Buying guide · Calcium fertilizers
The best calcium fertilizer depends on your water, your pH, and your calendar.
Six Greenway Biotech products carry calcium, from 3.2% to 24% by weight, and no single one is best. Calcium nitrate and Cal-Mag Plus dissolve and deliver in days, which is what a hydroponic reservoir or a container on RO water needs. Gypsum, dolomite lime, bone meal, and crustacean meal release over a season and build the soil's reserve, which is what an in-ground bed needs before planting. This guide compares all six on calcium per pound, what else comes in the bag, speed, pH effect, and organic status, matches them to real situations, and starts with the question most buyers skip: whether you need one at all.
24%
Most calcium per pound — Bone Meal 3-15-0, released over months
Days
Fastest to the root — Calcium Nitrate and Cal-Mag Plus, fully water-soluble
pH ±0
Calcium without a pH shift — gypsum and crustacean meal; lime raises pH
0
Chelate needed — calcium stays soluble across the garden pH range
01 · Before you buy
Do You Need a Calcium Fertilizer at All?
Most calcium symptoms are delivery problems. A test decides whether a product will help.
This guide compares six calcium products sold by Greenway Biotech and names non-Greenway alternatives where they fit better. Analyses come from product labels; the ppm and pounds-per-area figures are calculations from those analyses, not crop trials.
Blossom end rot on tomatoes, tip burn in lettuce, and hooked young leaves all look like calcium deficiency, and often are not. Calcium travels into fruit and new leaves only with the plant's water stream, so uneven watering, a heat spell, a stressed root system, or excess nitrogen and potassium can starve the fruit of calcium in soil that tests perfectly adequate[1][3]. Extension trials have not shown calcium sprays to fix blossom end rot once it appears, and adding calcium to a soil that already has enough changes nothing[3][4]. The mechanism is covered in our guide to the function of calcium in plants; the buying decision rests on three facts about your situation.
What does the test say? A soil test that reports calcium, magnesium, potassium, pH, and sodium (about $15–30 through most extension labs) is the only way to know whether calcium is short, whether pH needs to move with it, and whether lime or gypsum is the right form. For hydroponics, coco, and containers on treated water, the equivalent is a water report listing calcium, magnesium, sodium, alkalinity, and EC — an EC reading alone cannot tell you which ions it is made of. Ion-exchange-softened household water is a different case: it trades calcium and magnesium for sodium, so test sodium and feed from an unsoftened bypass or an RO source instead.
Reading the result: labs report calcium in parts per million or pounds per acre, sometimes with a percent base saturation, and each lab publishes its own sufficiency range — use that range, not a number from the internet. As one example of scale, Georgia extension treats 800 lb of calcium per acre or more, at a pH of 6.0 or above, as enough for proper crop growth[3]. Do not buy to a calcium-to-magnesium ratio: decades of trials found crops tolerate a wide range of ratios as long as neither nutrient is actually short, and correcting a "wrong" ratio on a high-fertility soil can mean tons of unnecessary calcium[7]. Sufficiency is the target; a ratio is not.
Where are you growing? In-ground beds hold a calcium reserve on their exchange sites; the products that build that reserve are slow minerals and meals. Reservoirs, coco, and soilless mix hold little or none, so calcium has to arrive dissolved in the feed, and reverse-osmosis water starts at zero.
When do you need it? Visible symptoms call for diagnosis, not a purchase. In soilless systems, correct a confirmed deficit through the complete feed; in soil, fix irrigation and root problems first and add soluble calcium only when a test and the crop's nitrogen budget justify it. A bed you are preparing for next month calls for an amendment worked in three to six weeks ahead. Buy for the calendar you actually have.
The short version
A garden that tests adequate for calcium at a workable pH needs even watering, mulch, and moderate nitrogen — not a calcium product. Where the test shows calcium low, the rest of this guide picks the form. Where you grow in water or soilless media, calcium is part of the feed by definition and the question is only which soluble source.
02 · The five things that matter
How to Compare Calcium Fertilizers
Calcium per pound is the number on the bag. Speed, co-nutrients, pH effect, and organic status decide whether it is the right bag.
