Moisture Sensor Landscape Irrigation Design Guide
What is Soil Moisture Measurement and Control?
Before we talk about specifics, let us take a step back and look at the living system in which we will be working.
The Soil, Water, Plant System
Evapotranspiration moves water out of the system and rain or irrigation replaces it. The soil provides storage for the water. Typically, the plant removes water according to the chart below.
Water is pulled by the roots into the plant. The soil is attracting the water as well. This competition between the plant and soil for the water creates tension. As the water is depleted by the plant, it first takes the water that is most easily released and continues to take water that is more and more tightly held by the soil. This tension can be measured by soil moisture measurement instruments. The higher the tension created, the harder it is for the plant to extract water from the soil. Eventually there will be no more “available water” for the plant. Most landscape plants suffer from just the opposite, no oxygen in the soil because of too much water.
In most cases, the ideal situation would be to irrigate until the root zone approaches saturation. Next, allow it to dry out to a reasonable level before irrigating again. This allows the soil to “BREATHE” oxygen in, which is good for both the plant and soil organisms. It also helps minimize the impact of insect and disease problems.
Measuring Soil Moisture
Soil moisture measurement consists of determining the level of moisture in the soil by a direct or indirect method. Control involves interpreting the measurements and taking the action or actions that are most beneficial. One direct method is accomplished by using a device called a tensiometer. (Figure 1-1) This sensitive instrument measures the amount of “tension”, “suction” or negative pressure in the soil. It uses units of pressure called centibars or kilo-pascals. One centibar is one hundredth of a bar. One bar is about 14.7 pounds per square inch (psi). So when you see a gauge that reads from 0-100 cb (centibars) it is roughly the same as reading 0-14.7 psi in negative pressure or suction. An indirect method of measuring soil moisture tension is by way of a Granular Matrix Sensor, called the Watermark from Irrometer Company, Inc. (Figure 1-2). This product is calibrated to a Tensiometer and has a range of 0-200 cb. Wire can be run up to 1000 feet or more. It is a low maintenance, low cost product.
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