Measuring soil moisture for growing purposes is typically done in one of two ways:

Irrometer soil moisture measurement is based on the tensiometric method, because of the fact that the amount of water is not as important as how difficult it is for the plant to extract it from the soil. 


Soil water tension (or matric potential) has to be overcome for the plant to move water in to its root system.  Different soil types will have different tensions even at the same volumetric measurement, making volumetric information relative to local conditions and often requiring site calibration for reading equipment.  Because we use soil water tension, there is no site calibration required when using our sensors.  


The chart to the right illustrates the relationship between soil water tension and soil water contents.  The contents are listed as percentages of the Available Water Holding Capacity (the amount of water which can be held in the root zone between the wilting point of the plants and the field capacity) depleted, derived from volumetric measurements.  Clearly demonstrated is the difference in volumetric based measurements for different soils at the same tension value, which is what really determines the availability of moisture for plant material.


Due to the fact that tensiometers have been used in research since the 1920's and have been commercially available (from the Irrometer Company) since 1951, years of published research by numerous universities and extension agencies have produced a wide field of reference for recommended tension levels to use with common crops and landscapes.  Soil moisture information provided by Irrometer equipment is inexpensive, accurate, and relevant.

The tensiometer is the only direct measurement system available, which means that it actually reads the physical forces at work in the soil. Tensiometers act like a dummy root, allowing the soil moisture to interact with the instrument through the ceramic tip.  Soil water tension outside of the instrument tries to remove the water from it, which creates a measurable tension inside the column.  This tension is read with either a mechanical gauge or a transducer attached to the instrument.  While this is the most accurate and proven method available, there is some maintenance required periodically to keep them full of water, and they must be removed from the field during the winter months to avoid freezing.