Most living-soil recipes include these five slow-release mineral amendments. They usually come with a paragraph of enthusiastic description and no data. Of the five, one is well supported (gypsum). Two are sound on their basic function but not on their extra claims (oyster shell and Azomite-type rock dust). One is far weaker than its reputation (greensand). And one carries a claimed mechanism that is real in the published literature but was studied in a different material from the one usually credited: holding nitrogen in the soil instead of losing it as ammonia (soft rock phosphate). Below, each one is split into three parts: what it is, what common living-soil recipes claim, and what independent evidence supports. For the recipe claims we mainly use True Living Organics by The Rev (Green Candy Press; first published 2012, later editions 2016 and 2024), one of the best-known living-soil books.
1. Why dry minerals at all
Living-soil growing keeps nutrients in forms the plant cannot take up directly and lets the soil biology release them as needed. Water-soluble mineral salts defeat that. They deliver nutrients instantly, and at high concentration they suppress or kill the microbes doing the work.
Slow-release rock minerals sit at the other extreme. They release over months to years, they do not spike electrical conductivity, and they act as a long-term reservoir in a soil you plan to recycle many times. The trade-off is that “slow” sometimes means “too slow to matter in one crop cycle”. Telling the two apart is the whole point of this article.
Safety first: fine rock dust, soft rock phosphate and gypsum can contain crystalline silica, which is a breathing hazard. Wear a dust mask when you mix them, and mix in a ventilated space.
2. Greensand: the one that does not do what people think
What it is: glauconite, a marine sedimentary mineral. Composition figures commonly quoted for the mineral itself are roughly iron 12–19%, potassium 5–7%, silicon 25%, oxygen 45%, magnesium 2–3% and aluminium 2%, plus many trace elements.
What common living-soil recipes claim: that greensand is a core amendment for container living soil, “a miracle”, a major mineral supplement for recycled soil and a major iron source, and that it improves soil structure and aeration. It is also often listed as a potassium remedy in deficiency troubleshooting.
What independent evidence shows: the potassium claim does not hold up as a fertiliser claim. Commercial greensand products are typically labelled around 0-0-3, and some analyse as low as 0-0-0.2. Heckman and Tedrow (Rutgers) reviewed the agronomic literature in 2004 and reported that almost all investigators concluded greensand has very little value as a nutrient source. Where plant growth did improve after greensand was applied, they attributed it more likely to changes in soil physical properties than to nutrition. That is a more nuanced picture than “useless”: the structure claim may have some basis, but the potassium claim does not.
The distinction to hold onto: elemental content and plant availability are not the same number. The mineral glauconite contains several percent potassium, but the fertiliser greensand delivers only a small fraction of that to a crop in one season. Confusing the two is the most common error in organic amendment marketing, and it is not limited to greensand. As section 4 shows, it applies to rock phosphate too.
Verdict: greensand has a defensible role as a very long-term mineral reservoir in soil you will recycle for years, and possibly as a soil conditioner. It is not a practical potassium source, and it will not fix a potassium deficiency this cycle. For that, use sulphate of potash (SOP) or langbeinite (Sul-Po-Mag). Both are mined products widely permitted in organic growing; check your certifier’s list if that matters to you. Kelp meal also helps. Keep greensand in your recycling mix if you like, but do not count on it.
3. Azomite: trace elements and manufacturer claims
What it is: a mined volcanic rock dust product (rhyolitic material), sold in powdered and granular grades.
What common living-soil recipes claim: more than seventy minerals and trace elements, higher yields, better cold tolerance and favourable cation exchange characteristics. The element lists that circulate with it include boron, calcium, chlorine, cobalt, copper, iron, magnesium, molybdenum, nitrogen, phosphorus, potassium, silicon and sulphur. A common practice is powdered Azomite in fresh soil mixes and granular in recycled ones.
Assessment: two items on that list deserve a comment. Rock dust contains negligible nitrogen, and its chlorine contribution is not meaningful either. Their presence on the list tells you it describes detectable elements, not useful nutrient supply. The “70+ minerals” figure and the yield and cold-tolerance claims are manufacturer claims. As far as we could find, independent peer-reviewed literature has not verified them. What is not in dispute is that finely ground volcanic rock dust slowly supplies a broad, dilute range of trace elements. Trace-element deficiency is also a real and under-diagnosed problem in containers where the same medium is reused.
Verdict: reasonable as trace-element insurance in a recycled-soil programme. Treat it as a low-risk, low-certainty input, not a performance product. A locally quarried volcanic rock dust or basalt dust is a possible substitute if a heavy-metal and elemental analysis is available. Basalt and rhyolitic material differ in composition, and heavy-metal content varies from quarry to quarry, so do not assume one rock dust equals another.
4. Soft rock phosphate and rock phosphate, and the nitrogen claim
What they are: related phosphate minerals in different particle sizes. Soft rock phosphate (SRP, also called colloidal phosphate) is the fine, powdered form; rock phosphate (RP) is granular. Both supply phosphorus plus a range of trace elements. Neither is acid-treated.
