The soil moisture readings are not what I expected. Is my sensor malfunctioning? Is my readout device broken?
These are some of the questions we occasionally get from our customers. Some are experienced cultivation managers who are new to sensors and some are newbie gardeners. Regardless of your experience and familiarity with growing plants and sensors, these are absolutely normal questions.
Please remember that good irrigation practice requires three elements:
Science, Experience, and Sensors.
Good irrigation practice requires three elements:
Science, Experience, and Sensors.
Soil moisture sensor is just a variable in the equation, and it is useless on its own. Nothing can replace a good “human” irrigator. Sensors, however, can be helpful if installed and used properly.
Those who experience unexpected sensor readings are highly likely to either blame the manufacturer for selling them a sensor that is not “good” or is not calibrated. This is while, in most cases, the root cause of the problem is lack of experience with sensors and/or not being familiar with the basics of irrigation scheduling.
In this article, I will try to provide some information on how to troubleshoot your moisture monitoring system (regardless of its brand) and take better readings.
Quickly Test Soil Moisture Sensor
Moisture values that are too high, too low, or do not change after a long period of time can always raise suspicion. The first step to troubleshoot your system is, of course, to make sure your sensor(s) and readout device are not broken. For example, if you are reading a moisture value of 10% in rockwool 24 hours after irrigation, you may want to check your monitoring system.
To make sure that your sensor is functioning properly, you only need a container full of water and your readout device. Start by taking a reading in the air and write it down. Then, gradually insert your sensor in the water (wait 60 sec between steps) and take more readings. The numbers should change from a minimum value (reading in the air) to a maximum value when the sensor is fully submerged. For example, the minimum and maximum values for the APAS T1 moisture sensor in rockwool are about 0% to 100%.
After this stage you can insert the sensor in a substrate (e.g. rockwool, coco coir) that has been given enough time to fully saturate, but with no plant in it. You can try different methods of installation (e.g. vertical, horizontal) that work with both your sensor and substrate. If the numbers in the water were as expected, but not in the substrate, you may be experiencing one of the issues that are explained in the following paragraphs.
Perched Water Table
If your moisture sensor is residing in a so called “perched water table”, it will show high moisture readings. A perched water table, as the name implies, means water “perches” or gathers at the bottom of your pot (or even rockwool cube). This means the substrate stays saturated for long and causes an upward movement of moisture due to capillary force. This not only does not allow the moisture level to decrease as quickly as you expect, but also damages plant roots in the long run due to lack of aeration in the waterlogged substrate.
In outdoor soil, this happens when rain or irrigation water stops or slows down and rests on a dense, less permeable layer of the soil. The fix is to wait for the water to be used by nearby trees, or relocate the sensor to a better spot. It would also be helpful to create a mound of soil above the spot where the sensor is installed. In container plants and rockwool, you should avoid watering too much and install the sensor in the unsaturated zone right above the perched water table.
Please remember, the main purpose of irrigation scheduling using moisture sensors is to avoid such situation!
Sensor Might Be Loosely Installed
This happens quite often in coco coir. You might need to fully insert your sensor into the substrate or completely bury it if there is room. You can bury it horizontally (recommended) or vertically. If you bury it horizontally in the middle of the pot, it will be less affected by the perched water. You also need to tie the sensor cable to something so using it when taking measurements does not move the sensor body that is in the soil.
Air Gaps in Sensor Installation
Air gaps can result in sudden reading changes. When the water fills the gaps, the sensor reads high. When the water goes away it reads significantly less. Water also loves air gaps and tends to find its way to those and gather around the sensor (preferential flow of water). It’s a common problem in sensor installation. Good installation needs experience. You might need to relocate your sensor several times, take some readings and settle for a spot that gives you the best results.
One Sensor Is Not Enough!
It is not easy to carry out irrigation scheduling or take soil moisture readings using only one sensor in a given irrigation zone unless you are experienced enough in using sensors. If you are serious about using sensors for a more efficient irrigation, always invest in more sensors. Knowing this, we have designed our SUMERIT irrigation node with two sensor ports. The node takes average of readings from two APAS T1 moisture sensors in order to decide if the soil is too dry. Both APAS T1 sensors need to be installed in the same irrigation zone.
If you do not have enough number of sensors, you can combine sensor readings with what is called “evapotranspiration” data. The basis for this approach would be daily “plant water use estimations”, which is a function of radiation (light), air relative humidity, air flow, and air temperature. Please read this post to learn about evapotranspiration. In this method, you apply water based on the daily estimations of crop water use. You cab also take substrate moisture readings occasionally to see if everything is ok, but not as a basis for irrigation scheduling.
Measurements Uncertainty
Be careful with interpreting soil moisture values. There is always some level of uncertainty in soil moisture measurements. In the case of the APAS T1 sensor, we recommend interpreting sensor readings with ±5% tolerance. For example, if you re reading 50% moisture, it should be reported as 50 ± 5%, which means the actual water content is in the range of 45% and 55%.
One of the ways to decrease the uncertainty is to average readings from several sensors installed in different spots. So, please be patient and open minded about sensor measurement errors. I have several blog posts on different sources of error in moisture measurements and we have discussed some of them already. Just revisit those for more information.
