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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

Drainable Porosity   

The drainable porosity is the pore volume of water that is removed (or added) when the water table is lowered (or raised) in response to gravity and in the absence of evaporation. Consider a soil that is saturated with the water table at the surface. If this soil has a subsurface drainage pipe (tile) buried several feet down and it is discharging to the atmosphere at some lower elevation, the drainable porosity water content will be released to the tile drain until the water table is lowered to the depth of the drain.    

Any nutrients or pesticides dissolved or suspended in this readily drainable pore space will also be carried along with this water, either flowing to the tile drain or continuing downward to the water table via deep percolation if no drainage restriction exists. In large pores, nutrients that might otherwise adsorb to the soil particles (ammonium or phosphate) will bypass the soil because of limited time for contact and chemical reactions to occur with the soil surface area. Soils with a wide range of different pore sizes (sandy loams) or soils with mostly small sized pores are better at filtering nutrients and pesticides as they leach through the soil profile. 

The combined aspect of low available water holding capacity and high drainable porosity for sandy soils causes these soils to have a high leaching potential. It will not take much rain or irrigation (or application of liquid manure) to replenish the available soil water and to raise the soil water content to a drainable state. Applying the proper amount (depth) of irrigation to these soils will both conserve water and enhance irrigation and nutrient use efficiency.