What common living-soil recipes claim: the notable claim is a mechanism. When nitrogen-rich materials such as blood meal or guano are mixed with lime, some nitrogen is normally lost as ammonia gas. The recipe claim is that SRP holds that nitrogen in the soil instead of letting it volatilise. It is also said to help calcium stay in the soil through cation exchange instead of leaching away.
What independent evidence shows: the underlying chemistry is real and has been studied, but the studied material is superphosphate: rock phosphate treated with sulphuric acid, not raw soft rock phosphate.
In a pilot study of pig manure and cornstalk composting (Wu et al., 2019), adding superphosphate at 10–30% of dry weight cut cumulative ammonia emissions by 23.8–48.1% compared with the control. Those are high addition rates. The usual explanation is that superphosphate is acidic: it lowers pH and binds ammonia as ammonium salts instead of letting it escape as gas.
The honest reading: the effect is established for acidulated phosphate. Soft rock phosphate is not acidulated, so the acid-driven part of that mechanism does not apply to it. We could not find published evidence that untreated soft rock phosphate gives a comparable nitrogen-conservation effect. Treat the SRP claim as plausible in direction but unverified for this specific material.
Rock phosphate and pH: the same warning as greensand. Raw rock phosphate dissolves well only in acidic soil. Agronomic guidance generally treats it as effective at about pH 5.5 or below, and much less useful above about pH 5.5–6. A living-soil mix carrying dolomite and oyster shell sits well above that range. In that soil, phosphorus release from SRP or RP depends mainly on microbes, root exudates and mycorrhizal fungi working at the particle surface, not on soil chemistry. The total phosphorus on the label is not what the plant gets this season. That is exactly the “total content ≠ plant-available” gap we marked greensand down for, and it applies here too.
Contaminants: rock phosphate can carry cadmium and uranium, and levels vary widely by deposit. Ask the supplier for the cadmium content before buying, and prefer sources that can show an analysis.
Verdict: SRP and RP are reasonable slow-release phosphorus and trace-mineral sources in a biologically active soil, as long as you accept that their release in a near-neutral mix is slow and biology-dependent. Phosphorus is the nutrient you most want in a slow form in living soil, because high available phosphorus suppresses mycorrhizal colonisation. Use them for that. Do not choose them for the ammonia-retention claim.
The common practical notes are sound. Finer is better, and micronised SRP is preferred because more surface area means more contact with biology. Including one of the two in each soil mix or recycling batch is common practice. Granular RP is mainly worthwhile if you recycle soil repeatedly.
5. Oyster shell: calcium plus a microbial habitat
What it is: crushed or ground oyster shell. It is sold as coarse “crushed shell”, roughly coin-sized pieces, and as fine “oyster shell flour”. It is mostly calcium carbonate (commonly quoted at about 95%), with small amounts of silicates and trace elements.
What common living-soil recipes claim: two separate functions. The first is calcium supply; in this system calcium is treated effectively as a macronutrient rather than a secondary nutrient. The second is that the porous surface of crushed shell acts like an artificial reef. It is said to create small bacteria-friendly spots with slightly higher pH while changing overall soil pH only within a narrow range, leaving the rest of the soil to fungi, which prefer areas away from the shell.
Assessment: the calcium carbonate content and the calcium supply function are straightforward and match what oyster shell is. The “artificial reef” argument is a plausible physical mechanism, but it is presented as reasoning, not data. We found no controlled study of it in a container growing medium. Flag it as unverified mechanism, uncontroversial material.
Verdict: a good calcium source, and one of the easiest amendments to find in coastal Southeast Asia, where shell is a cheap by-product of the oyster industry. The broader principle is the useful part: supply calcium from several sources rather than one. Bone meal, feather meal, lime and shell release at different rates, so using several gives steadier supply.
Caution on lime: do not use hydrated (slaked) lime in living soil. It is caustic: a saturated solution reaches about pH 12.4, and it can damage soil life and roots. Dolomite lime supplies calcium and magnesium and buffers pH. Pelletised (prilled) lime is not more soluble than other forms. It is finely ground lime pressed into pellets, so it reacts faster once the pellets break down. Reactivity depends mainly on how finely the lime is ground, not on what form it is sold in.
6. Gypsum: calcium and sulphur without touching pH
What it is: calcium sulphate dihydrate.
What common living-soil recipes claim: gypsum is a standard ingredient in soil-recycling recipes. It is named alongside soft rock phosphate as a preventive against sulphur deficiency in recycled soils. Dose quantities belong to the recipe itself, so see the original source or the recycling-recipe article rather than this assessment.
What independent evidence shows: this is the best-supported of the five. Extension guidance, for example Ohio State University Extension’s fact sheet ANR-20, is consistent:
- Gypsum is a moderately soluble (~2 g/L) source of calcium and of sulphur in sulphate form, both plant-available. It dissolves far more readily than limestone.
- Gypsum is not a liming material and does not meaningfully change soil pH. The sulphur is already fully oxidised as sulphate and does not form acid. This is the key functional difference from lime, and from elemental sulphur.
