Soil Sensors

Everything You Need to Know About Soil Monitoring Instruments

Why is Soil Monitoring So Important?

Soil is an important natural resource, just as the air and water that surround us are. Unfortunately it has been overlooked in the past and taken for granted with disastrous results, such as the North American dust bowl of the 1930s. Today, the role of soil health on our climate as a whole is taken more seriously, with researchers at organizations such as the USDA-ARS (US Department of Agriculture, Agricultural Research Service) looking into how exactly soil interacts with the rest of our environment. Because of ongoing research and general interest in soil health and sustainability growing every year, monitoring soil in a more substantial and quantifiable way is becoming more important. Monitoring soil in the past meant going out and physically handling the soil, taking samples, and comparing what was found to existing knowledge banks of soil information.

While nothing will replace actually going out and handling the soil for basic information, today’s technology makes it possible to remotely monitor soil and track parameters that simply can’t be easily or quickly measured by hand. Soil probes are now extremely accurate and offer an unparalleled look at what is going on below the surface. They give instantaneous information on soil moisture content, salinity, temperature, and more. Soil sensors are an important tool for anyone involved with soil, from a small-town farmer trying to increase his crop yield to researchers looking at how soil retains and releases CO2. More importantly, just as computers have increased in power and dropped in price due to economies of scale, advanced soil measurement systems can be found at prices that are affordable for everyone.

How Do Soil Sensors Work?

Soil moisture sensors (or “volumetric water content sensors”) measure the water content in soil, and can be used to estimate the amount of stored water in a profile, or how much irrigation is required to reach a desired amount of saturation. These sensors can be portable and used for instant measurements or installed for long-term monitoring.

No commercially available soil moisture sensors measure water directly. Instead, they measure changes in some other soil property that is related to water content in a predictable way. The other soil property becomes a proxy for water content. Common soil properties that change in relation to water content and are easy to measure include dielectric permittivity and matric potential.

Sensors that measure dielectric permittivity are the most common type of soil moisture sensor. These sensors use different technologies to measure the permittivity of the surrounding soil.

Regardless of the technology used in a sensor, the same principle applies: the bulk dielectric permittivity of soil changes with volumetric water content.

A simple way to think of permittivity is as stored electrical energy. The sensor generates an electric field in the soil. Because water molecules are polar, unbound water molecules in the soil rotate to line up with the electric field lines.

The rotation of unbound water molecules requires energy—stored as potential energy in the aligned water molecules. More water in the soil stores more energy, and the higher the bulk permittivity of the soil will be.

In order for any soil probe to work, no matter the type, it must make contact with the soil. The greatest accuracy will be obtained when the soil probe is fully surrounded by the soil, with no gaps or air holes between the probe and the soil. The probe then sends electrical signals into the soil, measures the responses, and relays this information to a data logger (or directly to the cloud with an Internet-connected wireless network).