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8.1.3 How does the soil moisture sensor work?
The sensor consists of two concentric electrodes buried in a special reference matrix material that
is held in place by a stainless steel case. The matrix material has been selected to reflect the
maximum change of electrical resistance over the growth range of production crops. Soil
moisture is constantly being absorbed or released from the sensor. As the soil dries out, the
sensor moisture is reduced and the electrical resistance between the electrodes is increased.
The Watermark provides accurate readings from 0 to 200 centi bars. This covers the entire soil
moisture range required in irrigated agriculture, even in the heavier clay soils. The Watermark
does not dissolve in the soil like a gypsum block. However, it does include internally installed
gypsum which provides some buffering for the effects of salinity levels normally found in
irrigated agricultural crops and landscapes. Because they are unaffected by freezing
temperatures, Watermark sensors do not require removal during the winter months in cold
climates.
8.1.4 What does the sensor reading mean?
The Watermark measures soil water tension or suction which is a direct indicator of how hard the
plant root system has to work to extract water from the soil. The drier the soil, the higher will be
the reading. By monitoring the sensors between irrigations, it is possible to measure the rate at
which the soil is drying out. The "rate of change" is as important as the actual reading in
determining when to irrigate to avoid moisture stress.
8.1.5 Determining "When' to Irrigate
Figure 8-1
shows how variations in soil affect the ability of the soil to store water (water holding
capacity).
Figure 8-1. Water holding capacities of different soil types
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