We have had a lot of questions about sensor installation types in soil and substrate especially with respect to potted plants. We have a blog post that explain how to install a soil moisture sensor in rockwool. In this post, we will look at potted and field installation types and some examples.
Soil moisture sensor installation type is greatly affected by its design. There are many sensors that are primarily designed for measurements in the soil and cannot be installed in soilless media. There are sensors that are optimized for measurements in soillees media (rockwool, coco coir, etc), but can also be easily installed in the soil. The APAS T1 soil moisture and temperature sensor from DurUntash Lab is a good example of such sensors.
Field soil moisture sensor installation methods may be categorized into the following:
Trench installation
Multiple hole installation
Potted soil moisture sensor installation is a bit different and can be carried out as:
In-pot installation
Side pot installation
Top-down pot installation
Drainage monitor installation
These methods are explained below.
Trench installation is labor-intensive
Installing sensors at different depths through the side wall of a trench is an easy and precise method, but the actual digging of the trench is a lot of work. This method puts the probes in undisturbed soil without packing or preferential water flow problems. But because it involves excavation, it is typically only used when the trench is dug for other reasons or when the soil is so stony or full of gravel that no other method will work. The excavated area should be filled and repacked to about the same density as the original soil to avoid undue edge effects.
Multiple-hole installation protects against failures
Digging a separate access hole for each depth ensures that each probe is installed into undisturbed soil at the bottom of its own hole. As with all methods, take care to assure that there is no preferential water flow into the refilled holes, but a failure on a single hole doesn’t jeopardize all the data, as it would if all the measurements were made in a single hole. The main drawback to this method is that a hole must be dug for each depth in the profile. The holes are small, however, so they are usually easy to dig.
Single-hole installation is the least desirable
It is possible to measure profile moisture by digging a single hole, installing one sensor at the bottom, then repacking the hole, while installing sensors into the repacked soil at the desired depths as you go. However, because the repacked soil can have a different bulk density than it had in its undisturbed state and because the profile has been completely altered as the soil is excavated, mixed, and repacked, this is the least desirable of the methods discussed. A single-hole installation may still be entirely satisfactory for some purposes. If the installation is allowed to equilibrate with the surrounding soil and roots are allowed to grow into the soil, relative changes in the disturbed soil should mirror those in the surroundings.
Side-pot installation is the least desirable
On the field scale, we install individual sensors at multiple depths to monitor the rooting profile. In the potted plant we want to monitor to depth whatever the size of our pot. The dimensions of the pot will define the rooting profile for mature plants. Additionally, for potted plants we have the option of inserting sensors through the side of the container. This method, however, is not recommended because the sensor might damage plant roots or the pot (depending on the pot, and sensor shape and size), create a pass for water to drain out of the pot, and will not be able to monitor the whole root zone.
In-pot installation
This method works a lot like installing soil moisture sensors in the field, but involve a lot less digging and can be carried out much quicker. The main issue here is the disturbed soil and we need to make sure it is pack, as much as possible, to its original density. Otherwise, you should just give the soil enough time to regain its structure. A good practice with this method of installation and others is to install soil moisture sensors at the beginning of the growing period. This will minimize any potential damage to the roots.
Top-down installation is preferred
Depending on the size or dimensions of the pot, we have the option to insert the sensor into the surface of the potting mix. This is handy for smaller pots and when we want minimal disturbance to the roots.
Sensor installation for monitoring drainage
For pot in pot or pot in ground we want to monitor the drainage. Look for the drainage holes and install sensors near these.