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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

Total Soil Water Storage Capacity

The total soil water storage capacity refers to when all the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded. A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels will have the lowest total porosity of around 30 to 40% by volume. Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40 to 60%. Restricted drainage conditions can cause the soil to attain its total porosity water content, at which time free water is observed and perched water tables develop (in layered soils) or the apparent water table is found near the surface.

When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is severely restricted. Most agricultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen diffusion to the roots. Without air-filled pores, the concentration of carbon dioxide and other gases like ethylene increase, producing toxic conditions and limiting plant growth. Root cells switch to anaerobic respiration, which is much less efficient than aerobic respiration in converting glucose molecules to ATP (adenosine triphosphate, the chemical energy within cells for metabolism and cell division).

As anaerobic (reduced) conditions develop in the soil, nitrification ceases and denitrification is enhanced. Corn plants will quickly yellow in response to this saturated soil state as nitrogen becomes limiting, and the plant tries to adjust by producing more adventitious roots. Prolonged anaerobic conditions in the soil starts to reduce manganese, iron (causing phosphorus to be more soluble), sulfur (producing hydrogen sulfide), and eventually methane gases. Hydrophytic (wetland type) plants are adapted to saturated soils because they are able to obtain oxygen through other forms of plant structure adaptations (i.e. pneumataphores, lenticels, aerenchyma).