Measurements in Wrong Spots
It is not possible to monitor all plants in your garden, field or orchard, and you do not need to do that. You need to pick spots that are “representative”, or are critical points. A pot that is expected to run out of water the fastest is a critical point. For example, a pot or rockwool cube with a larger healthy plant in it is a good spot to monitor moisture, because it uses up (evapotranspiration) and finishes the water faster than other plants. A spot that tends to get waterlogged is not appropriate. Please note that your sensors need to stay in one spot during the whole growing period. It is never a good idea to use a single moisture sensor (any brand) to carry out spot measurements.
Your sensors need to stay in one spot during the whole growing period. It is never a good idea to use a single moisture sensor (any brand) to carry out spot measurements.
Moisture Trends Do Not Lie
For irrigation scheduling purposes, we'd rather deal with moisture trends and ranges than absolute values. For example, a moisture range of 30% to 60% (volumetric water content) in a 6” x 6” x 6” rockwool cube might indicate a well-watered status. The numbers might increase and decrease after irrigation, but the trend line will show a decrease and by looking at it you can tell when it is time to irrigate.
Vertical Installation Is Preferred
Vertical sensor insertion in rockwool is our recommended method of installation. Rockwool cubes have a moisture profile that may change from saturation at the bottom to dry at the top. This is highly dependent on the brand and manufacturer. For example, each Grodan cube model has its own moisture profile (retention curve) that is described in its datasheet. Some cubes can hold more water than others.
The APAS T1 moisture sensor is designed to sense moisture along the length of its green blade. Therefore, it reports an average moisture value. To investigate the moisture profile of your cubes, you can insert the sensor horizontally at different heights. If installing the sensor vertically does not give you the number you’d like, you can install it horizontally right at the center of the rockwool cube.
Container Is Too Small
The sensor is installed in a plant container or rockwool cube that is too small (height < 4”) for the sensor. Thus, the sensor is only partially inserted and part of the sensor (e.g. green blade in the case of the APAS T1 sensor) is exposed.
Small rockwool cubes or pots have a small capacity for holding water and need to be irrigated regularly anyway. It might not be necessary to use a moisture sensor with small rockwool cubes.
Sensors Share a Common Ground
In outdoor soil or large plant containers, it is common to install several moisture sensors close to each other in a single trench or borehole and connect them to the same datalogger. This is a terrible practice, no matter what your intentions are (for example monitoring moisture at several depths).
Most inexpensive data acquisition systems with multiple sensor ports use a shared ground for all the sensors connected to them. While this might not create a problem for sensors installed in the air, it can lead to unreliable moisture measurements in the soil and soilless media.
If sensors are installed too close to each other, they might not provide reliable readings. This is because there is more than one pass (might be conductive - ground loop) between the sensor main electrode and the "ground". This means your sensors are not providing independent measurements and are affecting each other's readings. This situation is worse at higher moisture levels (lower ground resistance).
Cable Noise Can Be Avoided
The way that almost all dielectric soil moisture sensors are designed makes their "cable" part of their circuitry. This means the sensor cable behaves like an extension of the sensing part of the probe (could be prong, electrodes or blade) causing what is called "cable noise".
Naturally, the degree of the interference depends on how much of the cable is in contact with the substrate/soil, and to avoid it, you should minimize the contact. This problem is more pronounced when moisture levels are high. I have even seen moisture readings of some commercial sensors decrease (instead of increasing) after a heavy rainfall or irrigation event!
Note: Some sensors like the APAS T1 come with a snap-on ferrite core to the sensor cable that helps minimize the cable noise.
Substrate-Specific Calibration Might Not Be Necessary
Some soil sensor manufacturers sell soil moisture sensors not meant for measurements in soilless media as “soilless media sensors”. Then, they charge their customers several thousand dollars ($$) to calibrate the sensors for soilless media. They usually tell their customers that substrate-specific calibration is necessary and try to convince them that any problem with the sensors or inaccuracy in measurements are due to a lack of calibration.
If a sensor is developed for soilless media, it should not need any form of calibration unless it is especially formulated (custom-made). In addition, sensor calibration at its best can increase the measurement accuracy by only a few percent (%). Ironically, sensor calibration can even decrease moisture measurement accuracy. For example, if substrate temperature fluctuations are too high during the calibration (see this blog post).
There are a few important reasons why substrate-specific moisture sensor calibration is not as beneficial as you might think:
The degree of variability in both soil and soilless media, sensor installation errors, and sensor temperature dependency can be so high that can easily render any calibration useless.
There is no guarantee that sensor calibration equations will hold because they are developed in the lab under certain conditions that will not be met in the field.
So, do not waste your money and time on calibrating moisture sensors while there are more important factors affecting moisture measurements accuracy.
Miscellaneous
Here are a few other reasons why your moisture readings are too low:
The sensor is too close to the edges of the pot or rockwool cube. To resolve this issue, install the sensor as close as possible to the center.
The rockwool or pot was just irrigated and the moisture has not had a chance to fully distribute.
The same rockwool cube is reused or the sensor is inserted in the same opening/hole several times. If you decided to remove the sensor, next time, choose another rockwool for installation. The air gaps created by sensor re-installations, can cause measurement errors.
Sensor Optimized for Rockwool
The APAS T1 moisture sensor is optimized for moisture measurements in soilless media (e.g. rockwool) meaning that it is designed for ease of installation in rockwool, and sensor measurements are scaled and calibrated and reported according to the differences with the soil. Because of this optimization, the APAS T1 sensor does not need that an extra calibration step when used in rockwool.
