Soil Matric Potential

Capillary matric potential is sometimes referred to as tension or pressure head (ψ, hPa) is the cohesive attractive force between a soil particle and water in the pore spaces in the soil particle/water/air matrix. Typical ranges are 0 to 10,000,000 hPa where 0 is near saturation and 10,000,000 hPa is dryness. The drier the soil the more energy it takes to pull water out of it. Capillary forces are the main force moving water in soil and it typically will move water into smaller pores and into drier region of soil. This process is also called wicking.

Because of the wide pressure ranges that can be observed from very wet to very dry conditions, matric potential is often expressed as the common log of the pressure in hPa. The log of the pressure is called pF. For example 1,000,000 hPa is equal to a pF of 6.

Water potential is highly texture dependent. Clay particles have a larger surface area and thus will have a higher affinity for water than that of silt or sandy soils. The most common methods for measuring or inferring the matric potential include granular matrix sensors such as gypsum electrical resistance blocks, and tensiometers which measure pressure directly.

Heat dissipation-type matric potential sensors measure the matric potential indirectly by measure the heat capacitance of a ceramic that is in equilibrium with the soil. With heat up and cool down cycles of heating elements in the ceramic, the heat capacitance can be calculated and in turn calibrated to the matric potential. Heat capacitance-based matric potential sensors such as the ecoTech Tensiomark offer advantages in accuracy, range and maintenance over other technologies.

Soil water retention curve: soil matric potential verses soil moisture

Matric potential is important for irrigation scheduling because it can represent the soil water that would be available to a crop. Many unsaturated flow models require a soil water retention curve where water fraction by volume is plotted with the matric potential in a range of moisture conditions. A soil water retention curve can help understand the movement and distribution of water such as infiltration rates, evaporation rates and water retentions (Warrick 2003). 

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