• Dr Forest Cal-Mag Supplement — 750g kraft paper bag, Powdered Cal-Mag Supplement, Calcium 17.9% Magnesium 5.3% Sulphur 16%
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Dr Forest

Cal-Mag for Plants | pH Neutral Gypsum & Kieserite | Calcium Magnesium Sulphur Feed | Dr Forest

Cal-Mag for Plants | pH Neutral Gypsum & Kieserite | Calcium Magnesium Sulphur Feed | Dr Forest

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  • ✓Handcrafted and dispatched from Stockport
  • ✓Made with organic ingredients, no slaughterhouse by-products

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calcium, magnesium and sulphur in one pH-neutral feed

Dr Forest Cal-Mag is a pH-neutral calcium, magnesium and sulphur feed for plants, built on two naturally mined minerals — gypsum and kieserite — both permitted for use in organic growing. It supplies roughly 16% calcium, 4% magnesium and 18% sulphur in a balanced 4:1 calcium-to-magnesium ratio. Unlike garden lime, it raises calcium without lifting your soil pH, so you can correct a shortage without disturbing a pH you are happy with.

It is a small-batch blend handcrafted in Stockport, designed to sit alongside your Dr Forest fertilisers at 10% of the feed rate, so calcium and magnesium are never the nutrient that runs short. A trace of humic-stabilised boron — calcium's partner element — and a little fulvic acid round it out and help the plant take everything up.

For Plants & Soil pH Neutral ~4:1 Ca to Mg Mined Minerals Plant-Based, Peat-Free Made in Stockport
Garden use only — not a human supplement

This is a plant and soil product. It is not food-grade, not tested to supplement standards, and contains boron that is not intended for human consumption. If you are looking for a calcium-magnesium supplement to take yourself, this is not it.

~16%Calcium
~4%Magnesium
~18%Sulphur
~4:1Ca : Mg Ratio

What gardeners use it for

  • Alongside Dr Forest fertilisers — add at 10% of your feed rate so calcium and magnesium keep pace with everything else the plant is being fed.
  • Blossom end rot — keeps calcium well supplied to developing tomatoes and chillies, the crops most prone to it. Best paired with steady, even watering.
  • Calcium deficiency — tip burn in lettuce, distorted or curling new growth on heavy-feeding plants.
  • Magnesium deficiency — interveinal yellowing on the older, lower leaves while the veins stay green.
  • Soft-water gardens — rainwater and low-mineral tap water carry very little calcium or magnesium of their own.
  • Stage-feeding (veg & bloom) — calcium demand climbs as fruit and flowers set and cell walls are built.
  • Heavy clay soil — gypsum helps flocculate tight clay, improving drainage and root penetration over time.

This cal-mag vs dosing two products separately

Dr Forest cal-mag powder

  • One powder: calcium, magnesium, sulphur, boron and fulvic acid, pre-balanced to ~4:1 Ca:Mg.
  • pH-neutral mined-mineral base; mix it into the soil, top-dress and water in, or apply as a stirred suspension.
  • Boron held in a slow-release humic complex, so it does not wash straight through.

Weighing gypsum and Epsom salt yourself

  • Two products to measure, and the ratio is easy to get wrong.
  • Epsom salt is mostly water of crystallisation, so you carry less magnesium per gram.
  • No boron and no fulvic acid in the mix.

Handcrafted in small batches in Stockport.

what's in the blend

Four inputs, each doing one job. The two mined minerals carry the calcium, magnesium and sulphur; the boron and fulvic acid are small additions that make those nutrients work harder in the plant.

01

Naturally mined gypsum — 75%

Calcium sulphate dihydrate, roughly 23% calcium and 18% sulphur. A mined mineral permitted for use in organic growing, and the bulk of the calcium and sulphur in the blend. It dissolves to release calcium without raising soil pH, which is what sets it apart from lime.

02

Mined kieserite — 25%

Magnesium sulphate monohydrate, roughly 16% magnesium and 22% sulphur. Another mined mineral permitted in organic growing. Because it holds far less water than Epsom salt (one water molecule rather than seven), it carries more magnesium per gram and stores without caking.

03

Humic-stabilised boron — 4%

Boron fused into a humic-acid complex. Boron is the most easily leached of all the trace elements, so binding it to humic acid keeps it in the root zone and releases it slowly. It works as a calcium synergist — present at roughly 0.1% boron in the finished blend, a trace, deliberately kept small.

