Soil Organic Matter: Small Component in Soils, Big Impact to Soil Health

The WSU Farmers Network is hosting a workshop event on January 22, 2019, at 8:30 am – 3:30 pm at Banyans on the Ridge. The workshop will present in-depth information on various topics of soil health improvement, including increasing soil organic matter (SOM) using soil amendments such as manure, compost, and biosolid; why SOM is important for soil health; how and why no-till is better for soil health; and how tillage and cropping systems affect your soil microbiology properties relevant to nutrients availability.

Most of the soil in the Pacific Northwest contains 1.5-2.5% of soil organic matter in intermediate and high rainfall zones and only about 1% in drier regions. This is a small percentage component of soil, however, is very important for soil health and crop productivity. SOM is the foundation for providing the source of nutrients, supporting healthy microbes, and maintaining soil structure to the agroecosystems. SOM ensures continued resilience and sustainability in agricultural productivity.

SOM is subdivided into three pools or fractions depending on their turn over time: the active or labile pool can be decomposed within two years; the intermediate pool can take years to decades to be decomposed; the passive or stable pool is resistant to decomposition. SOM can supply and store elements such as carbon, oxygen, hydrogen, sulfur, nitrogen, phosphorus, potassium, calcium and magnesium, and more. The active SOM serves as a food source for soil microbes and the crop essential nutrients are released during the decomposition carried out by these soil microbes. All three pools are important for soil health. Increasing SOM can increase soil strength in terms of aggregate stability, water holding capacity and water infiltration, and reduce the chance for surface crusting. The stable fraction also allows for higher buffering capacity and higher ion exchange capacity, which correlates to how nutrients hold onto soil and how soil is resilient to changes in pH. Microbial communities are also benefitted by this fraction, as it serves as a food source for microbes and optimizes pore space for movement.

SOM is a dynamic property of a soil, and it can be reduced or built as a result of your cropping systems and management practices. In PNW SOM continuously decline in most of the cropland. For example, a research has shown a 196 to 428 lb/acre/year SOM loss in a Walla Walla silt loam soil managed in conventional tillage practice. A wide variety of management practices and soil amendments additions, including crop residue management, cover crops or the use of soil amendments such as manure, compost, biosolids, and biochar, can be adapted to increase SOM reservoir. Greater than 20 years of continuous dairy manure application in irrigated alfalfa-corn systems resulted in 40% increase in SOM, resulting 65% increase in plant available N, on a dairy farm in Columbia Basin. The use of perennial grasses or legumes in rotations also increases soil organic matter, due to reduced erosion and the large root mass. Legume cover crops or intercropping not only can increase SOM but also provide nitrogen nutrients. Conservation tillage practices, such as reduced tillage and no-till, preserves soil organic matter by decreasing erosion and by reducing soil mixing, which accelerates SOM decomposition.

For workshop and registration details, please visit the Management Matters for Soil webpage.

For workshop registration, please contact Keith Curran via email at or Carol McFarland via email at Both are in the Department of Crop and Soil Sciences at the Washington State University.

For general questions or comments, contact Dr. Haiying Tao, Assistant Professor, via email at, or graduate student Katherine Naasko via email at Both are in the Department of Crop and Soil Sciences at the Washington State University.

Washington State University