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How Montmorillonite Clay compares
Full guide and scienceMontmorillonite Clay (this product)
- One ingredient. You decide how much clay goes into a mix and nothing else comes with it
- The right choice when CEC and water-holding are the specific problems: sand, peat-free compost, hanging baskets
- Works as a compost additive, which a blended conditioner is not designed for
- For growers who build their own soil recipes and pair it with volcanic rock dust and humic acid as separate dials
Dr Forest Mineral Mix
- A blended mineral conditioner: gypsum, clay minerals, sea shell meal and basalt rock dust in one bag
- Already contains clay minerals, so most gardeners using it do not need to add more unless the medium is very sandy
- Adds calcium, sulphur and trace minerals alongside the clay
- The simpler choice for general beds, borders and re-amending old compost. See the Mineral Mix page for its current recipe
Customer highlight
The best clay mineral mix Ive found, as with all Dr Forest products I highly recommend them for organic growers.
Editorial highlight from marketplace and store reviews — separate from the full review list below.
Made by Growers · Backed by Science
Montmorillonite clay: the mineral that gives sandy soil and peat-free compost a memory
High CEC clay mineral Holds nutrients & water Permanent, never degrades Compost & lawn aid Mined, unprocessed, vegan Handcrafted in Stockport
Montmorillonite clay is a natural swelling clay mineral from bentonite deposits that adds cation exchange capacity (CEC) to soils and growing media that have almost none. Its plate-like layers carry a negative charge, so they hold calcium, magnesium, potassium and ammonium against leaching and hand them back to roots as the soil solution is drawn down. Water sits between the same layers and is released as the medium dries. Dr Forest supplies it as fine grey-green granules, packed in small batches in Stockport, Greater Manchester, with nothing added.
It is for sandy soils, peat-free composts and any container mix that drains well but holds on to nothing. Being a mineral, it does not rot down or wear out: what you add this spring is still working in ten years. Pack sizes run from 1.5 kg to 15 kg, and the bigger packs cost less per kilo.
700–800 m²Surface area per gram
80–150CEC, meq per 100 g
2.5–5 mlPer litre of soil mix
PermanentDoes not degrade in soil
What montmorillonite clay is used for in the garden
- Sandy soils and peat-free composts — adds the exchange sites that hold calcium, magnesium, potassium and ammonium in the root zone instead of letting them wash through with every watering
- Water retention in free-draining pots and beds — the clay swells as it takes up water between its layers and gives it back slowly, which softens the wet-dry swing that stresses plants in containers
- Getting more from every feed — nutrients that would have leached from a low-CEC medium are held until the plant wants them, so each application lasts longer
- Building stable soil organic matter — clay surfaces bind with humic substances to form organo-mineral complexes that protect carbon from decomposition for years
- A composting aid — sprinkled between layers it holds ammonium that would otherwise leave the heap as ammonia, so the finished compost keeps more nitrogen and smells less
- Sandy lawns — broadcast after aerating to build clay content in the root zone over two or three autumns
Why a straight clay mineral rather than the Mineral Mix?
Montmorillonite Clay (this product)
- One ingredient. You decide how much clay goes into a mix and nothing else comes with it
- The right choice when CEC and water-holding are the specific problems: sand, peat-free compost, hanging baskets
- Works as a compost additive, which a blended conditioner is not designed for
- For growers who build their own soil recipes and pair it with volcanic rock dust and humic acid as separate dials
Dr Forest Mineral Mix
- A blended mineral conditioner: gypsum, clay minerals, sea shell meal and basalt rock dust in one bag
- Already contains clay minerals, so most gardeners using it do not need to add more unless the medium is very sandy
- Adds calcium, sulphur and trace minerals alongside the clay
- The simpler choice for general beds, borders and re-amending old compost. See the Mineral Mix page for its current recipe
Handcrafted in Stockport
Weighed and packed in small batches at our workshop in Stockport, Greater Manchester. A mined mineral, unprocessed and vegan, with nothing added. It sits alongside the rest of our soil improvers; for where clay fits in a mineral programme, read what are organic dry amendments? Made by Growers. Backed by Science.
The Science
The science of montmorillonite: why a few hundred square metres in a gram matters
What montmorillonite actually is
Montmorillonite is a 2:1 phyllosilicate. Each layer is an aluminium octahedral sheet sandwiched between two silica tetrahedral sheets, and the layers stack with a gap between them (the interlayer) that water and cations can enter [1]. Substitutions inside the sheets leave the layers with a permanent negative charge, which is what attracts and holds positively charged nutrient ions. Bentonite is the rock; montmorillonite is the mineral that makes up most of it, and that is what is in the bag.
