Many commercial potting soils use vermiculite.

Are There Different Grades of Vermiculite?

by Rob Harris

When your regular garden soil isn't quite doing the trick for your plants, adding vermiculite to the soil might help. Tiny pieces of this mineral expand up to 30 times their original size when heated, creating large amounts of surface area in a small package. With several grade options available, the one that's best for you depends on your soil and where you'll be using the vermiculite.

Basic Vermiculite Grades

The two initial grades of vermiculite are crude and exfoliated. Crude vermiculite hasn't been heated and enlarged, so it's not ready for horticultural use. Exfoliated vermiculite is finished and ready for you to use. Both are graded by particle size as well: large, medium, fine, superfine and micron. Not all particle sizes are common for household use.

Horticultural Grades

When using exfoliated vermiculite in your garden, choose between large, medium and fine grades. Large grade particles can run slightly larger than 1/2 inch in diameter, while medium grades top out at 3/10 inch. The fine grade is tiny, rarely more than 1/10 inch in diameter. Smaller particles tend to become more compact than the larger grades, which help reduce soil compaction.

How It Helps Your Plants

Vermiculite works with dense and loose soils in different ways. The pockmarked surface of exfoliated vermiculite allows the particles to hold large amounts of water, slowly releasing it back into the soil to your plant's roots. This makes it ideal for sandy and loamy soils. It also helps aerate the soil and loosen dense soils such as clay. This can give roots better access to nutrients and prevent waterlogged soil, helping plants thrive.

Where to Use Different Sizes

Large grades work best in outdoor garden soil where you need to mix an additive throughout. Large particles provide more aeration at the same time they help with water retention. Medium grades typically improve potting soil mixes used for containers, whether for outdoor or indoor plants. Fine grades, which have lower surface area and can squeeze together without providing as much aeration as larger particles, finds its place in starter trays when you're germinating seeds. Keep vermiculite and soil amendments stored away from children and pets, and wear gloves when handling vermiculite.

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