What is a Granular Matrix Sensor?
Granular matrix sensors (GMS) are an option for measuring soil water to schedule irrigation. Irrigation of crops highly sensitive to water stress, like potatoes, onions, and many other horticultural crops require precision irrigation scheduling, determining both irrigation frequency and duration.
Soil Water Potential:
An expression of the energy level of water in the soil system. This is contrasted to the amount of water present in the system for which water content is the fundamental parameter. The soil water potential is of direct importance to plants because it is the force necessary to remove water from the soil. Soil water potential is usually expressed in centibars, cb, or kilo Pascals, kPa. The scientific units, kPa, were conveniently defined so that 1 kPa = 1 cb.
Soil Water Tension:
Soil water tension is numerical similar to soil water potential with the opposite sign. For example a "soil water tension" of 20 cb would be a "soil water potential" of -20 cb.
Soil Water Content:
The amount of water present in a given volume of soil.
A Granular matrix sensor for electronically measuring soil water has been patented (Larson, 1985: Hawkins, 1993) and is marketed as the Watermark soil moisture sensor (Irrometer Co., Riverside, CA). Granular matrix sensor technology reduces the problems inherent in gypsum blocks (i.e., loss of contact with the soil by dissolving, and inconsistent pore size distribution) by use of a granular matrix confined in a metal case. Granular matrix sensors operate on the same electrical resistance principle as gypsum blocks and contain a wafer of gypsum embedded in the granular matrix. The electrodes inside the GMS are embedded in the granular fill material above the gypsum wafer. The gypsum wafer slowly dissolves, to buffer the effect of salinity of the soil solution on electrical resistance between the electrodes. According to Larson (1985), particle size of the granular fill material and its compression determines the pore size distribution in GMS and their response characteristics.
For many soil types, growers have found that GMS are a useful tool to schedule irrigations. As plants use water from the soil, the soil dries and water is drawn out of the sensor. Sensor resistance increases. Upon irrigation or rainfall, the GMS takes up water and the resistance decreases.
Granular matrix sensors have been calibrated to soil water potential and used to estimate soil water potential for irrigation decisions. GMS can substitute for tensiometers in irrigation management when irrigation criteria based on soil water potential have been established (Shock and Wang, 2011). Because GMS do not require periodic maintenance during the growing season, they are ideally suited for sensing soil water potential to automatically start an irrigation system as we have been doing since 1995. GMS have advantages of low unit cost and simple installation procedures, similar to those used for tensiometers. Data acquisition with GMS can be remote from the measurement site by use of electrical wires or radio, so the plants and soil at the measurement site remain relatively undisturbed.