Since soil temperatures appear to be a significant factor in plant growth, I'm wondering if there's a standard procedure for taking soil temperature readings. Soil temperature is a spectrum throughout the root zone,very hot at the surface, and a good deal cooler 9-12 inches deep.
This makes me wonder about Arisumi's results where the entire root ball as well as the above-ground parts of the plant were essentially the same temperature. Roots have probably not evolved to tolerate such high temperatures, perhaps leading to the poor growth and performance seen in the higher temperature ranges—temperatures which are not uncommon in many parts of the US.
In Arisumi's experiment, were the plants held at the stated temperatures continuously, or was there a day/night swing?
