When I was in school, in crop physiology class (as I recall), I thought I learned that soil temp is lowest around 7:00 am at 2" deep, and around 11:00am at 4" deep.  In other words, it takes time for the change in soil temp to move down thru the soil profile.  That idea makes sense to me.

However, as I look at your data, that idea does not hold up.  It appears that the lowest temp in the day, and the highest temp in the day, occurs around the same time of day for both the 2" depth and the 4" depth.  It also appears that the lowest soil temp in the day, and the highest temp in the day occurs *about* 3 hours (+/-) after the highest air temp occurs.  That would make sense to me. I'm just surprised there is not the difference I expected for the 2" and 4" depths.

Thanks for doing that.