This article is part of a series of technical documents that we wrote to help you understand how our products work. This one is about Irrigation Management. Below is a list with all of our articles about this subject:
Soil Tension (this article)
Soil tension is another way to measure soil moisture. As the soil gets drier, the plants roots need to exert more force to extract water from the soil. Tension is a negative force that moves the water upward from the soil to the root, so if less water can be found on the soil the force needed to move it will be higher. In other words, tension and moisture are inversely related. The unity of measure for soil tension is kilopascal (kPa), a pressure unity (since pressure = the force exerted over an area).
Hence the tension of moist soil (CC) is lower than the tension of dry soil (PMP). However, there is some debate regarding the CC soil tension. According to the literature, it is common to consider that the CC moisture content is determined by a tension of 5 kPa on sandy soils, 10 kPa for medium-textured soil, and 33 for kPa for clay soil (FRIZZONE, 2017; SILVA et al., 1999. The tension for a PMP soil is usually 1500 kPa.
Finally, soil moisture has a linear behaviour, but soil tension has an exponential trend. This means that a soil with 32% of moisture content can have a tension of 33 kPa, but if it looses 1% of moisture, the tension can increase up to 42 hPa. Observe the blue line with diamonds:
Example of three different soil types and the exponential spacing of soil tension and linear spacing of soil moisture. Obs.: 1 kPa ≈ 0.01 bar ≈ 10 mca.
SILVA, E. M. et al. Manejo de irrigação por tensiometria para culturas de grãos na região do Cerrado. Planaltina: Embrapa Cerrados, 1999. 60p.
FRIZZONE, J. A. Necessidade de água para irrigação. Piracicaba: ESALQ/USP, 2017. 56p.
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