How Much Vermiculite Should I Add to Topsoil?

Properties

• Adding vermiculite particles to soil increases moisture retention, porosity, nutrient ion exchange, mineral retention and reduces potential for compaction. Vermiculite is a naturally occurring compound and is mined from ground for use in construction as well as in horticulture. Small amounts of asbestos fibers exist in vermiculite and are problematic when airborne during particle handling. Once moistened, the particles do not shed fibers as readily. In dry conditions, vermiculite is stable and doesn't degrade quickly.

Best Uses

• Vermiculite is used to create soil-less potting mixes for growing plants. Alternatively, pure vermiculite provides a porous but moist environment for seeds to germinate, but later can be transplanted into soil. Vermiculite on its own or in high concentrations isn't good for growing plants. Potting mixes typically comprise equal parts peat or coir, compost, perlite and vermiculite. This blend creates the best texture of medium for plants to grow, with good moisture-retaining and porous qualities.

In Soil

• Once vermiculite is added to garden topsoil, removing it is nearly impossible. Weigh the pros and cons of blending vermiculite into soil before proceeding. In sandy or gritty soils, vermiculite improves moisture and nutrient ion retention. In clay soils, vermiculite improves aeration or porosity but increases moisture levels further. When wet, vermiculite may become pulverized into grains that further bind to other particles. In loam soils, vermiculite increases moisture retention, often creating a soil that has more clay-like characteristics.

Tips

• Conduct a trial mix of vermiculite and garden topsoil, and monitor how the blend changes in texture. compaction and moisture levels. Initially try a mix of one part vermiculite to four or five parts of topsoil that's sand or loam. Do not add vermiculite to soils that already retain moisture or are of a fine particle texture, such as clay. Rather than solely adding vermiculite to improve your garden's topsoil, also add one to two parts of organic matter, such as compost or well-rotted manure. The organic matter coupled with a smaller amount of vermiculite can improve sandy topsoils particularly well. You may also add more vermiculite in incremental amounts to the soil to create the ideal topsoil mixture.