss, crusting, poor root growth, and low biological activity all contribute to the decreasing soil quality that many point to as an indictment of modern agriculture. The degraded soil requires more nutrients to replace those lost in runoff, and more tillage to treat crusting and allow short-term increases in infiltration and root proliferation (but furthering long-term soil degradation). Off-farm, the results are more sediments and nutrients in water, dust in the air, and the increased costs of dealing with these problems. When the soil surface does not function, the direct and indirect consequences on the profits, productivity, and environmental impact of agriculture are enormous.

The fix, according to research in several countries (Franzluebbers, 2013), is to manage the soil so that organic matter is concentrated in the top 2-3” of the soil. The benefits from doing this can be even greater than those from increased total organic matter. In one test, increased total organic matter improved water infiltration by 27%, but concentrating organic matter at the surface improved water infiltration by nearly 300%.

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

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

There is no standard for the depths used, but these are common.

SR-change-over-time
Change in stratification ratio of soil organic carbon with time under different tillage systems in Spain (from Franzluebbers, 2013)

Moldboard plowing, which decreases surface organic matter by inverting the soil profile, often has a SR near 1. 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 quality. As such, it has some advantages over other proffered soil quality indicators.

First, it is intuitive; it makes sense to put soil organic matter at the soil surface where many soil functions take place. Second, since commercial soil labs already measure SOM, all that is needed to calculate SR is an additional soil sample. Research also shows that SR is sensitive to farming practices across a diverse soils and climates.

Tilling a Field As Seen from Tractor
Chisel plow results in less intensive tillage.

Research has also found that a strategy of increasing surface SOM can be more beneficial in those hot, arid, low-SOM regions, like many irrigated areas of the West, where building organic matter is difficult. This is because SR, and associated improvement in soil function, can be increased without having to increase the total amount of organic matter in the soil.

I see one problem with SR; it does not account for surface crop residues that protect a functioning soil surface. An unprotected soil surface, battered by water droplets, will soon break down and form a crust, reducing water infiltration, and causing runoff and erosion. Or, buffeted by high winds, will be eroded by bouncing sand particles, creating dust of valuable organic matter, silt and clay particles. In addition to protection, a residue cover serves as a continual source of fresh organic matter to the soil’s surface, it moderates soil temperature, and reduces evaporation. Clearly, without crop residue covering the soil, the value of concentrated surface organic matter is diminished.

To amend this, I propose a “protected stratification ratio,” the SR given above multiplied by the percent residue cover. The latter is easily measured and accounts for the importance of maintaining a crop residue cover on the soil. This protected stratification ratio may be more sensitive to practices that do not overturn the soil (do not reduce SR), but which reduce residue cover, such as chisel plowing or some types of vertical tillage.

To concentrate organic matter at the soil surface, minimize or eliminate tillage. This will also increase surface residue cover, increasing both SR and protected SR. Tillage diminishes surface soil function by increasing decomposition of organic matter, by distributing it more evenly throughout the tilled zone, and by decreasing residue cover. Practices that increase SR 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 and leave the middle layer fractured but with structure intact. Organic amendments can be part of this strategy as long as they are left on the surface, or if desired, incorporated shallowly (2-3”).

Concentrating organic matter at the soil surface is a different way of thinking about soil quality. Combined with a protective cover of crop residues, a functioning soil surface provides many benefits without the need to increase total soil organic matter levels. Do your practices increase or decrease SR over time? It’s easy to measure, so farmers, why not take this opportunity to evaluate your soils?

  1. Franzluebbers, A. J. (2013). Pursuing robust agroecosystem functioning through effective soil organic carbon management. Carbon Management, 4(1), 43–56.
  2. 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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Andrew McGuire is an Irrigated Cropping Systems Agronomist for Washington State University Extension. He works with farmers in the Columbia Basin of Central Washington in improving soils through cover crops and high residue farming systems.