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Have you ever wondered what the light-colored specks in your potting mix are? Or questioned whether adding perlite or vermiculite to your growing medium is necessary, and if so, which one, or a combination of both, will provide the highest ROI? I have! Not realizing how much they differed in water retention, I once amended my drought-resistant lavender soil with too much vermiculite (and not enough sand to balance it), creating a water-logged disaster. The vermiculite absorbed and retained too much water and my dry-soil-loving babies were not pleased. Mediterranean plants need aerated, fast-draining soil and do not like wet feet. Lesson learned!
Perlite and vermiculite look similar and while both aid with soil aeration, each has key benefits and limitations. Let's examine each separately.
PERLITE
Usage: In gardening, perlite is used as a soil medium to improve drainage and aeration. It helps prevent soil compaction and promotes healthy root development by allowing air and water to move freely through the soil. Horticultural perlite is a lightweight growing media that typically ranges from 1/8-3/4 inches in size, with larger granules offering superior drainage and aeration. Industrially, fine perlite is used to help filter hazardous materials from our waterways, helping to prevent contamination of rivers and fish habitats.
Gardening Application: Useful when amending soil for plants intolerant of "wet feet" rather, preferring aerated, well-drained soil such as lavender, sage, and rosemary. Perlite will benefit moisture-sensitive house plants such as succulents and orchids. Amend to container and raised beds to help create an aerated, well-draining soil that most vegetables and herbs prefer.
Material: Perlite is made up of a lightweight volcanic rock material. When hot molten rock is spewed and quickly cooled, water gets trapped inside, creating a glass-like structure that is mined and processed into small, porous granules. Chemicals are not involved in the processing of this non-toxic material, but perlite can have high dust levels when dry. Perlite is a non-biodegradable, neutral soil media that will not break down or alter pH levels.
Sustainability: Perlite is considered a non-renewable resource because the volcanic rock deposits it comes from, are not readily replaceable. Studies indicate that less than 1% of global perlite has been mined and that the process of mining, crushing, and heating perlite into a usable media has a moderate environmental impact.
Cost: As a lightweight product, perlite can be economical to ship but is still rather expensive, especially when amending large amounts of soil. Purchasing in bulk will lower the price per quantity.
VERMICULITE
Usage: Vermiculite helps retain moisture in the soil while also improving aeration. It is often used in seed-starting mixes and as a soil amendment for plants that require consistent moisture. Vermiculite is used throughout construction, agriculture, and horticultural industries as insulation, soil amendment, packing material, and more.
Gardening Application: Useful when amending soil for moisture-loving plants such as hydrangeas, ferns, or peace lilies. In hot climates, easy-wilt plants such as lemon balm and mint will profit from the consistent moisture that vermiculite can provide. Hanging baskets containing water-loving plants that tend to dry out quickly may also benefit from vermiculite. Useful in seed germination and rooting when mixed with compost or other nutrient-forward medium. Top dressing seed starting trays can help retain vital moisture while preventing dampening-off. When needed, consider amending raised beds or containers containing fast-draining sandy soil with vermiculite to retain moisture
Material: Vermiculite is a mica-type mineral material that undergoes expansion when heated, resulting in lightweight, absorbent flakes. Vermiculite ore is mined all over the world and in its pure form, is non-toxic and sterile. Vermiculite mined from the Libby, Montana mine before it was shut down in 1990, was found to contain cancer-causing asbestos. Do not use vermiculite processed before 1990 and do not disturb attic vermiculite without having it tested.. Currently, US-sourced vermiculite must pass stringent testing and is considered safe.
Sustainability: A finite, non-renewable resource, extracted by open-pit mining, and formed through high-energy heat processing. Before 1990, about 80% of the world's vermiculite was sourced from the Libby Montana mine. Now closed due to asbestos contaminants, most US vermiculite is sourced from two remaining US mines, Brazil, and South Africa. Increased demand, slowing domestic production, and world supply chain issues may necessitate we seek viable alternatives.
Cost: Increased demand and supply chain issues have resulted in shortages, causing prices to increase. Search online for bulk ordering options and don't wait to purchase or it may not be available when needed.
To summarize, while perlite and vermiculite look similar, perlite is usually lighter in color and provides excellent aeration for plants needing moisture without waterlogging. Perlite will protect plant roots from soil compaction, allowing for adequate oxygen and nutrient uptake. Vermiculite is light brown to gold, provides some aeration, and excellent moisture retention. Think of vermiculite as a sponge, holding moisture and nutrients for plants to soak up as needed. Perlite and vermiculite may be utilized together by mixing perlite in soil for root aeration and top dressing with vermiculite to lock in moisture. Identify your plant's ideal growing requirements and experiment to see what works best for your gardening projects. Happy planting!