1. Calcium per pound — and why the highest number is rarely the fastest
Calcium appears on the guaranteed analysis as a plain percentage, below the three N–P₂O₅–K₂O numbers. Bone Meal 3-15-0 leads at 24% calcium, with Dolomite Lime (22.7%) and Gypsum (22.5%) close behind; Calcium Nitrate 15.5-0-0 carries 19%, Crustacean Meal 4-0-0 12%, and Cal-Mag Plus 2-0-0 3.2%, all per the product pages retrieved August 24, 2026. Concentration and speed run in opposite directions: the three most concentrated products are minerals and meals that release over a season, while the two soluble salts deliver the calcium they carry within days. Neither is better; they answer different questions.
2. Speed and form
Water-soluble salts (Calcium Nitrate, Cal-Mag Plus) dissolve completely, work in reservoirs, fertigation, drenches, and sprays, and correct a tested shortfall mid-season. Minerals and meals (gypsum, dolomite, bone meal, crustacean meal) are applied dry, worked into soil, and depend on moisture and, for the meals, on soil biology above about 50°F. They are for bed preparation, not emergencies, and none of them belongs in a reservoir or a sprayer.
How fast is fast? A soluble salt is in the root zone with the next irrigation and in the plant within days. Gypsum dissolves only to about 2.5 g per liter, so it releases over weeks as rain and irrigation move through it — the reason it needs no soil biology and works in cool soil, and the reason it is useless in a sprayer. Lime dissolves over months, faster the finer the grind and the more acid the soil, which is why the product page asks for a retest at three to six months. Bone meal and crustacean meal depend on soil microbes, which means warm soil above about 50°F and partial first-season availability; the crustacean meal page budgets 40–60% of its rated nitrogen in the year of application, and the calcium follows the same slow curve. A calendar, not a preference, decides between the two families.
3. What else is in the bag
No calcium product is calcium alone, and the co-nutrient is often the deciding factor. Calcium nitrate brings 15.5% nitrogen, welcome in a growing crop and unwelcome late in the season. Gypsum brings 18% sulfur. Dolomite brings 11.8% magnesium and raises pH. Bone meal brings 15% available phosphate (P₂O₅), which a high-phosphorus soil does not need. Crustacean meal brings 4% slow nitrogen, 1.3% magnesium, and chitin. Cal-Mag Plus brings magnesium and EDTA iron. Match the passenger to what your test says is short, not just the calcium. The passengers are not small: at the product-page rates, crustacean meal's 5–10 lb per 100 sq ft adds 2–4 lb of nitrogen per 1,000 sq ft, and bone meal's 2.5–5 lb per 100 sq ft adds 3.75–7.5 lb of P₂O₅ per 1,000 sq ft — a season's worth of either on many beds.
4. Effect on pH
Dolomite is a liming material: it raises soil pH as it releases calcium, which is right on acid soil and wrong above about 7.0. Gypsum and crustacean meal supply calcium with essentially no pH change[4]. Calcium nitrate's solution is near neutral, though nitrate uptake tends to raise root-zone pH over time in soilless systems[5]. Most vegetables take up calcium well between about pH 6.0 and 7.0; below 6.0 availability drops, and by pH 5.0 dissolved aluminum damages roots[2][5]. If your test says the pH needs to move, the calcium source should move it in the right direction; if the pH is fine, pick one that leaves it alone.
5. Organic status
Bone Meal 3-15-0 is repackaged from OMRI Listed® material, per its product page. Mined gypsum is generally allowed in certified organic systems subject to certifier review, and dolomite lime is a mined mineral on the same footing. Crustacean meal is ground crab and shrimp shell. Calcium Nitrate and Cal-Mag Plus are mineral nitrate salts, and Cal-Mag Plus carries an EDTA chelate, so neither fits a certified organic program. Check your certifier before relying on any of them.
Calcium does not need a chelate
Iron, zinc, manganese, and copper lock up as pH climbs, which is the case for chelates in our guide to sulfate vs. chelated fertilizers. Plants absorb calcium as the simple Ca²⁺ ion, so chelation is not required for uptake. Source solubility and tank compatibility still matter — concentrated calcium precipitates with phosphate, sulfate, or carbonate — which is where complexed calcium products can earn a place in foliar or tank-mix formulations. Judge them on calcium delivered, compatibility, and price, not on the word chelate.
