Translated and edited from the original: True Living Organics by The Rev (Green Candy Press; 1st ed. 2012, 2nd ed. 2016, 3rd ed. 2024). The recipe claims below come from this book. Each claim is checked against the independent sources listed under References.
Most living-soil recipes add five slow-release rock minerals. This part covers the three that supply calcium and control soil pH: oyster shell, lime and gypsum. It then shows which amendment fixes which problem in poor tropical soil. It ends with a summary table of all five minerals and sourcing notes for Thailand and Southeast Asia.
Here is the short verdict for this part. Gypsum is the best supported of all five minerals. Oyster shell is a good calcium source, but its “microbial reef” claim is unproven. Hydrated lime does not belong in living soil at all.
Part 1 covered the other two minerals and rock dust safety. Greensand is a weak potassium source. Azomite-type rock dust is low-risk but unproven. Rock phosphate releases phosphorus only slowly above pH 6.
The recipe claims come mainly from True Living Organics, often shortened to TLO. It is one of the best-known books on living soil. Its author writes under the pen name The Rev. Each mineral gets three questions: what it is, what the recipes claim, and what independent evidence shows.
This is Part 2. Part 1: Dry Minerals for Living Soil: Greensand, Azomite, Soft Rock Phosphate, Oyster Shell and Gypsum
1. Calcium and pH: why these three belong together
In living soil, soil organisms release nutrients from organic and mineral amendments over time. Slow-release minerals feed that system without killing the microbes, as soluble fertiliser salts can.
TLO treats calcium almost as a main nutrient, one the plant needs in large amounts. Standard plant nutrition calls it a secondary nutrient. In this system, how you supply calcium therefore matters a lot.
The three calcium sources in this part change pH in very different ways. pH is the acid–alkaline scale; 7 is neutral. Oyster shell and dolomite lime raise and steady the pH. Gypsum leaves it almost unchanged. Hydrated lime pushes it dangerously high.
Safety: gypsum powder can contain crystalline silica, a fine mineral dust that can reach deep into the lungs. The US Occupational Safety and Health Administration (OSHA) says long-term exposure can cause silicosis, an incurable lung disease, and also lung cancer. Mix it outdoors or in a ventilated space. Wear a dust mask made for fine particles, rated at least N95 or an equivalent standard. Moisten the soil before adding the powder, and clean up with water, not a dry broom.
What to do: measure the pH of your current mix with a meter or test kit before you pick a calcium source. The reading tells you whether you can raise the pH or must keep it where it is.
2. Oyster shell: calcium, plus a claimed home for microbes
What it is. Oyster shell is crushed or ground shell. It is sold as coarse crushed shell, in pieces roughly the size of a coin. It is also sold as fine oyster shell flour. It is mostly calcium carbonate, often quoted at about 95%, with small amounts of silicates and trace elements.
What recipes claim. Recipes give oyster shell two separate jobs. The first is supplying calcium.
The second job is acting like an artificial reef. The porous surface of crushed shell is said to create small spots where bacteria thrive at slightly higher pH. Overall soil pH is said to change only within a narrow range. The rest of the soil is left to fungi, which are said to prefer areas away from the shell.
What the evidence shows. The calcium job matches what the material is. Oyster shell is mostly calcium carbonate, so it supplies calcium.
The reef idea is a plausible physical mechanism. But it is presented as reasoning, not data. No controlled study of it in a container mix could be found. Treat it as an unverified mechanism in a well-understood material.
Verdict. Oyster shell is a good calcium source. It is one of the easiest amendments to find in coastal Southeast Asia, where shell is a cheap by-product of oyster farming.
The broader lesson is the useful part: supply calcium from several sources, not one. Bone meal, feather meal, lime and shell release calcium at different speeds. Using several gives the plant a steadier supply.
Translator’s note: buying shell in Thailand and Vietnam. Crushed shell and shell flour are both sold through the poultry-feed supply chain. Feed-grade shell can carry salt. Salt raises EC (electrical conductivity, the meter reading of dissolved salts) and stresses roots and microbes.
Rinse feed-grade shell in a bucket of water before it goes into a soil mix. Repeat until the rinse water’s EC reads close to your tap water. That reading tells you the salt is gone.
What to do: use oyster shell as one of two or three calcium sources. Rinse it first if it was sold as poultry feed.
3. Lime: which kind, and why hydrated lime does not belong
Do not use hydrated lime (slaked lime) in living soil. It is caustic. A saturated solution reaches about pH 12.4. At that strength it can damage soil life and roots.
Dolomite lime supplies both calcium and magnesium and steadies the pH. It is the usual liming choice in living-soil mixes.
