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Competency Area 2: Soil hydrology AEM

PO 12. Know the relationship between soil water content, soil water tension and soil pore size and the following soil parameters (and qualitatively understand how these parameters vary for different soil types) and their relationships to plant growth and the fate and transport of nutrients and pesticides.

  1. Field Capacity
  2. Permanent Wilting Point
  3. Available Water Capacity
  4. Total Soil Water Storage Capacity
  5. Drainable Porosity
  6. Soil Texture and Structure
  7. Macroporosity/Preferential Flow

Macroporosity/Preferential Flow

Macropores refers to those soil pores through which water flows primarily in response to gravity. Macropores occur in coarse sands and gravels, soil structural cracks, or may form as the result of worm holes, other small burrowing microorganisms, decaying roots, and some tillage operations. Since water can be infiltrated quickly and flows rapidly downward in macropores, it is also termed preferential flow.

The significance of macropores and preferential flow is that nutrients and other dissolved and suspended substances can be rapidly transported down past the root zone without substantial filtration or other biochemical remediating interactions. Although the magnitude of macroporosity in soils is generally small, when only a small concentration amount of a nutrient, pesticide or other contaminant creates great risk to water quality, the environmental threat may still be significant. Macroporosity is generally beneficial to air and water exchange, soil health, and to providing more optimum conditions for plant growth, but it has also lead to water quality impacts when dissolved and suspended materials are transported to tile drain outlets or groundwater.

Since water flow into and through macropores is most prevalent when soils are already in their wettest state, avoidance of applying potential contaminants during this time and prior to rainfall is one method of minimizing unwanted impacts. Applying nutrients or other materials at lower rates will also reduce concentrations of contaminants occurring in preferential flow.