Re: soil mosture
basically, you should ask: what property you want or you need to know. "Soil moisture" is rather a colloquial term than designating an exact property. In enviromental engineering (don't know about agriculture), water content is mostly defined as percentage of dry material contained in a sample, a gravimetric measure. It can be excactly determined only by drying the sample.
As an approximate value, a capacitive measurement is often used, but it has to be calibrated with similar material. In contrast, vapour pressure respectively dew-point, as measured with the said hygrometer, is a completely different property,
although the two are apparently related.