- Unlike lime, whose dissolution slows as pH rises, gypsum keeps supplying calcium in neutral-to-alkaline soil.
- Its classic agricultural use is displacing sodium in sodic soils so the sodium can be leached away.
- It adds little where calcium is already sufficient.
Verdict: the amendment to reach for when you need calcium or sulphur without moving pH. In a living-soil mix already carrying dolomite and oyster shell, gypsum lets you add more calcium without pushing pH up further. Its sulphur is also the more reliable half of the gypsum-plus-SRP sulphur-deficiency pairing; SRP contributes less.
7. Summary table
| Amendment | Primary function | Evidence quality | Practical verdict |
|---|---|---|---|
| Greensand | Long-term mineral reservoir; possible soil conditioner | Potassium claim not supported: a 2004 literature review found very little nutrient value | Optional in recycling programmes; never count it as a K source |
| Azomite / rock dust | Broad, dilute trace elements | Manufacturer claims independently unverified; nitrogen and chlorine negligible | Low-risk insurance; local basalt dust only with a heavy-metal and elemental analysis |
| Soft rock phosphate / rock phosphate | Slow phosphorus, trace elements | Poor chemical solubility above ~pH 5.5–6, so release is biology-dependent; ammonia-retention claim studied only for acidulated superphosphate | Use for slow P in active soil; ask for cadmium content |
| Oyster shell | Calcium, pH buffering | Composition sound; microsite mechanism unverified | Cheap and good in coastal Asia; rinse feed-grade shell; use with other Ca sources |
| Gypsum | Calcium + sulphur, pH-neutral | Well supported by extension guidance | The pH-neutral calcium/sulphur tool; never plaster or unverified phosphogypsum |
8. Sourcing in Thailand and Southeast Asia
These are general horticultural inputs used across vegetable, fruit and ornamental growing. Practical notes:
- Oyster shell: widely available and cheap. Both crushed and flour grades exist in the poultry-feed supply chain. Feed-grade shell can carry salt, so rinse it well before it goes into a soil mix.
- Gypsum: widely available for agriculture and industry. Buy agricultural-grade calcium sulphate dihydrate. Do not use construction plaster: it is usually hemihydrate, which takes up water and sets hard. Avoid phosphogypsum unless you have a verified analysis, because this fertiliser-industry by-product can carry radioactivity and heavy metals.
- Dolomite lime: readily available. Ask about particle fineness, because that, not the form, decides how fast it reacts.
- Rock phosphate: available in the region, though “soft rock phosphate” as a distinct micronised product is harder to find than granular RP. Ask for particle size and cadmium content, not just the name.
- Volcanic rock dust / basalt dust: available from quarries and far cheaper than imported branded rock dust. Use it only if you can get an elemental and heavy-metal analysis.
- Greensand: essentially an import item. Given the evidence above, it is the one we would drop first if shipping cost matters.
Whatever you buy, ask for a guaranteed analysis and read it as total elemental content, not as what the plant will get this season. That gap is the single most important idea in this article.
Which of these have you actually seen make a visible difference, and over how many recycling cycles? Mineral amendments are the hardest category to evaluate honestly, because their claimed timescale is longer than most growers keep records for. If you have run the same soil batch for three or more cycles with and without a given rock dust, that is data almost nobody has.
Related reading on the forum: topic 1091, and topic 1030 on mycorrhizae, where the phosphorus–mycorrhizae point is covered in more depth.
References
- The Rev. True Living Organics. Green Candy Press; 1st ed. 2012, 2nd ed. 2016, 3rd ed. 2024.
- Heckman, J.R. & Tedrow, J.C.F. (2004). Greensand as a soil amendment. Better Crops with Plant Food 88(2): 16. https://my.ucanr.edu/sites/nm/files/76652.pdf
- Wu, J., He, S., Li, G., Zhao, Z., Wei, Y., Lin, Z. & Tao, D. (2019). Reducing ammonia and greenhouse gas emission with adding high levels of superphosphate fertilizer during composting. Environmental Science and Pollution Research 26(30): 30921–30929. Client Challenge
- Dontsova, K., Lee, Y.B., Slater, B.K. & Bigham, J.M. (2005). Gypsum for Agricultural Use in Ohio: Sources and Quality of Available Products (ANR-20). Ohio State University Extension. https://ohioline.osu.edu/factsheet/anr-20
- Crop Nutrition resource library. Phosphate Rock. Phosphate Rock | Mosaic Crop Nutrition
This article is general horticultural information for readers acting legally in their own jurisdiction. Cannabis rules differ in every country. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568, and cultivation requires a licence under that framework. Flower may not be sold or advertised online, and recreational use is not permitted. The Cannabis Control Act is still a draft: the Cabinet approved it on 22 September 2026 and sent it to the House. In countries where cannabis is banned, including Vietnam, this article does not encourage growing it; the amendments discussed here are ordinary inputs for legal crops. Nothing here is legal advice.