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Quick Guide: Soil Moisture Sensors

Precision irrigation improves yields, reduces labor, and conserves water. Using soil moisture meters for irrigation scheduling, especially for agriculture, is one of the easiest and most cost-effective ways to save water and reduce costs. There are many soil moisture sensors available today, and learning the basic differences between them will make it easier to select the right one for your application.

Types Of Soil Moisture Sensors

The most common types of soil moisture sensors include gypsum blocks, tension meters, capacitance, volumetric, and neutron probes. These sensors either measure soil tension when placed in the soil or measure volumetric water content.

Soil Moisture Meters That Measure Tension

These sensors measure tension in the soil and report how much energy it takes plants to pull water from the soil. When there is adequate moisture, it is easier for plants to pull water, and as the soil dries, plants work harder to draw water.  The unit of measure is a centibar. When soil is full of water, the soil’s tension is close to zero, and as water evaporates, drains and used by plants centibars increase.  Depending on the plant and the soil when the centibars reach a specific number, its time to water.

Think of gypsum block soil moisture sensors as two electrodes buried in plaster. When water enters the block of the gypsum, it becomes less resistant. The blocks have two wires extending out of the block and connecting to a handheld reader or data logger. This data logger provides a reading that indicates when to water depending on plants and type of soil. It takes some time for the gypsum to absorb moisture, so there is a lag with these sensors, and they break down over time, so you need to replace them every couple of seasons. They are relatively easy to install and not expensive.

Capacitance Soil Moisture Sensors

These sensors measure water volume in the soil by releasing an electrical charge into the soil, which measures the soil’s dielectric constant. 

The soil has different dielectric constants based on the amount of water in the soil. This reading happens quickly and can be communicated to Jain Logic software for irrigation scheduling purposes. Growers don’t have to drive around looking at dials; they get all the information they need on their computers, phones, tablets, or pads.

Pros And Cons Of Capacitance Sensors

These sensors are durable, robust, and low maintenance. They also provide soil moisture measurements at various soil depths if you want. You can purchase them in various lengths (great for trees with deep roots) and fix sensors at each 4″ section to observe water movement through the soil. Some offer soil temperature and soil salinity as well as fertilizer measurements too. They can be attached to Jain Logic software providing the ability to view soil moisture remotely from any Internet connected device. These devices are more expensive, but the added value you receive more than makeup for the added cost.  

Pros And Cons Of Soil Tension Sensors

Tensiometers are accurate and affordable.  Centibars are easy to read on the dial, so it is easy to know when to water. The challenge with them is they require more regular maintenance than common volumetric or capacitance probes.  If you miss a refill or refill incorrectly, you will get bad data. Additionally, these devices only have one depth they report from, so to get the same number of readings as the other probes for a 48″ root zone, for example, you would have to buy, install and maintain 12 tensiometers at every site you wanted to monitor soil moisture.   As you can see, this is not a scalable method of measuring soil moisture over an entire operation like sealed probes are. 

Soil moisture sensors provide a window to view water at multiple points in the root zone without having to take multiple samples or using a backhoe or other digging devices to see below the soil surface.  We know how much water the soil is holding at any point in time. We track trends over time; every year is different, and even small differences can affect how we chose to irrigate in the coming year.  Some capacitance probes have inputs that are temperature sensitive, and report soil temperature to assist in timing our chemigation injections.  This is a tool that provides near real-time insight into what our soil profile is doing. 

Take a look at this informative episode of the Jain Irrigation training series on soil moisture sensors.

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