A Gusseted Thermogradient Table to Control Soil Temperatures for Evaluating Plant Growth and Monitoring Soil Processes

The overall goal of this video, is to demonstrate the setup, use, and applications of a one dimensional thermogradient table, with gussets and soil. We have developed a new thermogradient table design to provide greater temperature control by welding aluminum strips, or gussets, at intervals perpendicular to the surface of the table. This design facilitates heat flow above the surface of the tables, so that soil, or containerized samples placed between the gussets can equilibrate to a great enough temperature across the table.

This table design can test the affects of temperature on biological and physical processes on replicate samples in a single experiment. Compared to the previous gradient table design, the new table design has nine, three inch high gussets welded to the surface, over the length of the table. Now, LED light fixtures emit photosynthetically active frequencies from the sides of the table, to support seedling growth when the table is closed.

The insulated enclosure covering the new table design, is made of white PVC, and is resistant to warping and cracking. When in operation, two climate controlled circulating baths pump water at a minimum of 10 liters per minute, through the table floor. The temperature of each bath is controlled independently and can be heated or chilled to obtain the precise temperature required for the experiment.

Before operation, ensure the air filters and the fluid reservoirs of the circulating baths are clean. Now, fill each bath to the top of the reservoir tank with an equal mixture of distilled water and antifreeze typically. Next, at both ends of the table, connect the inlets and outlets of the baths to the outlets and inlets of the table.

The tubing must be thick walled, flexible, and have inelastic walls that will not expand under pressure, or kink when bent. Secure the tube attachments using collared screw hose clamps, to prevent leaks. Then, wrap the tubing with insulation;pipe foam insulation works well, to minimize the temperature change of the circulating solution.

Now, open all the valves, and turn on the pumps to check for leaks, kinks, or collapsed tubing. Make adjustments as needed. Finally, check the lighting fixtures, to ensure they are also operating properly.

To begin preparing for an experiment, first line the bottom of the table between the gussets with a hydrophilic material such as greenhouse capillary matting, non-glossy newspaper, or paper towels. This will help distribute the irrigation water more evenly across the table. Next, between the gussets, fill the table uniformly with soil.

For example, a native or synthetic soil might be used. While filling the table with soil, avoid creating air pockets, that would interfere with temperature equilibration and uniform hydration. Now, open the valves and turn on the pumps and temperature controllers, initially, set the temperature of the cooling bath five degrees Celsius below the desired low temperature, and set the warming bath five degrees Celsius above the desires high temperature.

This will account for heat loss and gain during circulation. Next, prepare wireless minature data loggers to record soil temperatures. Wrap each temperature logger in para-film, to avoid water damage.

Then scatter them in the growing media, to monitor the temperature at different positions on the table. Next, set the tilt of the table by adjusting the feet, so there is a very slight slope towards the drain corner. Place a container beneath the drain, to catch any irrigation runoff.

Then, wet the growing media uniformly, around three quarters of its water holding capacity. Wetter soils will conduct heat more efficiently. Be prepared to water the the soil at the warm end of the table more often.

Now, allow the table to equilibrate for 24 hours. Later, adjust the bath temperatures to achieve a desired soil temperature range. Take a quick temperature reading with a thermister temperature sensor, a thermal cupel, or an infrared thermometer.

After adjusting the baths to the desired temperatures, plant the seeds for the experiment. The table may be operated without the lid, when comparing plant growth at different soil temperatures. The inter-transparent covers are transparent plastic sheets, and are used to improve soil temperature equilibration, mitigate evaporative losses, and allow light transmission for plant growth.

The second cover, is a more substantial insulated lid, which should be used when external light, or additional space, is not needed. Throughout the experiment, monitor the system closely for power outages, bath malfunctions, leaks, dried out soil, or excessive fluctuations in table temperatures. Also, be sure to regularly check the reservoir baths and add water and or antifreeze as needed.

For these experiments, the gusseted thermogradient table was stable just 12 hours after set up. It showed acceptable temperature variation at four different positions, measured at three soil depths. For lighting, the side mounted LED grow lights produce all the photosynthetically active frequencies required by the plants, however, other lighting could be used.

For example, when studying the germination of wheat seeds, supplemental overhead fluorescent lighting was used in addition to the LED lighting. The germination experiment was preformed on tomato, melon, oat, and lettuce seeds. From such data, the temperature range for emergence, peak emergence temperature, and fastest emergence temperature are easily calculated.

The gusseted thermogradient table design can be used in a variety of applications beyond the demonstrated germination of seed. For example, with some additional tools, the evolution of carbon dioxide and other gasses can be measured. Because the table can also be operated at subfreezing temperatures, it can be used to simulate frozen road surfaces to test the affects of different de-icing treatments.

Another interesting application for the gradient table, is it can be used for friction testing including subfreezing friction testing.