Soils are one of the most complex biological communities. A teaspoon of soil has more than 180 million bacteria, according to USDA’s Hal Collins.
Soils are one of the most complex biological communities. A teaspoon of soil has more than 180 million bacteria, according to USDA’s Hal Collins.

Managing soil organic matter is usually not a priority for most farmers who must worry about yield, pest management, labor, and other things that impact the bottom line. But practices that improve soil organic matter contribute more than any other resource to a farm’s long-term productivity, says a U.S. Department of ­Agriculture soil scientist.

Adding soil amendments, such as compost, crop residues, and farm or municipal wastes, not only builds soil organic matter, but can supply nutrients, help stabilize the soil, and reverse trends of soil compaction, erosion, and reduced water infiltration, according to USDA’s Dr. Hal Collins, stationed at the Agricultural Research ­Service laboratory in Prosser, Washington.

Soil organic matter makes up from 1 to 8 percent of the soil, but in eastern Washington, with its sandy and silt loam soils, organic matter is around 1 to 1.5 percent of the soil, Collins said during statewide grape talks last ­winter in Grandview. Washington’s aridisol soils, with the native shrub-steppe of plants like sagebrush, bitter brush, rabbit brush, and bunch grasses, receive only a scant six to eight inches of precipitation annually, making ­decomposition of vegetation difficult.

In most soils under agricultural production, soil organic matter declines through time, principally due to years of plowing and cultivating, Collins said. But under eastern Washington irrigated agricultural practices, farmers have actually increased levels of soil organic matter by adding crop residues and soil amendments, and growing perennial crops, he said.

Collins noted that the sagebrush and other high desert native vegetation adds only about 1,000 pounds per acre per year of soil organic matter compared to 3,000 to 12,000 pounds that are added from irrigated agricultural practices.

“Through cultivation, we’ve actually increased soil organic matter because of the high amount of crop residues incorporated,” he said, referring to eastern Washington growers. “We’re one of the few places where this occurs. In most places, there’s a decline.”

Soil problems

But even with the addition of organic matter, soil compaction can still be a problem. Compaction, a major source of yield reduction, restricts root growth, limits nutrient availability and aeration, and can lead to poor root health and increase root pathogens.

Collins reported on a study that showed how compaction developed in a brand-new vineyard. In the first year of the vineyard, compaction zones, as shown by water infiltration rates, were nonexistent, he said. But three years later, significant compaction zones had developed from tractors and equipment driving up and down the rows.

Erosion is also an issue. Bare ground, without some sort of cover crop in a vineyard or orchard, increases soil erosion, Collins noted, adding that erosion then results in a loss of physical, chemical, and biological properties of the soil. “The biggest problem with erosion is that the impacts are difficult to measure. But you will have an increase in inputs like water, fertilizer, pesticides, labor, and energy due to erosion.”

Microorganisms in the soil control the soil transformations and are responsible for mineralization (release) and immobilization (holding) of nutrients. Organisms take up and retain nutrients as they grow. Inorganic nutrients are usable by plants and are mobile in the soil, while organic nutrients are stored in soil organisms and organic matter.

He recommended that growers use stable organic amendments like compost to build soil organic matter quickly.

A 2006 study he cited showed a 40 percent increase in soil organic matter after three years of adding stabilized compot (30 tons per acre).

To improve soil structure and stability, Collins suggested that growers use materials that are high in carbon, incorporating materials like raw wastes, green manures, or cover crops. High-carbon materials stimulate soil microorganisms to make polysaccharides and mucigels to bind soil particles and improve soil structure. However, the high-carbon materials will reduce the availability of nitrogen in the soil.

To improve the nutrient status of soils, use high-­nitrogen materials (minimally processed organic wastes high in nitrogen, manures, or green manures), or plant legume cover crops.

But he warned that growers must know the nutrient analysis of the material they are adding. Commonly available dairy manure adds very little nitrogen, he said. Typical solid dairy manure contains 10 pounds of nitrogen per ton compared to liquid dairy waste that has about 24 pounds of nitrogen per ton.