Measuring how much water a plant needs

Rather than measuring the water content in the soil, Watermark soil water-potential sensors help farmers determine how much water the crop needs, says SASRI researcher Ashiel Jumman. Glenneis Erasmus reports.

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Watermark soil water-potential sensors are a good tool for continuous soil water monitoring, Ashiel Jumman, a researcher from the South African Sugarcane Research Institute (SASRI) in Mount Edgecome, told delegates at the South African National Committee on Irrigation and Drainage’s symposium, held at Langebaan, Western Cape.

Ashiel and another SASRI researcher, Neil Lecler, tested the Watermark sensors’ effectiveness in measuring soil water potential levels in KZN sugarcane fields. They used the Watermarks in combination with H8 Hobo four channel data loggers (see box: Recording the data: the science of loggers).

Watermarks pass the test
Before the study there was strong evidence the farmers in the study were under-irrigating their crops, and could improve yields with more water. The Watermark sensors proved this right. Not only did the data provide evidence of substantial water stress during critical growth periods, it also showed there was potential to save water early in the growing season.

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Ashiel said this proves the monitoring system can add value – the relatively inexpensive information it provides can help inform irrigation management decisions and optimise water use.Compared to neutron probes and time domain reflectors (TDR), the sensors are compact, easy to use and relatively cheap. They don’t require specialised knowledge to record and interpret measurement data or, unlike neutron probes which use radioactive materials, strict operation, transportation and storage safety programmes.Capacitance sensors are relatively cheap, but the small volume of soil they scan might not be representative of the whole sample, says Ashiel.

Strong advantages
He adds that volumetric soil water sensors, including capacitance sensors, neutron probes and TDR, indicate the amount of water present in a given volume or depth of soil. In contrast, soil water potential meters – such as the Watermark – measure the energy the crop needs to suck up water. As such, they’re a more direct indicator of potential crop stress, and whether or not the soil is too dry.

The Watermark can operate over a broader range than tensiometers. “Tensiometers are limited to soil water potentials above –75kPa,” Ashiel explains. “If the soil dries out to below –75kPA, air enters the device and breaks the vacuum with which tensiometers operate, so they’re high maintenance apparatus.” Watermarks are also more robust than gypsum blocks. Gypsum blocks break down and dissolve over time, so the calibration relationship between gypsum block readings and soil water potential isn’t fixed.

The downsides
Watermarks’ one disadvantage is that they’re sensitive to soil temperature, which must accordingly be monitored and factored into the calibration equation. Ashiel adds that downloading data can be tedious and time-consuming if data is required frequently, for example to make irrigation application decisions. But he proposed that remote access to data via General Packet Radio Service (GPRS), for example, might help remedy this.

What and how
The Watermark sensors consist of two concentric electrodes in a porous reference matrix material, wrapped in a synthetic membrane. The membrane helps protect them and makes them last longer. The sensors also have a stainless steel mesh and rubber outer jacket, making them more durable.“The porous sensor retains water in the same way as the soil,” explains Ashiel. “It gets wetter and drier as the surrounding soil does.”

“Water moving between the soil and the sensor changes the electrical resistance between the electrodes inside the sensor,” he continues. “A calibration equation can use this electrical resistance to calculate soil water potential.”
E-mail Ashiel Jumman at  [email protected] or call the South African Sugarcane Research Institute on (031) 508 7400.  

Recording the data: the science of loggers

H8 Hobo four channel data loggers, from the Onset Computer Corporation, record the data collected by sensors like the Watermarks. When they tested the Watermarks in cane fields, South African Sugarcane Research Institute researchers Ashiel Jumman and Dr Neil Lecler used them in combination with H8 Hobos. The H8 Hobos are readily available, relatively inexpensive and only require a small watch-type battery. They’re also small and inconspicuous, making it highly unlikely they’d be tampered with or stolen.

The Hobo logger uses DC current to excite the sensor, while the Watermark sensors are better suited to using high-frequency AC excitation. DC excitation can cause electrode polarisation over time, as cations or anions migrate to the electrodes. But Ashiel points out the Hobo excites all sensors simultaneously, completing reading in as little as 10 to 40 milliseconds, so there’s little time for migration to occur. However, electrolysis occurs at the electrodes when the excitation persists for more than two milliseconds. This results in micro gas bubbles, which alter the water medium’s resistance and therefore the sensor reading.

Ashiel explains the resulting distortion in the sensor readings could be addressed by using a different calibration relationship for each channel of the logger. The combination of the Watermark sensor and Hobo logger was calibrated in a pressure plate chamber by Prof Simon Lorentz and JJ Pretorius, both from the School of Bioresources Engineering and Environmental Hydrology at the University of KwaZulu Natal.