You never really gave any details on how you wish to take the measurement. Ground moisture will vary by depth and time of day, season and so forth. To get a precise measurement of water content, you will need to dig up a sample, weigh it, remove the water (heat), and weigh the dry soil. The difference will be the amount of water removed. I don't remember the conversion numbers if you are looking for a volume figure, but shouldn't be hard to look up.
For continuious monitoring, the resistance method is most likely what you'll be using. Carbon rods will not corrode, you shouldn't have anything in your soil that will have any effect. If you do, might want to contact the EPA, and avoid consuming your vegetables (or whatever type of plant you grow)...
Regardless of the method of measurement, the results will vary by the minute, its natural.
Might take a look at some county extention websites, usually have a lot of agricultural information. Some universities have websites with research information. You will probably find more useful information there, than in this forum. Once you find the what and how to measure, we can help with the electronics.