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Competency Area 2: Soil hydrology AEM

PO 11. Understand factors that affect:

  1. Soil Infiltration
  2. Evaporation and Transpiration
  3. Leaching
  4. Runoff
  5. Soil Water Storage


Leaching is the removal of soluble material(s) from soil or other permeable material by the passage of water through it. Leaching depends on whether or not there is a soluble material dissolved in the soil water, and whether or not this soil water is moving, such as via deep percolation. Thus, many different soil chemical, physical and biological properties and interactions can influence the degree of leaching. For example, if one adds common table salt (NaCl – sodium chloride) to the soil, it can easily dissolve into separate sodium (cation) and chloride (anion) ions. Since soil (i.e., clay) particles exhibit a negative charge, some of the sodium may bind to the soil, whereas the chloride will not. When percolation occurs, the chloride is then more readily leached (transported) downward through the soil.

Different soil types have different percolation (infiltration or hydraulic conductivity) rates. So when adding NaCl to a gravelly soil, as compared to a silt loam, more chloride will be leached through the gravel because it has a higher percolation rate. Also, more sodium will be leached through the gravelly soil, compared to the silt loam because the gravelly soil also binds less of the sodium (has a lower cation exchange capacity). Similarly, nitrate-nitrogen is more subject to leaching than ammonium-nitrogen because of its solubility and availability in the soil water, and more of each will be leached through a gravelly soil because of its higher percolation rate.