Because the interlayer surfaces count as well as the outside of each particle, the total surface area is estimated at 700–800 m² per gram [1], which is why a small dose does a lot of work. The same structure is why it swells: water moves between the layers and is held there until the surrounding medium dries and pulls it back. The calcium form expands far less in water than the sodium form [8]; the FAQ covers which form this is. Humic substances bind to those same surfaces, the mechanism behind clay-humus complexes; see what is humic acid? for the other half of that partnership.
Cation exchange capacity by clay mineral
| Mineral | CEC (meq per 100 g) | Swells in water |
|---|---|---|
| Kaolinite | 3–15 | No |
| Illite | 10–40 | Very little |
| Montmorillonite | 70–100 (Carroll); 80–150 (Jemai et al.) | Yes |
| Vermiculite | 100–150 | Limited |
Figures from Jemai et al. (2024) and Carroll (1959) [1, 2]. Montmorillonite holds roughly five to thirty times more exchangeable cations per gram than kaolinite, a common clay in garden soils. It is not the highest of all minerals; vermiculite edges it. What montmorillonite offers is high CEC, reversible swelling and a price that makes sense by the kilo. Sand contributes almost no exchange capacity at all, which is why sandy soils need feeding so often.
Sand lets nutrients through. Clay holds them until the roots ask.
Mechanisms of action
01
Cation exchange holds fertiliser in the root zone
Negatively charged clay surfaces hold Ca²⁺, Mg²⁺, K⁺ and NH₄⁺. When a root takes up nutrients it releases H⁺, which displaces a stored cation from the clay for the root to absorb. In a column study on sandy soil, Hussain et al. (2022, PLOS ONE) found that bentonite at 2–4% cut nitrate in the leachate by 12–19% across three nitrogen sources, and by 20–25% at the 4% rate with calcium nitrate [3]. Less in the drain, more in the soil.
02
Water-holding that lasts for years
Mi et al. (2020, Scientific Reports) applied bentonite once, at 6–30 tonnes per hectare, to a sandy loam in northern China and followed it for five seasons. Field water-holding capacity and plant-available water in the top 40 cm were significantly higher throughout, and millet grain protein rose by up to 28% over the control [4]. One application, five years of measurable effect.
03
Better plant growth on sand
Farghali and Tammam (2026, BMC Plant Biology) grew maize and senna in three desert sandy soils amended with 1.5%, 3% and 5% bentonite by weight. Soil water content, shoot length, root length, chlorophyll and fresh weight all improved over the unamended control [5]. Garden rates are lower than this, but the direction matches the field trials above.
04
Ammonium capture in the compost heap
Protein-rich material breaks down to ammonium, and in a heap with nowhere to hold it, ammonium leaves as ammonia gas. That is the smell, and it is nitrogen lost. Li et al. (2012, Bioresource Technology) composted pig manure with bentonite and found it sped up organic matter breakdown and raised the total nitrogen of the finished compost; they recommend keeping bentonite under 2.5% of the mix [6]. A few handfuls per barrowload is well inside that.
05
Clay protects organic matter from decomposition
Six et al. (2002, Plant and Soil) identified close association with silt and clay particles as one of three main routes by which soil carbon is protected from microbial breakdown [7]. A sandy soil has few of those surfaces, so compost added to it disappears quickly. Add clay and the organic matter has somewhere to bind and stay.
Scientific references
- Jemai, R., Chalghaf, R., Boubakri, S., Djebbi, M.A., Naamen, S., Ben Rhaiem, H. and Ben Haj Amara, A. (2024). Montmorillonite: properties, characteristics, and its harnessing in environmental applications. In: Recent Advances in Montmorillonite. IntechOpen. doi:10.5772/intechopen.1004763
- Carroll, D. (1959). Ion exchange in clays and other minerals. Geological Society of America Bulletin, 70(6), 749–779.
- Hussain, Z., Cheng, T., Irshad, M., Khattak, R.A., Yao, C., Song, D. and Mohiuddin, M. (2022). Bentonite clay with different nitrogen sources can effectively reduce nitrate leaching from sandy soil. PLOS ONE, 17(12), e0278824.
- Mi, J., Gregorich, E.G., Xu, S., McLaughlin, N.B. and Liu, J. (2020). Effect of bentonite as a soil amendment on field water-holding capacity, and millet photosynthesis and grain quality. Scientific Reports, 10, 18282.
- Farghali, K.A. and Tammam, S.A. (2026). Advantageous effects of bentonite on growth performance and metabolic compounds of two mesophytic plants in desert sandy soils. BMC Plant Biology, 26, 124.
