Three essential nutrients, one ancient mineral.
Our Sul-Po-Mag Crystals (also known as Langbeinite, or by its chemical name Sulphate of Potash Magnesia) are one of the few naturally occurring minerals that deliver Potassium (22%), Magnesium (11%), and Sulfur (22%) in a single chloride-free, pH-neutral package. Each crystal is already in the water-soluble sulfate form that your plants can use.
Mined from ancient marine deposits formed roughly 250 million years ago, our Sul-Po-Mag is a clean, reliable single-source amendment to keep on hand for any garden, farm, or living soil build.
Why You'll Love This Product
▪️ Chloride-Free Performance. With less than 3% chloride, Sul-Po-Mag won't damage fine roots or compromise the storage quality of chloride-sensitive crops. Especially well-suited for strawberries, blueberries, peppers, citrus, potatoes, and cannabis — crops where chloride buildup affects flavor, yield, or shelf life.
▪️ Three Nutrients, One Particle. Each crystal contains all three nutrients in a fixed ratio, so distribution stays uniform across your beds, containers, or fields. Compare this to mixing three separate amendments, where coverage and consistency can vary.
▪️ pH-Neutral at Normal Application Rates. When applied as directed, our Sul-Po-Mag won't raise or lower your soil pH. This sets it apart from most other Mg or S sources. Dolomite lime raises pH, while elemental sulfur and ammonium sulfate lower it. If you've already dialed in your pH, langbeinite crystals let you feed K, Mg, and S without undoing your work. A real advantage in established living soils, no-till beds, and acid-loving crops like blueberries.
▪️ Steady, Gradual Release. The nutrients in Sul-Po-Mag are in the water-soluble sulfate form your plants can use, but the dense crystal structure itself dissolves gradually. Nutrients are released steadily as soil moisture and root activity break the crystals down — feeding plants over weeks rather than flushing through with the next watering.
▪️ No Nitrogen. Most "complete" fertilizers (anything with an N value) push leafy growth, which can be a problem during flowering and ripening when you want energy going into fruit, not foliage. Our Sul-Po-Mag delivers K, Mg, and S without that trade-off.
▪️ Low Salt Index. Sul-Po-Mag has one of the lower salt indexes among concentrated potassium fertilizers, making it gentler on roots and soil microbiology. Useful if you've invested in building a living soil and want to avoid disrupting microbial activity.
Application Guide by Crop
Application rates are general guidelines. Adjust based on soil test results and crop requirements.
🌾 General Broadcast Rate For larger areas, apply 5 to 10 lbs per 1,000 sq ft (≈ 2.5 to 5 kg per 100 m², or 220 to 435 lbs per acre). Use the lower end for maintenance on established soils, the higher end when correcting a confirmed deficiency or prepping new beds. Broadcast evenly, lightly work into the top 5 to 8 cm (2 to 3 in) of soil, and water in.
🍅 Tomatoes, Peppers, Eggplant Mix 15 ml (1 tbsp) per plant into the planting hole at transplant. Side-dress with 5 to 10 ml (1 to 2 tsp) per plant at first flower. Especially useful on sandy or leached soils. Many growers report improved flower set after application.
🍓 Berries (Strawberries, Blueberries, Raspberries) Apply 60 g per m² (2 oz per 10 ft²) in
early spring, lightly worked into the top 3 cm of soil. Reapply lightly mid-season. A good fit for blueberries because it's chloride-free and won't disturb their narrow pH window.
🍎 Fruit Trees (Apples, Pears, Stone Fruit) Apply 115 to 225 g (¼ to ½ lb) per inch of trunk diameter, broadcast at the drip line and watered in. Once annually in early spring. Use the higher end on sandy soils, heavily-cropping trees, or where a soil test confirms low K or Mg. Supports fruit sugar development, color, and storage life.
🥬 Leafy Greens & Brassicas Apply 60 to 100 g per m² (2 to 3 oz per 10 ft²) at planting. Sulfur is particularly important here. It contributes to the characteristic flavor and pungency of kale, mustard, broccoli, and arugula.
🥔 Potatoes Apply 100 g per m² (3 oz per 10 ft²) at planting. Chloride-free is important for potatoes, since high-chloride fertilizers can affect starch quality and storage life.
🌿 Cannabis For living soil amends, use 1 tbsp per gallon of soil, mixed at build. For top-dress during flower, apply 5 to 10 ml per 4 L of soil (1 to 2 tsp per gallon), in weeks 3 to 5 of bloom. Supports terpene development and flower density. Pair with our Flower Bloom 2-7-4 for full-spectrum bloom support.
🪴 Container Plants & Houseplants Mix 5 ml per 4 L (1 tsp per gallon) into the soil at potting. Top-dress monthly at half rate during active growth.
🌱 Soil Mixing (Bulk) Up to 1 kg (2.2 lb) per cubic yard of custom soil blend.
A Bit of Geology Worth Knowing
Langbeinite is one of the few minerals on Earth where Potassium, Magnesium, and Sulfur naturally occur together in the same crystal, with the chemical formula K₂Mg₂(SO₄)₃. The largest commercial deposits sit deep beneath the desert near Carlsbad, New Mexico, formed when an ancient inland sea evaporated during the Late Permian period. The mineral is recovered from roughly 800 to 1,400 feet underground, washed, and crushed into usable crystalline form. Nothing has been added, blended, or chemically altered. That's why it's approved for use in certified organic programs.
How It Fits in Your Garden
Sul-Po-Mag is the only product in our catalog that delivers Potassium, Magnesium, and Sulfur in one naturally-mined mineral, chloride-free and pH-neutral at normal application rates. If you've ever looked at our shelf and wondered which potassium product is right for you, here's the simple answer:
▪️ Reach for our Sul-Po-Mag when your soil pH is already where you want it, but you need K, Mg, and S together. Common in sandy, acidic, or heavily-cropped soils, and during the bloom and fruiting stages.
▪️ Reach for our blended fertilizers (Flower Bloom 2-7-4, General Purpose 4-4-4) when you want a complete, hands-off feeding program. Sul-Po-Mag is already in those blends, doing its work.
Not sure which product is right for your soil? We offer professional soil analysis and personalized recommendations. Reach out and we'll help you build a feeding program tailored to your crops and growing conditions.
A Word of Caution, and Why a Soil Test Helps
Sul-Po-Mag is a precision tool, not a general booster. It works best when soil tests confirm low Potassium and Magnesium and Sulfur together. A common pattern in sandy, acidic, or heavily-cropped soils. If your soil already has high magnesium (common where dolomite lime has been used heavily), adding more can lock out calcium and potassium uptake.
If you're not sure where your soil stands, we can help. We offer professional soil analysis. We'll interpret the results, identify what your soil actually needs, and recommend the right amendments and rates for your specific crops. A soil test takes the guesswork out of choosing inputs and helps you spend your fertilizer budget where it'll have the most impact. Contact us to get started.
This product complies with Canada's Organic Production Systems General Principles & Management Standards and Permitted Substances Lists (CAN/CGSB-32.310-2026, CAN/CGSB-32.311-2026).
ANALYSIS:
- Soluble Potash (K2O): 22.0%
- Magnesium (Mg): 11.0%
- Sulfur(S) 22%