03 · Product by product
The Six Calcium Fertilizers, Compared Honestly
Analyses and rates from each product page, retrieved August 24, 2026. The label on the bag governs.
Calcium Nitrate 15.5-0-0 — the fast, soluble standard
A fully water-soluble salt supplying 19% calcium with 15.5% nitrogen, 14.5% of it as nitrate[1]. It is the reference calcium source for hydroponics, fertigation, and mid-season correction because it dissolves completely and reaches the root within days. The nitrogen is the trade-off: every gram per gallon adds about 50 ppm calcium and 41 ppm nitrogen, so the 3–4 g per gallon that reaches 150–200 ppm calcium in zero-calcium water also brings roughly 125–165 ppm nitrogen — a formulation input to subtract from the base feed, not a stand-alone rate — and a 2 lb per 100 sq ft bed rate delivers 3.1 lb of nitrogen per 1,000 sq ft in one pass, a full season's nitrogen for many vegetables. Buy it for reservoirs, containers on RO water, and tested shortfalls in a crop that still wants nitrogen; skip it late in the season or on a plant already pushing leaf. Never mix its concentrate with phosphate or sulfate concentrates. Calcium Nitrate 15.5-0-0 →
Cal-Mag Plus 2-0-0 — calcium, magnesium, and iron for soft water and coco
A water-soluble concentrate supplying 3.2% calcium, 1.2% magnesium, 2% nitrogen, and 0.1% EDTA iron, dosed from a stock of 2.5 lb per gallon at 2.5–10 ml per gallon of reservoir water by growth stage and starting water. At 10 ml per gallon it adds roughly 25 ppm calcium and 9 ppm magnesium — a supplement within a complete program, not the whole calcium supply, so count it together with your water and base feed against the target. It earns its place on RO, distilled, rainwater, or naturally low-mineral water, in coco coir (which binds calcium at every watering), and under intense indoor lighting where all three nutrients run short together. It is not a soil amendment and not for certified organic programs. Cal-Mag Plus 2-0-0 →
Gypsum (calcium sulfate) — calcium and sulfur with no pH shift
Mined calcium sulfate dihydrate supplying 22.5% calcium and 18% sulfur. Its defining property is that it supplies calcium without moving pH, which makes it the soil calcium source when the pH is already where it should be, or too high for lime. Its second job is displacing sodium from sodic (high-sodium) soils so it can leach away. It is not a general fix for compacted clay, and it can increase leaching of magnesium, potassium, iron, and manganese, so it belongs only where a test shows the need[4]. The product page's 2–4 lb per 100 sq ft into the top 6 inches is a starting range where a test shows the need; Washington State notes there are no broad, science-based home-garden gypsum rates, so a lab recommendation beats a rule of thumb[4]. Containers: 1 tablespoon per gallon of potting mix. Not for reservoirs or sprayers. Generally allowed in organic systems subject to certifier review. Gypsum →
Dolomite Lime — calcium and magnesium that raise acid pH
Ground calcium–magnesium carbonate supplying 22.7% calcium and 11.8% magnesium while raising soil pH. It is the right buy when a soil test shows low pH and low magnesium together, applied 3–4 weeks before planting at the rate your lab's lime requirement calls for — labs derive it from buffer pH and target pH, and the product page's texture-based range runs 2–10 lb per 100 sq ft — never more than 5 lb per 100 sq ft (50 lb per 1,000) in a single pass, with a retest in 3–6 months. It is the wrong buy on soil above about pH 7.0, on soil already high in magnesium, and under blueberries, azaleas, rhododendrons, or potatoes. Over-liming locks out iron, manganese, zinc, and boron, so lime only on a test. Dolomite Lime →
Bone Meal 3-15-0 — the most calcium per pound, with phosphate attached
Steamed bone meal supplying 24% calcium, 15% available phosphate (P₂O₅), and 3% nitrogen, repackaged from OMRI Listed® material per its product page. It releases over months through soil biology, working best at pH 7.0 or below and in soil above about 50°F. The phosphate is the decision point: at 2.5–5 lb per 100 sq ft it is a good organic calcium-and-phosphorus source where soil phosphorus tests low to moderate, and the wrong product where phosphorus is already high, which is common in long-fertilized garden beds. Two tablespoons in a tomato or pepper planting hole is the product-page rate for transplants. Bone Meal 3-15-0 →
Crustacean Meal 4-0-0 — organic calcium and slow nitrogen without a pH change
Ground crab and shrimp shell supplying 12% calcium, 4% slow-release nitrogen, 1.3% magnesium, and chitin, with minimal effect on soil pH. It fills the gap between gypsum (calcium, no nitrogen) and blood meal (nitrogen, no calcium) for organic beds that need both, worked in at 5–10 lb per 100 sq ft four to six weeks before planting, or 1–2 tablespoons per gallon of potting mix. Nitrogen availability is partial in the first season, and it is an amendment, never a spray. Crustacean Meal 4-0-0 →
Three products that are not calcium fertilizers
Tomato Fertilizer 4-18-38 contains no calcium by design, because calcium and phosphate precipitate together in concentrated solution; it is the base feed that pairs with a separate calcium source. Azomite 0-0-0.2 lists 1.8% calcium on its analysis — a trace-mineral remineralizer, not a calcium source. Epsom salt is magnesium sulfate and supplies no calcium; adding it to a plant with blossom end rot can increase magnesium competition with calcium.