04

Fulvic acid — 0.5%

A low-molecular-weight biostimulant that complexes calcium and magnesium and helps carry them into the roots and around the plant. A small inclusion that improves the uptake of everything else in the blend.

Guaranteed analysis (approximate)

Nutrient Elemental Oxide form
Calcium (Ca) ~16% ~22% CaO
Magnesium (Mg) ~4% ~6.6% MgO
Sulphur (S) ~18% ~45% SO₃
Boron (B) ~0.1% (trace) —
Ca : Mg ratio ~4 : 1 —

Values are nominal, calculated from the mineral inputs. Confirm against the figures on your product label before quoting them as guaranteed.

how to use Dr Forest cal-mag

Water quality first

Use rainwater or stand tap water for 24 hours where you can. The fulvic acid binds calcium and magnesium from hard tap water before it reaches the plant, so soft water gets the best out of the blend.

Applying it with water

If you water it in, treat it as a suspension, not a clear solution. The calcium, magnesium and sulphur dissolve, but the boron is held in a humic form that does not — it settles to the bottom. So stir the powder into your water, leave it to stand about 30 minutes (the kieserite is slow to dissolve), then keep it moving and pour the whole lot onto the soil, sediment and all, so the boron goes down with it. Apply to the root zone only — never through a sprayer or a fine rose, which will clog and leave the boron behind.

Application rates

A note on the dose

These rates look modest next to instant liquid feeds, but the whole dose reaches the roots: you pour the full suspension onto the soil, where the gypsum releases its calcium steadily rather than in one quick flush. Gram for gram it delivers more calcium than a typical bottled cal-mag — it just feeds the soil slowly, the way it should.

With Dr Forest fertilisers — the simple rule

Rate: 10% of your fertiliser dose | Frequency: every feed

Whatever you are feeding, add cal-mag at one tenth of that weight. Feeding 5 g of fertiliser? Add 0.5 g of cal-mag. Feeding 10 g? Add 1 g. Either top-dress it when you feed, or stir it into the same water and pour the lot onto the soil. This keeps calcium and magnesium in step with the rest of the feed without overdoing it.

Top-dressing established plants

Rate: ~30 g per m², or a level teaspoon (about 5 g) per established plant in a large pot | Frequency: every 4–6 weeks in the growing season

Scatter over the surface, scratch lightly into the top centimetre and water in. Keep it light — it is a calcium and magnesium top-up, not the main feed.

Mixing into a soil or potting mix

Rate: ~1–2 g per litre of potting compost, or ~50 g per m² forked into the top 10–15 cm of a bed | Frequency: once, at planting or when mixing up

Blends a season’s worth of calcium and magnesium evenly through the root zone. Useful in peat-free and coir-based mixes, which hold little calcium of their own, and for opening up heavy clay before planting.

Watering it in (used on its own)

Rate: 1 g per litre routine, up to 2 g per litre for heavy feeders or poor soils | Frequency: every 2–3 weeks

For gardeners not on a Dr Forest feed who want steady calcium and magnesium. Stir it into the can as a suspension, keep it moving and pour the whole lot — sediment included — over the root zone. Not for sprayers or fine roses.

Correcting a visible deficiency

Rate: 1–2 g per litre, watered in | Frequency: repeat every 2–3 weeks until new growth recovers

Stir and pour the whole suspension over the root zone. Calcium is immobile in the plant, so it fixes new growth, not the leaves already damaged — and steady, repeated supply matters more than a single strong dose. Watch the fresh growth that follows to judge whether it has worked.

Sensitive crops — go lighter

A few plants are sensitive to boron, which builds up slowly in the soil with repeated use. On these, halve the amount or apply half as often. The boron-sensitive group is beans and peas; strawberries and other soft and cane fruit (raspberries, blackberries, currants, gooseberries); top and stone fruit (apples, pears, cherries, plums, peaches); grapevines; and onions, garlic and leeks. Most other plants — including tomatoes, chillies and the bulk of vegetables — are fine at the standard rate.

Step by step

  1. Measure. Weigh the dose — 10% of your feed weight, or about 1 g per litre on its own.
  2. Disperse. If top-dressing, mix the powder through an equal volume of dry soil or mix first so it does not clump on the surface.
  3. Mix to a suspension. If watering it in, add to water and stir. The fulvic and kieserite dissolve (kieserite is slow — give it about 30 minutes), but the boron will not — it settles. Keep the can moving so the boron stays carried through.
  4. Apply to the soil. Pour the whole lot — sediment included — over the root zone, or top-dress it when you feed. Never through a sprayer or fine rose.
  5. Water in. If you top-dressed, follow with plain water to carry it down to the roots.
Do not double the boron

Boron is needed only in trace amounts and becomes toxic just above sufficiency — the gap between "enough" and "too much" is narrow. The 10% rate keeps boron comfortably in the safe band. Do not stack this with a separate boron product, and do not exceed the rates above to "make up" for a deficiency. Pick the one method that suits your setup rather than layering several at once. With boron, more is not better.

