The Amount of Vermiculite for Garden Soil

Seed-Starting Medium

• Experts at the University of Arizona recommend using vermiculite as part of a potting-medium mixture to start seeds in. A mixture containing 1/3 potting soil, 1/3 vermiculite and 1/3 peat provides good aeration and moisture retention for seeds. Alternatively, a mixture of 4 quarts of vermiculite, 4 quarts of shredded sphagnum peat moss, 2 tbsp. of ground-up limestone and 1 tbsp. of superphosphate works to start seeds. Try using these mixes in pots to grow seedlings to transplant to the garden. The mixtures also work to fill outdoor raised beds or planter boxes, but keep in mind that grow plants will require some fertilizer if planted in one of these mixtures.

Seed-Spreading Mixture

• It is hard to spread tiny seeds evenly across the garden soil. The National Gardening Association recommends mixing 4 parts vermiculite or sand with 1 part small seeds before scattering seeds. This mixture helps distribute tiny seeds, like carrot and wildflower seeds, more evenly. Use just 1 part vermiculite and 1 part seeds to help distribute larger seeds evenly.

Seed Covering

• Overly dense and crusted soils make it difficult for seedlings to sprout through the surface of the soil. The National Gardening Association recommends covering newly planted seeds with vermiculite instead of soil. Vermiculite is especially helpful as a cover for tiny seeds and in very compacted soils. Use the same amount of vermiculite to cover seeds as the seed packages recommend for soil.

Adding Vermiculite to Soil

• Vermiculite helps aerate and loosen up garden soil. Consider tilling some vermiculite into heavy clay garden soils to improve drainage, water retention and the release of nutrients from fertilizers into the soil. The amount of vermiculite needed for garden soil varies with the soil density; add more vermiculite for heavier soil and less vermiculite for soil that only needs a little bit of loosening. To start out, spread an inch-thick layer of vermiculite on top of the surface of the soil and mix it in with a tiller or garden fork. Add more if water still puddles up on the surface of the soil or if the top layer of soil forms a crust as it dries.

Warnings

• The U.S. Environmental Protection Agency warned that vermiculite packaged before 1990 might contain asbestos. New vermiculite should be safe, but avoid using old vermiculite.