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
Couldn't load pickup availability
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.
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.
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.
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.
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.
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.
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
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.
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
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
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
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
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)
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
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.
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
- Measure. Weigh the dose — 10% of your feed weight, or about 1 g per litre on its own.
- 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.
- 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.
- 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.
- Water in. If you top-dressed, follow with plain water to carry it down to the roots.
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.
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.
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.
How it works
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
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
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
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
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
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
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
- 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.
- Cakmak, I. & Yazici, A.M. (2010). Magnesium: A forgotten element in crop production. Better Crops with Plant Food, 94(2), 23–25.
- 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.
- Brown, P.H., Bellaloui, N., Wimmer, M.A., et al. (2002). Boron in plant biology. Plant Biology, 4(2), 205–223.
- 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.
- Marschner, H. (2012). Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press.
- 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
Share
