Thursday, March 1, 2012

Soil Health: Level 2 - Soil Structure

Soil macroaggregates create a soil structure which is permeable (by air and water) and stable (resistant to compaction and erosion).
  • Soil aggregates result when Mycorrhizal Fungi bind soil particulates together. To be more specific Mycorrhizal hyphae, or filaments, are responsible for increasing a plants overall root mass. These hyphae extend throughout the soils surrounding a plant, and in their search for water and nutrients end up binding soil particles together. As hyphae die and begin to decompose they release Glomalin into soil systems. Glomalin is a glue-like protein which significantly increases aggregate formation, by gluing organic matter to soil particles. This process, of binding Labile carbons (as you recall, organic matter contributes to the Labile Carbon pool) to soil particles, traps these rapidly decomposing carbons in the soil; thereby storing them for future use and preventing these carbons from being released into the atmosphere.  
  • Humic Acids have been shown to increase aggregate formation. Please see Level 2 - Carbon Compounds for a detailed description of Humic Acid's role in aggregate formation.  When Humic Acids combine with clay particles, we get the Humus-Clay Domain where most of the Nutrient holding and water holding capacity of the soil takes place!
  • Once these aggregates have been formed, as a result of both Mycorrhizae and Humic Acids, water and air can easily penetrate the soil. Macroaggregates can also be described as Water Stable Soil Aggregates. The term Water Stable Soil Aggregate can be a bit confusing. First of all, "Water Stable" means that a soil which contains water stable aggregates will not collapse when water enters the system (these aggregates remain in tact during a slake test, which is a measure of the disintegration of soil aggregates when exposed to rapid wetting); as compared to a soil which does not contain these water stable aggregates and will collapse, thereby preventing water from penetrating. Once you have fostered the creation of water stable soil aggregates, water can enter the systems and form small pools in the aggregates. These pools hold moisture, gases, bacterial colonies and labile carbons, all of which increase soil and plant health.
This image shows a macroaggregate (on the left) holding its structure during a slake test. The cylinder on the right does not clearly illustrate how a compacted soil will prevent penetration of water, but it does show how much soil will be lost as water runs off a compacted soil.
  • Tilling of soil will destroy these structures, increase erosion and create soil compaction. Tilling of soil reduces stability, by destroying the structural web created by Mycorrhizal hyphae.  Tilling destroys worms passages and other structural spaces created by soil organisms. Worm passages are tunnels which help the soil breath, by letting in water and oxygen and releasing other gases. 
  • Please see No Till and Water for interesting information on this topic.
Written by:
Aurora Fabry-Wood
Staff Biologist