Pelletised (prilled) lime is not more soluble than other forms. It is finely ground lime pressed into pellets. It reacts faster only because the fine particles spread out once the pellets break down.
How fast any lime reacts depends mainly on how finely it is ground, not on the form it is sold in. Finer particles have more surface touching the soil.
Translator’s note: read the bag, not the common name. In Thai farm supply shops, several liming products sit side by side. The everyday names do not always match the chemistry. As a guide, ปูนขาว is usually calcium hydroxide (hydrated lime), ปูนมาร์ล is mainly calcium carbonate mixed with clay, and ปูนโดโลไมท์ is dolomite, a calcium–magnesium carbonate. Check the chemical name on the label. Do not buy any product listed as calcium hydroxide for a living-soil mix.
What to do: when you buy dolomite, ask the seller how finely it is ground. Fineness, not form, decides how fast it works.
4. Gypsum: calcium and sulphur without changing pH
What it is. Gypsum is calcium sulphate dihydrate, calcium sulphate with water bound into the crystal.
What recipes claim. Gypsum is a standard ingredient in soil-recycling recipes. Recipes pair it with soft rock phosphate to prevent sulphur shortage in recycled soil. Doses belong to the recipe itself. See the original book for amounts.
What independent evidence shows. Gypsum is the best supported of the five minerals. Ohio State University Extension, the farm advisory service of Ohio State University (USA), publishes fact sheet ANR-20 on gypsum. Extension services turn university research into practical guidance for farmers. The guidance is consistent:
- Gypsum is moderately soluble, about 2 g per litre. It supplies calcium and sulphur in sulphate form, and plants can take up both. It dissolves far more readily than limestone, the rock most farm lime is made from.
- Gypsum is not a liming material. It does not change soil pH in any meaningful way. Its sulphur is already fully oxidised as sulphate, so it cannot form acid. This is the key difference from lime, and also from elemental sulphur.
- Lime dissolves more slowly as pH rises. Gypsum keeps supplying calcium in neutral and alkaline soil.
- Its classic farm use is in sodic soils, soils holding so much sodium that structure and plant growth suffer. Gypsum’s calcium displaces the sodium so water can wash it away.
- It adds little where calcium is already sufficient.
Verdict. Use gypsum when you need calcium or sulphur without moving pH. A living-soil mix already carrying dolomite and oyster shell has a raised pH. Gypsum adds more calcium without pushing it higher.
In the gypsum-plus-SRP pairing against sulphur shortage, gypsum’s sulphur is the more reliable half. Soft rock phosphate contributes less.
Translator’s note: grades to buy and avoid. Gypsum is widely sold in Thailand for farming and industry. Buy agricultural-grade calcium sulphate dihydrate.
Do not use construction plaster. It is usually hemihydrate, calcium sulphate with less bound water. It takes up water and sets hard in the pot. Avoid phosphogypsum, a by-product of phosphate fertiliser factories, unless you have a verified analysis. It can carry radioactivity and heavy metals.
Translator’s note: outdoor beds in the rainy season. Gypsum dissolves far more easily than lime. In open beds under heavy monsoon rain, it may wash through faster than in a covered container. Outdoor growers should check calcium symptoms again after long wet spells rather than assume the gypsum is still there.
What to do: if your mix pH is already at or above your target, use agricultural-grade gypsum as the calcium source. Leave out more lime or shell.
5. Improving poor tropical soil: which amendment fixes which problem
Readers often ask which of these minerals to use on poor, worn-out tropical soil. The table matches each problem to the amendment that fits, using the evidence in both parts of this guide.
| Problem you measured or see | Amendment to consider | Why it fits | Watch out for |
|---|---|---|---|
| Acidic soil that needs calcium and magnesium | Dolomite lime | Supplies calcium and magnesium; steadies pH | Choose finely ground; fineness decides speed |
| Calcium needed, but pH already high enough | Gypsum | Calcium without moving pH | Agricultural grade only |
| Sulphur shortage | Gypsum | Plants can take up its sulphate | Soft rock phosphate adds less sulphur |
| Steady calcium over a long time | Oyster shell plus other calcium sources | Different sources release at different speeds | Rinse feed-grade shell |
| Low phosphorus | Soft rock phosphate or rock phosphate | Slow phosphorus that does not harm root fungi | Dissolves well only at about pH 5.5 or below; ask for cadmium content |
| Trace-element shortage in a reused mix | Volcanic or basalt rock dust | Broad, dilute, slow supply | Only with an elemental and heavy-metal analysis |
| Potassium shortage in this crop | Sulphate of potash, langbeinite or kelp meal | They release potassium within the crop | Greensand will not fix it |
| Too much sodium (sodic soil) | Gypsum | Calcium displaces sodium, which water then washes out | Needs enough water to flush the sodium |
| Poor drainage or aeration | Aeration amendments such as perlite or pumice (topic 1402) | They improve air and drainage directly | Greensand helps structure at most a little |
Translator’s note: measure before you add. Each row starts with a problem you have measured or seen, not a guess. For containers, a pH meter and an EC meter cover most of it. For field soil, a laboratory soil test gives pH and nutrient levels before you spend money. Thailand’s Land Development Department (กรมพัฒนาที่ดิน) offers soil analysis to farmers. You register online at osd101.ldd.go.th/osdlab and send a sample to one of its labs. Basic analyses for farmers are free. Without a measurement, you cannot tell whether an amendment helped.
