First tests with E-5 soil moisture sensor and ECH2O handheld monitor (not strictly a datalogger since it doesn't store readings). An identical set was recently tested with Kenyan students by VeSeL member Cecilia Oyugi. It can relate to soil information, for example here.

The readings are uploaded in a simple form which feeds a Google spreadsheet.

I stuck the sensor in this pot of rosemary. The first reading - on this slightly drizzly day - was 14.5 percent water content in the soil. Soils typically are 50 percent dirt and 50 percent air pockets which can be filled with water. Rosemary is generally happy with a wide range of soil moisture levels (see this dissertation for more), and although my soil is on the dry side as I suspected, Rosemary can be grown successfully in drought conditions, according to this article.

Soil moisture levels are typically measured by running a small voltage through two metal conductors placed close together, and measuring the voltage passing through them - this will vary according to the medium they're placed in, and water will ease the flow of electrons from one post to the other. A simple DIY soil moisture sensor can be made using two galvanized nails - see for example this article or this article.