Several different methods have been developed to measure soil moisture (see https://agrismartgree n.com/talajnedvesseg/talajnedvesseg-ertelmezese-es-merese/ for details).
The soil moisture meters available on the market measure soil moisture either in volumetric percentage by volume or in tensiometric percentage. In practice, 4 types of soil moisture measuring instruments are widely used:
Instruments measuring tensiometry, matric potential, water potential:
- tensiometers
- plaster, Granular Matrix, ceramic coated sensors
Volumetric moisture meters:
- FDR capacitive sensors
- TDR electromagnetic sensors
How the tensiometer works
The tensiometer works like an artificial root, measuring how hard it is for the plant to suck water out of the surrounding soil. Tensiometers consist of a ceramic element, a plastic or metal tube and a vacuum gauge with an airtight connection. Water can diffuse through the ceramic element into the soil and back into the tensiometer. Water always flows into the lower pressure range. When the soil dries, the tensiometry increases (in the negative direction) and water leaks out of the tensiometer. Since the device is closed, the vacuum created is just the amount that balances the external tension. This value is indicated by the attached vacuum gauge.
A tensiometer measures the force needed to break water away from soil particles.
They are available with different lengths of tubing to allow measurement at several levels in the root zone.
Tensiometers are the simplest moisture measuring instruments for irrigation decision-making.
The required suction force is indicated in pressure units (mbar, hPa, kPa or cbar).
The higher the value, the more force the plant’s roots need to exert to absorb water. Since water uptake is also a prerequisite for nutrient uptake, as the crop grows, it can absorb less and less nutrients and its growth slows or stops, which in the long run can lead to yield loss or dieback.
In the previous blog, there is a table with the maximum stress-free tensiometry limits recommended for each plant. https://agrismartgreen.com/talajnedvesseg/talajnedvesseg-ertelmezese-es-merese/
Using a tensiometer to control irrigation by keeping the soil below the water potential of the plant provides the optimum conditions for the plant in terms of water uptake.
In terms of water uptake, there are 4 conditions that can occur in a given soil-plant type site:
- Saturated soil: there is water but no air, and the water runs off into the deeper layers,
- Water capacity: exactly how much water a soil structure can hold without runoff,
- Useful water: the plant can take up water in a stress-free range,
- It is impossible: even if there is water, the plant cannot absorb it from the soil.
| Saturated soil with air deficiency | Outdoor water capacity | Usable water resources Optimal water supply | Setting point, stress situation, yield loss, dehydration |
| Tensiometry: 0 – 10 kPa | 10 – 33 kPa soil dependent | 30 – 1500 kPa plant dependent | 30 – 1500 kPa plant dependent |
Types of typical strain gauges available on the market:
| Principle of measurement | Device type | Manufacturer | Measuring range (kPa) | Sensed parameters | Cost | Image from | Direct water pressure measurement | Ceramic-headed tube, dial gauge | Irromatic
Bambach | 0 – 60
|
Visual manometer
tensiometry reading |
* |  |
|---|---|---|---|---|---|---|
| Direct water pressure measurement | Ceramic-headed tube, digital dial gauge | Bambach | 0 – 75
|
Display visual
tensiometry reading |
* |  |
| Direct water pressure measurement | Ceramic-head measuring tube vacuum sensor | Bambach | 0 – 100
|
Stress proportional to tension,
manual measurement or data logger
|
** |  |
| Direct water pressure measurement | T5 mini ceramic head, pressure sensor | METER (EMS) | 0 – 160 | Stress proportional to tension,
manual or data logger |
***** |  |
| Electrodes embedded in plaster, ceramics, impedance | Watermark
200SS |
Irrometer | 0 – 200 | Signal proportional to tensiometry
manual or data logger |
* |  |
| Ceramics
embedded capacitance measurement |
ATMOS-21 (MPS-6) | METER
(Decagon) |
9 – 500 | Tension and temperature SDI-12, data logger | **** |  |
| Polymer pressure measurement | Full Range | UGT | -100 – 1500 | Tension and temperature RS485, SDI-12 data logger | *****
|
 |
| Heat capacity measurement | TensioMark | ecoTech | 0 –106 | Tension and temperature SDI-12, analogue, data logger | *****
|
 |
The theoretical upper limit of a water-based ceramic head tensiometer is around 80 – 90 kPa, because water boils at 20C and -92.3 kPa and cannot measure any further due to bubble formation. Another limiting factor for ceramic tensiometers with water content is the use in too dry soil, where the equilibrium is significantly disturbed and water leaks through the ceramic body into the soil and air leaks into the tensiometer. In this case, after the appropriate amount of watering, which will bring the soil back into the range of use, the tensiometer must be refilled and calibrated to resume use.
