- Author: Ben Faber
So, every few weeks the question comes up of whether to install soil moisture meters which leads to the question of which to buy and install or have installed. And then come the questions of what do the readings mean and why aren't the readings consistent. Or maybe this question arrives after the grower has installed the sensors or system and the values don't conform to a known or knowable pattern.
The first question to the grower is why they want to install soil moisture sensors or a system. Everyone has a different answer which I've always found interesting. Usually it boils down to having more or better information, although it's hard to beat a good old soil auger. Which takes time and labor.
So once that is cleared up, it comes down to what area they want to monitor. Is it an acre, 10 acres, 50 acres, 100 acres, 200 acres, 1,000 etc.? What are the different irrigation blocks, soil types, aspects? How complex is the area that is to be monitored? Do they need one monitoring site or many? Can the information be gathered in the field, or does it need to be accessed from a distance? Linked by hardwire, infrared, cell phone, wifi, satellite, etc.?
Then the question is does the grower do the installation or is it done by a company? And then whatever the case is, who maintains the system and for that matter, who maintains the information? What software is used and who interprets it?
And what sensors are being used: tension, electrical resistance, conductance, capacitance, electromagnetic…….? The list seems to go on and there are no models and brands coming out on a regular basis. And how reliable are the sensors? What's their lifespan? And what are they measuring and in what units? How affected are the readings by salinity and what soil volume are they measuring? And how important is their placement?
This last point is so often overlooked. The sensor needs to be in the active root zone where water is being taken up. Not where it's convenient to read, not where the plants cant use the water. Placement is so often overlooked.
And then how much do you want to spend? $100 per installation, $1000? With a monthly or yearly maintenance fee or none? Who responds when there are problems?
Wow, so yeah, there are lots of questions. Here's a chart that might help categorize some of the questions:
Method |
Cost |
Ease of use |
Accuracy |
Reliability |
Salt-affected |
Stationary |
Gypsum block |
L |
H |
H |
H |
L |
YES |
Tensiometer |
L |
M |
H |
M |
L |
YES |
Portable tensiometer |
M |
M |
H |
M |
L |
NO |
Solid-state tensiometer |
M |
H |
H |
H |
L |
YES |
Time domain reflectometer |
H |
M |
H |
H |
M |
BOTH |
Neutron probe |
H |
L |
H |
H |
L |
YES |
Feel (soil probe) |
L |
H |
H |
H |
L |
NO |
Gravimetric (oven) |
L |
M |
H |
H |
L |
NO |
Conductance |
L |
H |
M |
M |
H |
BOTH |
Capacitance |
M |
H |
M |
H |
M |
BOTH |
H, high; M, medium; L, low
And the good Almond Doctor might help some more:
http://thealmonddoctor.com/2015/07/10/soil-moisture-sensing-systems/
And maybe some of these publications can help sort out what questions to ask
http://calag.ucanr.edu/Archive/?article=ca.v054n03p38
http://calag.ucanr.edu/Archive/?article=ca.v054n03p47
http://anrcatalog.ucanr.edu/Details.aspx?itemNo=21635
Khaled Bali, our Irrigation Specialist at Kearney Research and Education Center near Fresno, is part of a group in the process of evaluating different types and models of soil moisture sensors. He should have a publication that can more accurately sort through the many sensor choices that are available at this time. But in time, there should be more models on the market and new update will be necessary.
The question, though, is to ask yourself how irrigation is being done and how it can be improved. The basics of design, maintenance, distribution uniformity and how scheduling is being currently done – when and how much to apply. Definitely, soil moisture sensors can help, but you gotta know how to use them and maintain them, just like the whole irrigation system.
A grower who uses tensiometers told me that people think of soil sensors as though they were reading a book. Something cut and dried. A simple plot line that you follow. Irrigation is not a book. There are many other subplots to irrigation than just reading the digital face. Looking at the weather, evapotranspiration, the tree, how fast the moisture is depleted, how deep the moisture is being pulled from all contribute to the "sensors" used to irrigate. Use them all. Even though this grower has irrigators on 250 acres of trees, he also checks the orchard tensiometers at least once a week on his own to confirm all of his senses.
