Others have recommended that I dry soil using an oven, and then re-apply water to attain the required level of moisture (10%). One recommendation was to use 1mL water per 10g dry soil to reach 10% soil moisture, however I think this assumes soil density. How else would you ascertain how much water to add per unit weight of soil?
Thanks!
If you are setting up a soil experiment, you should avoid drying and rewetting the soil because this can introduce all sorts of unwanted effects (e.g. major changes in respiration, onset of hydrophobicity...). Rather take a few sub-samples of the soil, weigh these before and after drying in an oven, and use the change in weight to calculate what the water content of the soil is right now. After that, you can calculate how much water you need to add to reach 10% water content.
You don't need to assume density to make these calculations if you are aiming for a weight-based moisture content (0.1 g water / g soil doesn't depend on the soil density). If you want a volume % water content, then you do need to know the soil density.
Reply / answer given by Kyle Mason - Jones is correct, you can calculate moisture in % as well as in weight.