What to do: pick one problem, add one amendment for it, and record the result over the next crop. Change one thing at a time so you know what caused the change.
6. Summary table: all five minerals
| Amendment | Main job | Evidence | Practical verdict |
|---|---|---|---|
| Greensand | Long-term mineral store; possibly improves structure | Potassium claim not supported: a 2004 review of the research found very little nutrient value | Optional in recycling programmes; never count it as a potassium source |
| Azomite / rock dust | Broad, dilute trace elements | Manufacturer claims independently unverified; nitrogen and chlorine negligible | Low-risk safeguard; local basalt dust only with a heavy-metal and elemental analysis |
| Soft rock phosphate / rock phosphate | Slow phosphorus, trace elements | Dissolves poorly above about pH 5.5–6, so release depends on biology; ammonia claim studied only in acid-treated superphosphate | Use for slow phosphorus in active soil; ask for cadmium content |
| Oyster shell | Calcium, pH buffering | Composition sound; “microbial reef” mechanism unverified | Cheap and good in coastal Asia; rinse feed-grade shell; use with other calcium sources |
| Gypsum | Calcium and sulphur, pH-neutral | Well supported by university extension guidance | The tool for calcium and sulphur without moving pH; never plaster or unverified phosphogypsum |
7. Sourcing in Thailand and Southeast Asia
Translator’s note: all five are general farm inputs used in vegetable, fruit and ornamental growing across the region. For where to buy, ask the official distributor of a branded product in Thailand. You can also ask other growers in the General Nutrients Q&A section.
- Oyster shell: widely available and cheap. Crushed and flour grades are both sold through the poultry-feed supply chain. Feed-grade shell can carry salt, so rinse it well before it goes into a mix.
- Gypsum: widely available for farming and industry. Buy agricultural-grade calcium sulphate dihydrate. Do not use construction plaster, which is usually hemihydrate and sets hard. Avoid phosphogypsum unless you have a verified analysis, because it can carry radioactivity and heavy metals.
- Dolomite lime: easy to find. Ask about particle fineness, because fineness, not form, decides how fast it reacts.
- Rock phosphate: available in the region. Micronised soft rock phosphate is harder to find than granular rock phosphate. Ask for particle size and cadmium content, not just the name.
- Volcanic or basalt rock dust: sold by 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, drop it first if shipping cost matters.
Whatever you buy, ask for the guaranteed analysis, the nutrient percentages the seller states on the label. Read it as total elemental content, not as what the plant will get this season. That gap is the most important idea in this guide.
Translator’s note: fertilisers sold in Thailand come under the Fertilizer Act B.E. 2518 (1975), amended by the Fertilizer Act (No. 2) B.E. 2550 (2007). If a mineral is sold as a fertiliser, check its registration and label. The topic Verifying a Fertiliser Is Legally in Thailand explains how.
8. Share your records
Mineral amendments are the hardest inputs to judge honestly. The claimed effect takes longer to show than most growers keep records. You may have run the same soil batch for three or more cycles, with and without a mineral from this guide. If so, post the details below this topic: the mineral, the dose, the number of cycles and what you saw. Very few growers have data like this. You can also ask questions about your own mix here.
For the phosphorus and root-fungi link, see the forum topic on mycorrhizal fungi and other root microbes.
References
- The Rev. True Living Organics. Green Candy Press; 1st ed. 2012, 2nd ed. 2016, 3rd ed. 2024.
- 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
- 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
- Crop Nutrition resource library. Phosphate Rock. Phosphate Rock | Mosaic Crop Nutrition
This article is general horticultural information for readers acting legally where they live. 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, in force since 26 June 2025. Flower may be sold only with a PT 33 (ภ.ท.33) prescription, and advertising or selling flower online is prohibited. 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.