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How Do Soil Moisture Sensors Work?

If your job includes managing or growing plants, vegetables and, actually, anything else related to agriculture, then you probably know something about soil moisture sensors. However, do you know how do soil moisture sensors actually work?

If you don’t, then you will find out here, don’t worry! I will explain how do soil moisture sensors work, what are they, and what types of moisture sensor technology exist.

So, bear with me and let’s begin!

What is Moisture Sensor?

Water is one of the crucial elements that exist on this planet. Everything depends on it. Humans, animals, plants.

Just as people need to consume water regularly, the same goes for plants. So, if you are in the business of growing plants, fruits or vegetables, you need to take that into account, of course. In order for your plants to grow healthy and beautiful, a proper irrigation system needs to be established.

That’s where soil composition comes into play. Different types of plants require different types of soil, so before you start growing things, you need to check the composition of the soil you have and that’s where soil moisture sensors come in.

It is used to measure and determine the amount of water content that exists in the soil. That way, when the measuring process is completed, usually in around 2 days, you can get the idea whether that place has good enough soil for the growth of plants you intended.

How Do Soil Moisture Sensors Work

So, how do soil moisture sensors meters work? It’s quite easy- soil moisture meter works by measuring soil’s electrical conductivity. Wetter soil will conduct electricity better. There are various types of moisture sensors, and finding the one that has a scale or chart where you can adjust measurements to a certain soil type would be a bullseye for you.

To make things easier for you, I’ve made a list of best soil moisture meters that I already tested. Or if you are bored of lists, here is the one that I’m currently using. Frankly, it does a good job.

There are 3 main types of soil moisture sensor technology. Each operates a bit differently and is meant for a different purpose, and therefore, it costs accordingly.

1. Volumetric moisture sensors

The first type of soil moisture sensors we are going to mention is the volumetric sensor. It can directly measure the exact amount of water which is in the soil. There are several subtypes of this sensor:

  • Neutron moisture probe
  • Heat dissipation sensor
  • Di-electric sensor

The third one is the most common of them all. The dielectric sensor can be used to tell us what amount of dielectric constant is present in the soil. That is an electrical property, which is entirely dependent on the soil composition and of the moisture content present in the soil.

Volumetric soil moisture sensors are the most technologically advanced of all types, thus, they cost the most.

General costs for all components needed is from $500-$1300+, which includes a $100 or more for each sensor and $300-$1200 for the electronic reader.

Despite being very expensive, they are the most reliable choice if you need high efficiency and precision. They are capable to provide data in real time instantly, which is very important for people who grow a lot of crops.

The most common usage of these sensors is when it comes to measuring the soil quality for some highly valued plants.

2. Tensiometer

The only way for the soil to keep moisture is to keep the water in it. And that can be done either by friction or tension. Tensiometers are the specific type of soil moisture sensors used exactly for the purpose of measuring this tension, which exists between particles contained in the soil itself and molecules that come from water.

Plants need to access this water from the soil, and they can do that only by reaching out with their root and overcoming this soil tension in order for the water particles from soil to get to the root itself.

How does a tensiometer look like, you may ask?

Well, it is a vertical tube that contains water and has a porous end. The porous end is supposed to be put inside the ground at a certain depth.

Types of Moisture Sensor Technology

The soil then pours the water out of the porous end, thus creating a vacuum. If the soil is dry, the vacuum is stronger and vice versa.

Tensiometers are very cheap equipment, which is also very simple to use, so people shouldn’t have any troubles using them.

They cost around $70-$150 per unit and the only downside is that they require a bit more careful maintenance. Since the water should be regularly swapped with clean water and it is not recommended to keep tensiometers in the soil through the entire winter, and cold weather.

Apart from that, they should be used in a bit moisty type of soil, so they could provide more effective and true data. If the soil is too dry, they become practically unusable.

3. Solid state sensors

This type of soil moisture sensors is considered to be the cheapest and the most basic you can get. There are 2 main subtypes of solid state sensors:

  1. Gypsum block
  2. Granular matrix sensor

The way they operate is this: they use two electrodes to pinpoint the exact electrical resistance present in the soil. Why is the electrical resistance so important, you may ask?

Well, the higher electrical resistance is, the less water soil contains, because water is known to being able to transport electricity, thus, when the soil contains a lot of water, its electrical resistance is much smaller.

After the electrical resistance properties of the soil are properly measured, it is time to measure the soil tension. That way, you will find out which place and which soil is suitable for your plant.

Solid state sensors cost around $30-$70 and are very easy to use since they don’t require a lot of thinking and too much calibration.

Their main downside is that they just need to be properly set up, which can sometimes be a bit harder. Apart from that, some of their parts, like gypsum blocks should be regularly changed when needed.

Another downside is that solid-state sensors are not very effective when it comes to measuring soil conditions of sandy soil types. It’s because those types of soil are made of larger particles.

And lastly, they are very ineffective in highly salty soil, because data they show could be wrong, due to a higher percentage of salt, which can temper with the readings.

And as you may have guessed out of all of these flaws, solid-state sensors are not effective in an arid climate and arid soil conditions.

Types of Moisture Sensor Technology


Which soil moisture sensor should I get out of these 3 types?

There is really no definite answer to this question. In order to choose the right sensor, you should first keep a clear mind and see what are the conditions you would put that sensor in. Some sensors are good with drier soil, some just won’t work if the soil is not moist enough. So, you need to define all other factors and consider them before buying and choosing the right one for you.

I am not an expert, do I need to pay someone to read the data for me?

Definitely not! Soil moisture sensors usually give pretty straightforward data. So, even if you are not an expert, you would be able to read the data and come to a useful conclusion.

Final Thoughts

Hope I’ve answered your question and that’s clearer now how soil moisture sensors work. If you have more questions on the topic, feel free to share with us in the comments below!!