Why are we concerned about soil structure? Soil structure stabilizes our soil to resist erosion by wind and water while aiding in nutrient holding capacity and cycling. Over the past century, we have observed high rates of soil erosion in the Palouse resulting in soil degradation. The key to this is the loss of soil structure due to aggressive tillage and the reduction of organic matter (OM). One of the most important components of soil structure is OM which influences stability, aids in water storing capacity, nutrient cycling, and helps to buffer soil acidity. These are all fundamental to maintaining soil quality and health.
Soil structure is the arrangement of the soil solid particles (sand, silt, and clay) allowing for open pore space between the particles. The ideal soil components include: air 25%, water 25%, minerals 45%, and OM 5%. Structure is determined by how the individual units are arranged, bound together, or clumped to form aggregates resulting in the arrangement of soil pore spaces between them. The clumping is the result of OM (carbon) and biology activity such as earthworms and fungi.“ Carbon is the food for the soil that builds the aggregates and makes the glue. Green plants take in carbon dioxide from the air and secrete carbon exudates into the soil.” Jay Fuhrer, Natural Resources Conservation Service, Menoken, ND. “When it comes to the glue, the fungi do most of the work. The fungi are comprised of long filaments that branch through the soil. Tillage breaks the filaments making it hard for the fungi to survive.”This soil structure provides stability to the soil and resists erosion while influencing water infiltration, water holding capacity, air movement, microorganism activity, root growth, and seedling emergence. “Without soil structure, future operations may compact the soil by squeezing out the pore spaces between the soil aggregates.” Paul Jasa, Extension Engineer, UNL.
Tillage, one of the leading causes of soil structure breakdown, has been heavily used in the past. The loss of structure reduces the infiltration of water by more than a 10-fold rate and adds to soil compaction. Dr. Stewart Wuest, USDA ARS reported ½” per hour water infiltration on conventionally tilled soils while no-till soils allowed up to 5” per hour infiltration.
Figure 1. A diagrammatic representation of well structure and poorly structured soils. Source: Victorian Department of AgricultureThe reduction of tillage over time will help to re-build soil structure, but it will take time to recover from past traditions. The removal of crop residue will ultimately slow the recovery as OM will continue to decline adding to the stability problem. OM is a friend of the soil and is critical to soil health and quality.
View the soil aggregate stability test demo (video) by Doug Sieck’s Ranch in North Central South Dakota.
For questions or comments, contact Paul Carter via email at email@example.com, Associate Professor, WSU Extension and Columbia County Director.