Works well combined with…

Pairs with the Dr Forest Veg 4-4-4 and Bloom feeds for stage-feeding (veg & bloom), where calcium demand rises through fruit and flower set. The Dr Forest Fulvic Acid Powder is the natural companion if you want to push uptake further in soft-water gardens.

Storage

Keep dry and sealed in the compostable pouch. Gypsum and kieserite both absorb moisture and will cake if left open. Store cool and dry, out of reach of children and pets. Kept dry, it lasts several years.

Five common mistakes

  • Treating it like a fertiliser. It is a supplement to a feed, not a feed in its own right.
  • Over-dosing the boron. Stick to the 10% rule and do not layer boron products.
  • Expecting it to repair old damage. Calcium is immobile — judge it by the new growth.
  • Using hard tap water. The fulvic underperforms; switch to rainwater where you can.
  • Leaving the pouch open. It draws in moisture and clumps.

the science behind the blend

Calcium and magnesium are the two structural cations a plant draws on most heavily, and they compete for the same uptake sites at the root. Supply them badly out of balance and one suppresses the other. This blend pairs a pH-neutral calcium source with a soluble magnesium source at a sensible 4:1 ratio, then adds the two elements that make calcium work — boron and fulvic acid.

Feed the soil's calcium and magnesium together, in balance, and you stop either one becoming the limit.

How it works

01

Calcium and sulphur without a pH swing

Gypsum (CaSO₄·2H₂O) dissolves to calcium and sulphate ions and is essentially pH-neutral. Unlike lime, it raises plant-available calcium without raising soil pH, which makes it the right calcium source where pH is already where you want it.1

02

Fast magnesium from kieserite

Magnesium sulphate monohydrate dissolves to release Mg²⁺, the central atom of every chlorophyll molecule. Magnesium is the element most often overlooked in feeding programmes, yet it powers the sugar transport that fills fruit and flowers.2

03

A balanced 4:1, not a magic ratio

Severe cation imbalance causes antagonism at the root, but the old idea of a single "ideal" calcium-to-magnesium ratio is overstated — the real goal is sufficiency of each nutrient across a wide workable range. A 4:1 supply delivers plenty of both without one crowding out the other.3

04

Boron, calcium's partner

Boron governs the movement of calcium through the plant and cross-links the pectins that hold cell walls together. Deficiency shows first at the growing points and in poor fruit and seed set — the same places calcium problems appear, which is why the two are dosed together.4

05

Holding boron in place

Boron is the most leachable trace element and washes out of low-humus soils quickly. Fusing it into a humic complex keeps it in the root zone and releases it gradually, so a small, safe dose lasts rather than draining away after the first watering.5

06

Fulvic acid as an uptake aid

Low-molecular-weight fulvic acid complexes mineral cations and acts as a biostimulant, improving the uptake and translocation of calcium and magnesium into and around the plant.6

07

Sulphur, the quiet third nutrient

Both mined minerals are sulphate sources, so the blend also supplies sulphur — a secondary nutrient needed for protein and enzyme synthesis and increasingly short in modern soils.7

Scientific References

  1. Shainberg, I., Sumner, M.E., Miller, W.P., et al. (1989). Use of gypsum on soils: A review. Advances in Soil Science, 9, 1–111.
  2. Cakmak, I. & Yazici, A.M. (2010). Magnesium: A forgotten element in crop production. Better Crops with Plant Food, 94(2), 23–25.
  3. Kopittke, P.M. & Menzies, N.W. (2007). A review of the use of the basic cation saturation ratio and the "ideal" soil. Soil Science Society of America Journal, 71(2), 259–265.
  4. Brown, P.H., Bellaloui, N., Wimmer, M.A., et al. (2002). Boron in plant biology. Plant Biology, 4(2), 205–223.
  5. Canellas, L.P., Olivares, F.L., Aguiar, N.O., et al. (2015). Humic and fulvic acids as biostimulants in horticulture. Scientia Horticulturae, 196, 15–27.
  6. Marschner, H. (2012). Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press.
  7. Hawkesford, M.J. & De Kok, L.J. (2006). Managing sulphur metabolism in plants. Plant, Cell & Environment, 29(3), 382–395.

