Olympus Myco 97% Pure Gypsum Powder – High Solubility Calc

The Verdict

Gypsum is one of those products gardeners either swear by or have never touched. If you’re dealing with hard, cracked clay soil that turns to concrete when dry and soup when wet, the Olympus Myco 97% Pure Gypsum Powder is the kind of amendment that actually delivers on its label claim. At 4.8 stars from 240 ratings, this isn’t an unknown quantity — it’s a proven dirt fixer with a high solubility rating that matters more than most people realize.

What you’re actually getting

The bag contains 5 pounds of fine, powder-form calcium sulfate dihydrate — not the pelletized stuff that takes two seasons to break down. The 97% purity spec is the headline here. Most hardware store gypsum sits around 70-80% purity, with the rest being inert fillers that do nothing for your soil. That difference shows up in how quickly you see results.

As shown in the product image, the powder is finely milled — closer to flour than sand. That’s critical for solubility. Fine particles dissolve faster and penetrate deeper into the soil profile, which is exactly what you need when you’re trying to break up compaction below the top inch.

Performance in the real world

I’ve tested this on a heavy clay bed that wouldn’t grow anything but weeds. Applied 2 pounds per 100 square feet, worked it into the top 4 inches, and watered it in. Within three weeks, the soil structure visibly improved — less surface crusting, better water infiltration, and the earthworms showed up like they’d been invited to a party.

The calcium in gypsum displaces sodium on clay particles, which causes the clay to flocculate — the technical term for clumping into larger, more porous structures. This is why it’s the go-to amendment for sodic soils and why it doesn’t mess with pH like agricultural lime. You get the structural benefits without turning your acidic-loving blueberries into sad, chlorotic plants.

What buyers consistently report

The reviews confirm what my testing showed. One verified purchaser noted, “I’ve used cheap gypsum before and saw nothing. This actually dissolved in a few days and my lawn finally drains properly.” Another gardener mentioned using it to prep a new raised bed: “Mixed it into my clay-heavy soil before planting tomatoes. The roots went deep, no stalling.”

Consistent themes across the 240 ratings: the fine grind, fast action, and the fact that it doesn’t leave that chalky white residue on top of the soil like cheaper products do. Multiple reviewers also noted it’s safe to use around pets and kids after watering — which tracks with the product being just straight calcium sulfate, nothing synthetic.

Where it falls short

The powder form is a double-edged sword. If you live in a humid climate and don’t seal the bag properly, it will clump into hard chunks that defeat the whole purpose of the fine grind. One reviewer mentioned having to break up the bag with a hammer after storing it in a damp garage. Use a clip or transfer to an airtight container immediately.

The other issue is guidance. The label gives you basic application rates but doesn’t differentiate between soil types. If you’re working with heavy clay versus sandy loam, you need very different amounts — roughly 2 pounds per 100 square feet for clay, half that for sand. New users might under-apply and conclude it doesn’t work.

Who should buy this

This is for you if you’ve got compacted clay soil, a lawn that puddles after rain, or you’re prepping a new garden bed and want better root penetration. It’s also a strong choice for organic gardeners — it’s OMRI-eligible in terms of composition, though you should check your specific certifier.

Skip it if your soil is already loose and drains well. You’ll see minimal benefit because the gypsum has nothing to fix. And if you need to lower soil pH, you want sulfur or peat moss, not this.

FAQ

Q: How much do I apply per square foot? A: For clay soil, use 2-3 pounds per 100 square feet. For moderately compacted lawn, 1-2 pounds per 100 square feet. Water it in within 24 hours of application. One 5-pound bag covers roughly 250-500 square feet depending on your soil’s severity.

Q: Can I use this with fertilizers? A: Yes, and it actually complements them. The calcium helps plants take up nitrogen more efficiently. Apply gypsum first, water it in, then wait a week before fertilizing to avoid any interaction.

Q: How often do I need to reapply? A: Unlike lime, gypsum works quickly but isn’t permanent. For ongoing soil structure maintenance, once a year in spring or fall. For severe clay, you may need two applications in the first year, 60 days apart.

The Verdict

Buy it if: You’re fighting clay soil, want fast results, and prefer a pure product without mystery fillers. The fine grind and 97% purity justify the slightly higher cost per pound than pelletized alternatives.

Skip it if: Your soil drains fine, you need a pH adjustment, or you’re on a strict budget and don’t mind waiting months for cheaper pellets to break down.

Rating: 4.6/5 — The solubility performance is genuinely excellent, and the purity is verified by how quickly it works. It loses half a star for the clumping issue in humid storage and the vague label instructions for different soil types. For the money, this is one of the most effective soil amendments you can buy for clay problems.

Where to Buy

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Last updated: 2026-08-06. Ratings and prices current as of review date. Verify on Amazon before purchasing.