This test is very simple. Just put a big handful of soil into a pint jar and fill it up with tap water, put a lid on it and shake it a couple of minutes to put the soil in a solution. Then let it sit a few hours until it has settled. This will only tell you how much of the basic soil elements there are in the sample, i.e. sand, silt, clay, organic matter, nothing specific. It is used as a guideline as for what to add to soil. You were worried about moisture levels dropping. If you have a lot of sand in that soil that is probably the reason your garden soil dries out rapidly. And to fix this you add finished compost or well dried and aged manure. I worry about that soil having stuff in it that is not good, for you and your plants. It may be perfectly safe and then it may not.