Quiz · Two minutes
Which calcium source fits your garden?
Five questions on where you grow, your pH, and what else is short. The situation table below gives the same answers in one screen.
04 · The decision
The Best Calcium Fertilizer by Situation
Ten situations, one recommendation each. Every row assumes a test has shown calcium is actually short — except the reservoir rows, where calcium is part of the feed by definition.
| Situation | Best choice | Why |
|---|---|---|
| Hydroponics on RO, distilled, rain, or low-mineral water | Calcium Nitrate, with Cal-Mag Plus for magnesium | Zero-calcium water needs the full 150–200 ppm from the feed; about 3–4 g of calcium nitrate per gallon gets there and brings ~125–165 ppm nitrogen to subtract from the base feed |
| Hydroponics on hard tap or well water | Water test first; reduce calcium inputs | Hard water may already carry most of the calcium; dose the balance, not the full target |
| Coco coir | Cal-Mag Plus at every watering, pre-buffered | Coco binds calcium and magnesium from every feed; 5–7.5 ml per gallon, 10 ml per gallon soak for new coco |
| Containers on potting mix | Check the mix label first; a soluble source in the feed for the deficit | Many mixes already carry lime, gypsum, or a starter charge; add only what the label and feed analysis leave short |
| In-ground beds, pH below 6.0 | Dolomite Lime (or calcitic lime if magnesium is high) | Raises pH and supplies calcium in one pass; 3–4 weeks before planting, on a test |
| In-ground beds, pH 6.0–7.5, test shows calcium or sulfur short or sodium high | Gypsum, at the lab's rate | Calcium without moving a pH that is already right; never lime here, and no gypsum without a demonstrated need[4] |
| Alkaline soil, pH above 7.5 | Gypsum or a soluble source; test first | Calcium is usually abundant; the real problem is iron and zinc lockout |
| Sodic (high-sodium) soil | Gypsum, with leaching irrigation | Calcium displaces sodium from exchange sites so water can carry it away[4][6] |
| Organic beds needing calcium and phosphorus | Bone Meal 3-15-0 | Only where phosphorus tests low to moderate; otherwise gypsum or crustacean meal |
| Organic beds needing calcium and nitrogen, pH fine | Crustacean Meal 4-0-0 | Calcium, slow nitrogen, and chitin without a pH shift; four to six weeks ahead |
| Blossom end rot showing now | Even watering, mulch, moderate nitrogen; a soluble source only if a test shows calcium short | Sprays have not been shown to help; damaged fruit will not recover[2][3] |
| Pairing with Tomato 4-18-38 | Calcium Nitrate or Cal-Mag Plus in hydro and containers; gypsum in beds | 4-18-38 has no calcium by design; add the calcium source separately, never as a mixed concentrate |
Two situations deserve a note. Hard water is the one most people get wrong: a water report showing 60 ppm calcium means a reservoir already holds a third of a fruiting crop's target before any fertilizer goes in, and dosing the full label rate on top pushes EC up for no benefit. And organic growers with an active deficiency have no fast option; bone meal, gypsum, and crustacean meal all take weeks, so the organic strategy is to amend on a test before planting and rely on even watering during the season.
