A soil temperature control system for ecological research in greenhouses

We designed and tested a soil temperature control system for plant ecophysiological experiments in greenhouses and growth chambers. The system consists of a plywood box, polyethylene liner, insulation, seedling containers, a water pump, and a flow-through heater or chiller. One hundred and twelve seedling containers (11cm diameter, 13.5 cm high) are mounted in the plywood box. There is a hole at the bottom center of each container to allow the free drainage of irrigation water and fertilizer solution. The space between containers is filled with water that is circulated through the chiller/heater. The water is also circulated within the plywood box by a water pump to increase the uniformity of temperature. The system was tested for three soil temperatures (5, 20, and 30°C) over a period of four months. The containers were filled with a peat-moss vermiculite mixture and planted with tree seedlings. The test showed that the soil temperature was almost equal to the water temperature for all three soil temperatures (regression slop = 0.99, intercept = 0.12,r ² = 1.00). The average soil temperatures were within (0.41°C of the set values. The soil temperature of the 112 containers within the same box followed a normal distribution with a small standard deviation (0.34°C for the 30°C treatment). There was a temperature gradient from the top to the bottom of the container (< 1°C). The direction of the temperature gradient was determined by the direction of temperature difference between the soil and the ambient air. When the soil temperature was lower than air temperature, the soil temperature decreased from the top to the bottom of the container, and vise versa. The soil temperature was higher during the day than at night (difference < 1.5°C). Funding for this research was provided by Lakehead University, CFI, ORDCF, and NSERC research grants to Dang, and an NSERC PGS A to Cheng.