Soil Science

Infiltration

Soil moisture tension key

Infiltration is the entry of water into the soil through the soil surface. The presence of macropores dramatically increases the infiltration rate. Water movement through smaller micropores is much slower.

The soil is saturated when all the pores are filled with water. The soil moisture tension at saturation is 0 kPa.

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Drainage

Soil moisture tension key

Under the influence of gravity, water is drained from the macropores of a saturated soil.

Micropores are able to hold water against the force of gravity due to capillary forces (adhesion between water molecules and solid molecules).

The soil moisture content after 1-2 days of drainage is called field capacity, and the soil moisture tension at this time is approximately 10 kPa.

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Evapotranspiration

The soil is dried below field capacity by evaporation and transpiration. As the soil dries, water moves from zones of low soil moisture tension to zones of high soil moisture tension. The rate of this soil water movement (unsaturated hydraulic conductivity) can be very slow.

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Evapotranspiration - continued

Soil moisture tension key

Plants are able to extract water from the soil until the soil moisture tension reaches about 1500 kPa. The moisture content of the soil at 1500 kPa is termed permanent wilting point.

Only pores smaller than 96 nm (0.0001 mm) equivalent diameter can still hold water against a tension of 1500 kPa.

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Evaporation

Soil moisture tension key

Evaporation is able to dry soil down to soil moisture tensions of 10,000 kPa, called the air-dried soil moisture content. At this moisture content, all water is tightly bound to the colloidal material (clays and organic matter in the soil).

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