- Author: Ben Faber
Soil moisture sensors fall into two broad categories, volumetric and tensiometric methods. One tells you how much water is in the soil and the other tells you how tightly the soil holds on to the water. Volumetric methods require a calibration of the sensor to the soil, whereas tensiometric is good to go when installed. For both methods, the grower learns to keep soil moisture within a given range of values and, in theory, the plant is kept in a better condition with improved health and yields and potential improved irrigation efficiency.
The most common volumetric methods rely on utilizing the dielectric constants of the soil and water, with water’s dielectric constant being much greater than soils’. The velocity of an electromagnetic wave or pulse depends on soil moisture content. These sensors have become widely used because they have a good response time, do not require maintenance and can provide continuous readings, allowing for automation. There are several different methodologies used: Time Domain Reflectometry, Frequency Domain Reflectometry (Capacitance), Amplitude Domain Reflectometry (Impedance), Phase Transmission, and Time Domain Transmission. There is quite a range in: (1) prices for these different devices, (2) requirements for calibration with soil moisture content, and (3) requirements for close soil contact. Some devices are affected by the chemical nature of the soil. Even if they are not calibrated they can be used as relative change in moisture content over time.
The tensiometric methods include: Tensiometers, Gypsum Blocks, Granular Matrix Sensors, Heat Dissipation and Soil Psychrometer. These techniques require the sensor to come into equilibration with the soil moisture and generally are unaffected by soil salinity. Gypsum blocks and Granular Matrix are not very responsive in sandy soils and require good soil contact. These methods are less affected by salinity and do not require soil calibration, because they are reflecting the soil moisture tension the roots are seeing.
All soil moisture sensors need to be placed in a position that represents the irrigated area. They need to be placed in the active root zone where water is applied and taken up and must be near trees representative of the whole irrigated area. They should not be next to a sick tree, a smaller tree compared to the other trees or be in an area that obstructs applied water (such as under a canopy that intercepts applied water). It is always best to reinforce sensor values with manual field measurements with a soil probe to ensure that sensor placement and response is truly reflective of what is going on in the field, before completely relying on the sensor values. As with all field equipment, occasional visual inspection of the field and sensor readings should be made to make sure the situation has not changed, such as an emitter has clogged or broken near the sensor.
Volumetric Sensors
|
TDR |
FDR |
ADR |
PT |
TDT |
Appx. Cost (including logger/reader)+ |
$400- 20,000 |
$100-3,500 |
$500-700 |
$200-400 |
$400-1,300 |
Field Maintenance |
No |
No |
No |
No |
No |
Affected by salts |
High levels |
Minimal |
No |
>3dS/m |
High levels |
Soil type not recommended |
Organic, salt, high cay |
None |
None |
None |
Organic, salt high clay |
+ Prices as of 2009
Tensiometric Sensors
|
Tensiometer |
GB |
GMS |
HD |
SP |
Appx. Cost (including logger/reader)+ |
$50-300 |
$400-700 |
$200-500 |
$300--500 |
$500-1000 |
Field Maintenance |
Yes |
No |
No |
No |
No |
Affected by Salts |
No |
>6 dS/m |
>6dS/m |
No |
maybe |
Soil Type not recommended |
Sandy |
Sandy, high clay |
Sandy, high clay |
Sandy |
Sandy, high clay |
These sensors can be purchased individually and installed by the grower, or increasingly there are companies that provide a monitoring station that includes soil moisture, sensors for estimating plant evapotranspiration , data logger and software that determines an irrigation schedule for the crop. Some of these systems are outright purchase and some are for lease. In the future, there will be affordable satellite imagery that can help in irrigation scheduling, showing how rapidly this technology is changing.