cal-mag for plants — your questions

No. This is a garden product for plants and soil. It is not food-grade, has not been tested to the heavy-metal and purity standards a human supplement must meet, and it contains boron that is not intended for people to ingest. For a calcium-magnesium supplement to take yourself, speak to a pharmacy — not this listing.
A calcium, magnesium and sulphur feed for plants, built on naturally mined gypsum and kieserite, with a trace of boron and a little fulvic acid. It supplies roughly 16% calcium, 4% magnesium and 18% sulphur in a balanced 4:1 calcium-to-magnesium ratio. It is a supplement to your feeding programme, used to keep these structural nutrients from running short.
Add it at 10% of your fertiliser dose, every feed. If you are feeding 5 g of fertiliser, add 0.5 g of cal-mag; for 10 g, add 1 g. Top-dress it when you feed, or stir it into the same water and pour the lot onto the soil. That keeps calcium and magnesium in step with the rest of the feed without overdoing it.
It is a strong tool for it. Blossom end rot is a calcium shortage at the fruit, and tomatoes and chillies are the crops most prone to it, so keeping calcium well supplied is a key part of preventing it. One honest caveat: blossom end rot is often less about how much calcium is in the soil and more about calcium reaching the fruit, which uneven or erratic watering disrupts. So feed cal-mag to keep the supply up, and water steadily and evenly alongside it — together they give you the best protection.
No. Gypsum is calcium sulphate and is pH-neutral, so it adds calcium without lifting your soil pH. That is the main reason to choose it over garden lime or dolomite, which both raise pH.
Lime (calcium carbonate) raises calcium and soil pH at the same time, which is fine if your soil is acidic but a problem if it is not. This blend raises calcium with no pH shift, so you can correct a calcium shortage without disturbing a pH you are happy with.
Kieserite is the naturally mined monohydrate form of magnesium sulphate. It holds far less water than Epsom salt (one water molecule versus seven), so it carries more magnesium per gram, stores without caking, and is permitted for use in organic growing as a mined mineral.
Most likely, yes. Dr Forest feeds carry some calcium and magnesium from minerals such as polyhalite, but the heavy biostimulant loading limits how much built-in calcium and magnesium there is room for. Adding cal-mag at 10% of the feed rate makes sure these two structural nutrients are never the limiting factor.
Yes — boron is toxic to plants in excess, and its safe window is narrow. The boron in this blend is a deliberate trace (~0.1%) held in a slow-release humic complex, and the rates given keep it well within the safe band. The rule is simple: do not stack it with another boron product, and do not exceed the stated rates.
Calcium symptoms appear in new growth — distorted young leaves, blossom end rot in tomatoes and chillies, tip burn in lettuce. Magnesium symptoms appear in old growth — interveinal yellowing on the lower, older leaves while the veins stay green. Calcium is immobile, magnesium is mobile, which is why they show up at opposite ends of the plant.
No. It is a soil product, and it does not make a clean solution — the boron and some of the gypsum stay as fine sediment that would clog a sprayer or fine rose and leave the boron behind. Calcium is also immobile in the plant, so getting it to the roots is the reliable route. Apply it to the soil, not the leaves.
It is built on mined minerals — gypsum and kieserite — that are permitted for use in organic growing, and it is made with organic-allowable inputs. It is not a certified organic product. If certification matters for your situation, check the inputs against your own scheme's rules, as boron is a restricted input under some organic standards.
It does for the fulvic acid. Hard tap water is already high in calcium and magnesium, which bind the fulvic before it can do its job. Use rainwater, or stand tap water for 24 hours, to get the best from the blend. The mineral feeding itself still works in hard water — it is only the fulvic that prefers soft.
Not fully, and it is not meant to. The calcium, magnesium and sulphur dissolve (the kieserite is slow, so give it about 30 minutes), but the boron is held in a humic form that does not dissolve — it settles to the bottom. If you water it in, treat it as a suspension: keep it stirred and pour the whole lot, sediment and all, over the soil so the boron goes down too. Or simply apply it dry and water in.
Treat it like any garden product: keep it out of reach of children and pets and do not ingest it. Used at the rates given, the mineral inputs are not a hazard to bees. It is a plant-based, peat-free soil and plant feed, not a pesticide.
Handcrafted in small batches in Stockport, Greater Manchester, blended and packed by hand.
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