The plentiful days this October with sunshine and warm air temperatures have kept soil temperatures rather mild for the month. The featured chart presents an hourly time series from an ISU Soil Moisture Network station near Ames. This station has a soil temperature reading at a number of levels from two inches down to a depth of 40 inches. The chart is a beautiful illustration of heat transfer processes in the soil. You first notice the difference in variability from the very noisy two inch depth to rather consistent 40 inch depth. The shallower soil depths are closer to the heating by the sun that occurs at the soil surface and conversely have a smaller buffer to the air for loosing heat at night. The heating during the day works downward with a lag as you can see the warming response lags. The effect of having multiple warm days in a row eventually works its way down to 40 inches and also conversely, the effect of multiple cooler days in a row as well.

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