- Li, R., Wang, J.J., Zhang, Z., Shen, F., Zhang, G., Qin, R., Li, X. and Xiao, R. (2012). Nutrient transformations during composting of pig manure with bentonite. Bioresource Technology, 121, 362–368.
- Six, J., Conant, R.T., Paul, E.A. and Paustian, K. (2002). Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant and Soil, 241, 155–176.
- Hang, P.T. and Brindley, G.W. (1970). Methylene blue absorption by clay minerals: determination of surface areas and cation exchange capacities. Clays and Clay Minerals, 18(4), 203–212.
Field studies above used agricultural rates on bare sandy soils. They describe the mechanism; the effect at garden rates will be smaller.
How to Use
How to use montmorillonite clay: rates for soil mixes, beds, compost and lawns
Measuring
A tablespoon is approximately 12–15 g of clay. Soil-mix rates are given in millilitres per litre of medium; bed and lawn rates in grams per square metre.
A permanent addition: add once, benefit for good
Fertilisers and biological amendments are used up and need replacing. Montmorillonite is a mineral that does not degrade. Once mixed into soil or growing media it stays there, holding nutrients, absorbing water and supporting structure indefinitely. Each application is a permanent upgrade to the medium. In containers, that is also the reason to stick to the rates: do not exceed 5 ml per litre, because too much clay in a confined volume can impede drainage.
Application rates
Soil mix: potting and container media
Mix thoroughly into potting compost, peat-free media or living soil blends before planting. Even at the lower rate (2.5 ml/L) the CEC improvement is significant: the 800 m²/g surface area means a small amount of clay contributes an enormous number of exchange sites. Use the higher rate (5 ml/L) for very sandy or fibre-based media with minimal native CEC. Do not exceed 5 ml/L in container mixes. Too much clay can impede drainage in confined volumes.
Top dressing: containers, pots and raised beds
Sprinkle over the soil surface and water in well. The clay particles work into the soil profile over successive waterings. Top dressing is for existing plantings where you cannot remix the soil; the clay incorporates gradually from the surface down. Use the lower rate for maintenance on media that already contain some clay, and the higher rate for very free-draining media that are losing nutrients rapidly. There is a method piece on how to top dress if you are new to it.
Outdoor beds, borders and vegetable plots
Scatter over the soil surface and fork or rake into the top 10–15 cm. Water in well. Use the higher rate (300–500 g/m²) for sandy and free-draining soils that are losing nutrients and drying out rapidly. Use the lower rate (100–200 g/m²) as annual maintenance on soils that have been previously amended. The effect is cumulative and permanent: each application adds to the CEC reservoir that remains in the soil indefinitely.
Composting aid
Sprinkle montmorillonite between layers of compost material, particularly when adding high-nitrogen materials (grass clippings, kitchen waste, manure) that tend to produce ammonia. The clay captures ammonium ions before they volatilise, retaining nitrogen in the compost and reducing odour. The finished compost will have a higher nitrogen content and a richer mineral profile than compost made without clay.
Lawn improvement: sandy soils
Broadcast over the lawn after scarifying or aerating. The clay particles need to reach the root zone, so application to recently aerated turf is far more effective than scattering on a dense, matted surface. Water in thoroughly. Repeat annually for 2–3 years to build a meaningful clay content in the root zone. Once established, the clay is permanent and continues working without further application.
Peat-free and fibre-based media conditioning
Fibre-based peat-free media have a very low CEC and hold nutrients poorly. Adding montmorillonite at 3–5 ml/L raises the effective CEC of the medium several fold, turning an inert sponge into a nutrient-retaining growing medium. Mix thoroughly before planting. Combine with sea shell meal or gypsum for calcium, and volcanic rock dust for trace elements. These media need all three to function as a complete growing medium.
Step-by-step application
- Measure the correct amount. For soil mixes: 2.5–5 ml per litre. For beds: 100–500 g per m². For compost: a few handfuls per layer. A tablespoon is approximately 12–15 g of clay.
- Mix or scatter evenly. For soil building, mix thoroughly into the growing medium. For beds, scatter and fork in. For top dressing, sprinkle and water in. Even distribution matters: clay clumps will create localised drainage problems.
- Water in well. Water activates the swelling and begins integrating the clay into the soil.
- Do not overdo it in containers. Too much clay in a confined volume can impede drainage. Stick to 2.5–5 ml/L for containers. In open ground, drainage is less of a concern and higher rates are safe.
- Store sealed and dry. Montmorillonite is hygroscopic. It will absorb moisture from the air and clump if stored open. If clumped, break apart and use as normal; the quality is unaffected. Indefinite shelf life.
