I am a little late coming in to this thread, but thought I would chip in with what I know. First a little disclosure; I am a distributor for a unique form of DE who's mineral name is Pozzolan. I sell it under the brand name Lassenite and it is the same material from the same mine as Higromite. It is diatoms fossilized in volcanic ash. It is 100% inert, so has no nutrient value for plants. It will hold in excess of 100% of it's weight in water while retaining an average of 15-20% non-capillary pore space. It has the best moisture release curve of any inorganic soil amendment available. Those are the basic facts on the material. Beware of using materials such as "Floor Dry" etc. These products are designed for absorbing oil, but might not be so great at releasing water to plants. The same holds true for expanded clays, shales granites etc. The reason for this is the pore size of the material. The key to finding the best inorganic soil amendment is to know the pore size. The smaller the pore size, the more difficult it is for plants to draw moisture from them. Materials with small pore sizes tend to hang onto salts as well. Some, such as zeolite based products, will actually attract salts. Lassenite on the other hand will not attract or retain salts. For more on this you can see the study done by MSU on my website
AquaFirst Technologies Inc.