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Maximize Living Roots

4 Spoke WheelThe principle of maximizing living roots in soil year-round is the fourth spoke. Roots nourish microbes by providing a food source or by Surface of a plant rootsreleasing nutritious compounds into the soil. It is estimated that plants release from 10-40 percent of the carbon fixed by photosynthesis through the roots. This carbon increases soil organic matter. Five different types of organic compounds released through roots are:

  1. Cells are continually falling off root tips as roots grow;
     
  2. Root tips produce an insoluble lubricating gel that helps the root penetrate small pores and provides the root tip protection against drying, helps gather nutrients, and binds soil particles together into aggregates that allow for better soil aeration and water percolation;
     
  3. Soluble compounds called exudates are produced and leach from the root surface. These leacheates include organic acids, amino acids, proteins, sugars, phenolics, and other substances easily decomposed by microorganisms. The exudates have many functions; for example, they can solubilize plant nutrients such as phosphate from the soil particles, change the redox state on the root surface making iron more available, desorb nutrients from clay surfaces, or chelate zinc;
     
  4. Sugars are fed directly by roots to fungi and bacteria that live in symbiosis with roots. Most well-known examples are rhizobia that fix atmospheric nitrogen in legume roots, and abuscular mycorrhizal fungi that form bushy structures inside root cells connected to hyphae that extend the reach of the root into surrounding soil (See the figure to the right).
     
  5. Dead cells are being lost from the root surface continuously as the roots develop. Subsequently, decomposer fungi and bacteria feed upon these dead cells located away from root tips.

Aggregate Stability-The importance of living roots for soil structure improvement was evident in a Penn State study in which aggregate stability was measured at different times of the year in different crop rotations (See the figure to the left). The study showed that aggregate stability increased during the period of active root growth and subsequently decreased when the root system declined. In the rotations with perennials, aggregate stability was always greater than in annual crop rotations due to prolonged presence of living roots.