Root temperature does play an important part in plant growth. Root zone temperature has been shown to have and influence on plant dry mass, uptake of nitrogen, phosphorus and potassium.
It was found with tomatoes that phosphorus and potassium uptake was increased with increasing root zone temperature. However, root zone temperature had no effect on nitrogen content of the plant.
It is important to note that when the night temperature is higher than during the day, plant growth is adversely affected. This applies both to air temperature and root zone temperature.
The optimum root temperature for most sub-tropical crops (tomatoes, peppers and brinjals) is in the region of 25°C. Root growth stops at temperatures below 10°C. Lowering the temperature of the root zone to 15°C reduces the uptake of nitrogen, phosphorus and potassium which caused some problems with fruit quality.
Lettuce gave excellent results with root zone heating with head mass, root mass and length increasing up to 30°C in the root zone.
The most vulnerable period during plant growth when heating the root zone is in the first 2 months of growth. Heating the root zone without close monitoring can cause the root zone temperature to increase above 35°C due to radiation. Care must be taken during this period not to overheat.
This technique should not be attempted by closed systems since the danger of water diseases increases with increasing water temperature.