Why clay makes feed go further
Every time you water a pot of sandy compost, some of the nutrients you applied wash out of the bottom. Montmorillonite intercepts them and holds them in the root zone until the plant is ready. Each feed lasts longer, less is wasted, and you feed less often. Over time it can pay for itself in fertiliser you no longer need to buy.
Works well combined with…
Montmorillonite builds CEC. It does not supply trace elements or NPK. Pair it with Volcanic Rock Dust for trace minerals and Humic Acid Flakes for clay-humus complex formation; the three together are the mineral foundation of a living soil. If you would rather buy one bag, the Mineral Mix already includes clay minerals alongside gypsum, sea shell meal and basalt. Add a balanced feed such as All Purpose 4-4-4 or Bloom 2-8-4 for NPK, and Grow-Kashi for the biology that colonises the clay surfaces.
Storage
Keep the bag sealed, cool and dry. It draws moisture from the air and will clump if left open; clumps break up and work as normal. Indefinite shelf life. Keep out of reach of children and pets.
Questions & Answers
Frequently asked questions about montmorillonite clay
Bentonite is the rock, a clay-rich deposit formed from weathered volcanic ash. Montmorillonite is the mineral that makes up most of it: the part that swells, holds water and carries the high cation exchange capacity. When a product is sold as bentonite clay for the garden, montmorillonite is doing the work. Ours comes from natural bentonite deposits with nothing added.
It matters. Sodium bentonite swells enormously and turns to a gel, which suits pond liners and drilling muds, not soil. Calcium bentonite swells modestly, keeps its structure and exchanges calcium freely with roots. This product is listed as calcium montmorillonite, the form suited to soil work. If you bought sodium bentonite for a pond, do not put the leftovers in your beds.
Not at these rates. Adding 2.5–5 ml per litre to a sandy or peat-free medium does not make a clay soil; it adds a small, useful amount of CEC and water retention while the medium stays well drained. In containers, do not go above 5 ml/L. In open ground, even 300–500 g/m² is a fraction of a percent of the soil volume. If your soil is already heavy and sticky, you want granulated gypsum, not more clay.
The Mineral Mix already contains clay minerals, blended with gypsum, sea shell meal and basalt rock dust, so it covers CEC, calcium, sulphur and trace elements in one bag. Buy the straight clay if you build your own soil recipes, want a compost additive, or have a sandy or peat-free medium that will not hold water and nutrients. For general beds and re-amending old compost, the Mineral Mix is the easier choice.
Very little. Montmorillonite sits around neutral, roughly pH 6.5–7.5 depending on the deposit, and at garden rates its effect on soil pH is negligible. If you need to move pH, use sea shell meal to raise it, or sulphur to lower it, alongside the clay. Gypsum adds calcium without shifting pH.
Yes. Fibre-based peat-free media drain fast and hold few nutrients. Mixing in 3–5 ml per litre when you prepare the medium raises its effective CEC several fold, so feeds last longer and baskets dry out less quickly. Pair it with a calcium source (gypsum or sea shell meal) and volcanic rock dust for trace elements, because the clay supplies exchange sites only.
Yes. A smelly heap is usually ammonia, produced when high-nitrogen material breaks down and the ammonium escapes as gas. Montmorillonite holds ammonium on its exchange sites so less leaves as ammonia. Composting trials with bentonite found the finished compost kept more total nitrogen. A few handfuls per barrowload between layers is plenty.
Yes. Montmorillonite is a geological mineral and does not decompose or wear out. Field trials that applied bentonite once measured higher water-holding capacity five years later. If you re-use potting soil from one season to the next, the clay you added in year one is still working in year five.
It is a natural mined clay with nothing added, and once watered in it is simply part of the soil. Like any fine dry mineral it is best not breathed in, so spread it on a still day and keep the bag out of reach of children and pets. It is vegan and contains no animal by-products.
Four sizes: 1.5 kg, 4 kg, 9 kg and 15 kg, and the price per kilo drops as the pack gets bigger. At 100–500 g per m² on beds, a 4 kg bag treats 8–40 m² depending on how sandy the soil is. At 2.5–5 ml per litre of potting mix, 1.5 kg goes a long way. For a lawn at 200–400 g per m², measure the area first and buy the 9 kg or 15 kg pack.
Mainly to give sandy soils and peat-free composts somewhere to hold nutrients and water. It's a swelling clay mineral with a cation exchange capacity of roughly 80–150 meq per 100 g, so its layers hold calcium, magnesium, potassium and ammonium that would otherwise wash through. Mix 2.5–5 ml per litre into potting compost and hanging-basket mixes, or rake it into hungry beds. It also helps keep compost heaps from smelling of ammonia. It doesn't decompose, so what you add this spring is still working years from now.
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