proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
Dear Peter and Colin Not only does one have to worry about roots near the sensor end, but one should also worry about roots behind the sensor and along the sensor wire all the way to the measuring device. Years ago we did a pot experiment using thermocouple psychrometers to measure the soil water potential. We had planned to force the rye grass roots vertically by having vertical holes in the pot. However, the plants took much longer to die than we had hoped for. The indications were that they were getting water from elsewhere in the profile. We abandoned the experiment and I had a look at the roots in a number of pots. The roots had moved vertically but they had also filled up the (horizontal) air space behind the psychrometers - we had many so this was lots of space. The hole diameter is roughly that of the sensor diameter, not the wire diameter. The air space is created when the psychrometers are inserted and they do need to be installed horizontally to reduce thermal gradients. A way around the problem would have been to fill the void at the rear of the psychrometers with soil. This would be tedious process. An alternative is to use a dowel rod, painted with water-proof paint, to fill the void. The wood is of low thermal conductivity and would not perturb the flow of energy and water as much as the psychrometer does. The rod would prevent roots from getting into the void space. The problem would be much more difficult in shrink-swell clays. Regards Michael J Savage SPAC Research Unit Department of Agronomy University of Natal Pietermaritzburg South Africa