Horticultural grade vermiculite. Very fine coarseness (granules slightly larger than sand). Vermiculite is a silicate of magnesium-aluminum-iron that is treated at high temperatures which causes it to expand. It is used as a soil amendment to increase the water and nutrient holding capacity of soil. Vermiculite aerates soil and can be used to treat compacted soil or soil that tends to compact easily. Vermiculite has a neutral pH of 7.