A great deal of what affects the accuracy of soil moisture sensor measurements (or at least the part that is in your control) has something to do with sensor installation. That’s why I’m going to provide tips that will help you install your sensors correctly, and by that minimize measurement errors. I will try to answer three important questions:
How to install soil moisture sensors?
How many sensors do we need?
Where to install the sensors?
How to install soil moisture sensors?
Here’s a list of what to avoid when installing a soil moisture sensor:
Soil disturbance
Air gaps
Foreign objects
Preferential flow
Root interference
Low elevation areas
Soils with low water holding capacity
Tops of hills
Dead plants
Below, I have provided more details on some of these titles.
Soil disturbance. To avoid measurement errors, minimize soil disturbance. Any change in the structure of the soil changes its density and capacity for holding water. The best way to deal with a soil that is disturbed during the installation is to just give it some time to regain its structure.
Air gaps. Leave no air gaps between the soil moisture sensor and the soil. Air gaps can significantly affect volumetric water content measurements and cause serious measurement errors.
Foreign objects. Keep surface debris out of your installation hole. A friend of mine, always uses a tarp to place the excavated soil on. Repack the soil to the native density as much as possible. Do not overdo this.
Preferential flow. Preferential flow of water can be a problem. Avoid creating an easy flow path for water down to the sensor during the installation process. Mound the soil above the hole to prevent preferential flow.
Root interference and other things. Sensor installation spots should be representative of the rest of the field. Do not install the sensors in places with the lowest water holding capacities, rocky spots or where you have dead or damaged plants. Avoid locations that are low areas or tops of hills.
How many sensors are enough?
There’s a simple rule of thumb: the more the better, but you also need to know where to install your moisture sensors. It won’t matter how many sensors you have in the soil If the installation spots are not chosen wisely.
Where to install sensors? How deep?
As a rule of thumb, we recommend installing two soil moisture sensors per spot at 1/3 and 2/3 the depth of crop rooting zone. If you’re going to install only one soil moisture sensor per measurement spot, install it in the middle of the root zone ( ½ of the crop effective root depth). In the case of installing two sensors, the shallow to tell “when to irrigate”, and the deep one “when to stop”.
If readings of the deep sensor go up faster than the shallow sensor, apply more water with each irrigation event. If readings of the deep sensor indicate high soil water content, there is a chance you’re putting too much water in the soil and causing deep percolation and nutrients leaching.
You may ask how am I supposed to know where the roots (or effective roots) are? As a general rule for soils without restrictive layers, about 70% of the roots are in the top 50% of the root zone.
If you have some extra sensors, you may install one sensor below the root zone. This sensor can tell you if you’re putting too much water in the soil (deep percolation).
In orchards irrigated using a drip or micro-sprinkler irrigation system, soil moisture sensors should be installed near the edge of the wetted soil volume. Make sure not to place the sensors too far from emitters (or dripper or micro sprinklers), or too close to them. If sensors are too far, the wetting front may not reach the sensors or may take too long, and this can lead to excess watering of your plants. Installing sensors too close to emitters can lead to under-watering of your plants. I recommend some trial and error to find the best distance from emitters in your soil.
Some useful tips
1) Flag sensor installation spots in the field so that you can easily find them and to know at what depth they’re installed.
2) Be patient with sensor measurements and data collection and stick with what you’re doing. It may a couple of years before you are good at interpreting your soil moisture sensor readings.
3) Talk to experts. Sensors alone are never enough. People with experience in using sensors for irrigating scheduling can help you get up to speed faster.
For more tips read my previous blog post titled “How to Make the Best of Your Soil Moisture Sensor?”
References
Peters, TR. Practical Use of Soil Moisture Sensors for Irrigation Scheduling. irrigation.wsu.edu
Werner, H. Measuring Soil Moisture for Irrigation Water Management.pubstorage.sdstate.edu