im not quite sure to be honest i have only heard and/or used it through foliar feeding as thats what it says to do in a book i dont think it would be usuable in a root drench but i have no facts to back it up... as for bwills its so hard to get rid of spider mites the best way to go about it probably would be to buy a bunch of lady bugs and let them lose and those suckers because they eat spider mites believe it or not, also you can vacuum your leaves gently if your not in flower, or you can purchase SM-90, or use a homeade hot pepper solution:
1/2 ounce (15g) dried or 4ounces (60g) fresh habanero peppers
2 tablespoons (30mL) of vegetable oil
1/4 teaspoon (1g) lecithin granules
1/4 teaspoon (1g) wetting agent
water to make up on pint (450mL)
.... thats a recipe it gives in the book for "capsaicin" which is the substance that gives hot peppers their heat.