Comparison: Best Expanded Shale Potting Soil (Updated | Nov 30th)

Focus on expansion ratio, nutrient content, and pH balance, as these affect how much soil you get and how well plants grow. Look for peat‑free, organic certifications if sustainability matters to you. Also consider the texture—light, airy coir promotes root aeration while preventing compaction.

Back to the Roots 51‑quart brick expands to over 2 cubic feet (about 4× its dry size), while WONDER SOIL’s bag expands roughly 4× when hydrated. Gro‑Med’s two 1.4‑lb bricks together produce 20 quarts, a 14× increase from the dry weight. InstaSoil and the 16‑pack wafers each expand about 5×, giving you 2–3 cubic feet from a single block.

WONDER SOIL includes worm castings, kelp, mycorrhizae, and perlite, delivering a ready‑made nutrient boost. Gro‑Med and InstaSoil also add plant food and mycorrhizae for faster germination. Standard coir bricks like Back to the Roots rely on added amendments from the grower, so you may need to supplement with fertilizer.

Several products are OMRI‑listed and explicitly peat‑free, such as Back to the Roots Organic Coir and the Back to the Roots 100 % Organic mixes. The 16‑pack coir wafers highlight renewable coconut fiber and a neutral pH, while Miracle‑Gro mixes contain peat but are certified by the Mulch & Soil Council. Choosing a peat‑free, OMRI‑listed option reduces environmental impact.

Bulk options like the 15‑lb Bonsai Starter Soil from Bo… typically offer the lowest cost per cubic foot because you get up to 7 quarts of expanded soil for a single purchase. Smaller bricks, such as the 1.4‑lb Gro‑Med pack, have a higher per‑cubic‑foot price but save space and are convenient for starter gardens. Multi‑bag bundles like Miracle‑Gro’s 16‑qt pack sit in the middle, offering a balance of volume and price.

Indoor growers often prefer the compact, low‑gnat formulas of Back to the Roots 6‑quart indoor mix or the Bonsai Starter Soil, which are formulated for clean indoor environments. Outdoor gardeners benefit from the larger expansion of WONDER SOIL and the 20‑quart Gro‑Med bricks, which handle heavier watering and provide better drainage for raised beds and containers.

Coconut coir naturally holds up to 10 times its weight in water while still allowing excess to drain, preventing root rot. Products that add perlite or vermiculite, like WONDER SOIL, improve drainage further. Pure coir bricks such as Back to the Roots expand into a fluffy medium that balances moisture retention with aeration, ideal for most houseplants.

Bulk packs generally deliver a lower cost per expanded quart and are ideal for seasoned gardeners who need large volumes. Smaller bricks offer convenience, less upfront storage, and are perfect for trial runs or limited‑space growers, though the price per cubic foot is higher. Consider your planting scale and storage space when deciding which format gives you the best overall value.