There are tensiometers equipped with techniques to shift the boiling point of the liquid used in the device, which can operate up to 100 – 160 kPa, but they are cumbersome to use and are mainly designed for laboratory research.
Or there are completely new tensiometers that can measure up to 1.000.000 kPa (see Full Range, TensioMark tensiometers). Because of their price, they are mainly used in research areas.
Advantages of conventional tensiometers
Inexpensive and accurate indication of water potential, easy to use to start irrigation. Neither the salinity nor the temperature of the water will affect the result. Installation is simple.
Tensiometric sensors are very suitable for detecting the water-holding thresholds of different plants before optimal stress conditions. In the case where no maximum tensiometric value is given for a given plant, the plant’s response can be used to infer the stress threshold and to control irrigation accordingly.
Disadvantages of Tensiometers
Response times can be slow, several hours, several days. Depending on typical soil conditions, it may be necessary to top up the tensiometer during the production season. If the average tensiometry remains below 15kPa, i.e. the soil is consistently moist enough, 1-2 times per season can be expected. If higher tensiometric values occur regularly, i.e. the typical soil condition is drier, air may leak from the soil into the pipe, making the measurement unreliable. In the event of prolonged dry soil, the water will drain out completely, the instrument will stop working, the tensiometer will need to be refilled and recalibrated. The water used must be treated to prevent algae growth. It is sensitive to frost and should be picked up for winter.
Use of tensiometers for irrigation control.
The tensiometer is a good indicator of the irrigation limit, but the amount of irrigation water that needs to be applied to fill the soil to the water capacity limit cannot be calculated from the tensiometer.
Volumetric water content sensors can be used for this purpose.
Volumetric sensors will be discussed in a future blog.
Tensiometers for data collection, automatic control.
There is a wide range of tensiometer versions with electronic sensors, which are also suitable for unattended data collection.
The Watermark sensor is the most widely used in tensiometre automation.
Using the Watermark gypsum-ceramic sensor
The ceramic-gypsum structure has a stainless steel mesh on the outside. The ceramic coating filters water and the resistance of the built-in electrodes increases as a function of the water deficit. Tension cannot be measured directly, as in conventional tensiometers, but can be determined from the resistance between the electrodes by a temperature-dependent approximate function.
The resistance value can be converted to water tensiometry (kPa, cbar) according to the conversion table provided by the manufacturer.
It has a measurement limit of 200 kPa, but dries out easily above 100 kPa tensiometry. The typical operating range is 10 – 100 kPa. The Watermark sensor has difficulty absorbing water when it dries out, requiring 3-4 watering cycles to recover its operating water content. It can be revived more quickly by removing it from the ground and placing the ceramic head in a water bath.
It also retains its over-wetted state for a long time, days may pass during which the water content of the ceramic-gypsum body adjusts to the water content of the surrounding soil. It is not frost-sensitive, there is no obligation to collect sensors for winter, and its life expectancy is 3-5 years.
Resistance measurements should be made with alternating current (AC), because the galvanic effect of DC measurement will quickly destroy the sensor. AC measurement requires a more sophisticated data logger, few of which are available on the market. Irrometer manufactures an interface unit that allows the Watermark sensor to be used with conventional data loggers.
The article was written by: Csaba Tóth, email: toth.csaba(at)tmarkt.hu
Next blog: Tensiometric difference based automatic watering devices.
Links:SmartGreen tensiometer price list (in preparation)
