uflnuceng shared a nice writeup on results of soil moisture testing on the Forums:

Ok, using the code above, I setup a DHT22 and an SHT10 as you can see in the image below. The DHT22 was used to give me ambient air temp and relative humidity, while the SHT10 was placed in different media to determine 1) its speed of change, 2) reliability of data reported, 3) consistency of results.

…Temperature was very consistent, and humidity differed within the margin of error of the sensor (+/- 1.5%).

I then placed the SHT10 sensor in some dry soil that hasn’t had a plant in it for a very long time.

As it should be expected, the soil was very dry, but being hydroscopic it contained a little bit more humidity than the air around it (23.4% relative humidity versus ambient air’s 15.6%).

I then picked a plant that the office didn’t mind me killing. I let it go without water for a few days, although by the finger touch the soil was still damp.

I then set the sensor to record every 5 minutes for several weeks and this is the resulting graph of that data….

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