Two uses fall outside the table. Orchard calcium is mostly a spray program for fruit disorders such as bitter pit in apples, and the usual spray material is calcium chloride, which supplies calcium with no nitrogen; calcium nitrate works as a foliar where extra nitrogen is wanted and the cultivar tolerates it, and soil calcium for bearing trees should be set from the product page with regional extension guidance rather than a blanket rate. Lawns rarely need a calcium product at all: gypsum on turf is justified where a soil test shows sodium or a real calcium shortfall at adequate pH, not as a routine annual application, and lime on turf is a pH decision made on a test[4].
05 · Side by side
Calcium Fertilizers Compared
| Product | Calcium | Also supplies | Speed | pH effect | Organic | Systems |
|---|---|---|---|---|---|---|
| Calcium Nitrate 15.5-0-0 | 19% | 15.5% N (14.5% nitrate) | Days | Near-neutral; nitrate uptake can raise root-zone pH | No | Hydro, fertigation, drench, foliar |
| Cal-Mag Plus 2-0-0 | 3.2% | 1.2% Mg, 2% N, 0.1% Fe EDTA | Days | Neutral in solution | No | Hydro, coco, containers, foliar |
| Gypsum | 22.5% | 18% S | Weeks (moderately soluble) | None | Generally allowed; certifier review | Soil, potting mix; not hydro or foliar |
| Dolomite Lime | 22.7% | 11.8% Mg | Months | Raises pH | Mined mineral; certifier review | Acid soil only |
| Bone Meal 3-15-0 | 24% | 15% P₂O₅, 3% N | Months | Slight rise | Repackaged from OMRI Listed® material | Soil, planting holes; pH ≤7.0 |
| Crustacean Meal 4-0-0 | 12% | 4% N, 1.3% Mg, chitin | Months | Minimal | Crab and shrimp shell; certifier review | Soil, potting mix, compost |
06 · Cost and coverage
How to Compare Calcium Fertilizer Cost
A calculation method, not a price list: prices change, the ratios do not.
The honest comparison is price per pound of calcium, and it favors large bags heavily because small bags are mostly shipping. The arithmetic is one line: bag price ÷ (bag weight in pounds × calcium percentage as a decimal). A 25 lb bag of gypsum holds 25 × 0.225 = 5.6 lb of calcium; a 5 lb bag of calcium nitrate holds 5 × 0.19 = 0.95 lb; a 5 lb bag of bone meal holds 1.2 lb. Run the line on the current price of the size you would actually order, because prices move and the ratio does not.
Coverage is the other half. At the product-page rates, 2–4 lb of gypsum treats 100 sq ft of bed and delivers 0.45–0.9 lb of calcium; 5 lb of dolomite treats 100 sq ft of loam and delivers about 1.1 lb of calcium and 0.6 lb of magnesium; 5 lb of bone meal at the top rate covers 100 sq ft with 1.2 lb of calcium and 0.75 lb of P₂O₅; 5–10 lb of crustacean meal covers 100 sq ft with 0.6–1.2 lb of calcium. In a reservoir, 5 lb of calcium nitrate mixes roughly 570–750 gallons at 3–4 g per gallon, and 5 lb of Cal-Mag Plus makes 2 gallons of stock — about 1,500 gallons of finished solution at 5 ml per gallon.
What the soluble salts cost you that the bag does not show
Calcium nitrate's price per pound of calcium looks high next to gypsum, and that is the wrong comparison: it buys speed and solubility, which the minerals cannot supply at any price. The better comparison is calcium nitrate against Cal-Mag Plus, where calcium nitrate delivers about six times the calcium per gram and Cal-Mag Plus delivers the magnesium and iron. On soft water most growers end up with both. The fertilizer calculator converts any of these rates to your bed size or reservoir volume.
Buy the size you will use. Calcium nitrate is hygroscopic and cakes into a block in a humid shed, so a home grower is better served by a bag that lasts one season than by a bulk bag that lasts three; keep it sealed and dry. Gypsum and dolomite store indefinitely if kept dry. Bone meal and crustacean meal carry an animal-origin odor that draws dogs and wildlife, so they belong in a sealed container off the floor. Every Greenway bag carries a 90-day guarantee and ships free in the continental US over $100, which is usually the point at which the larger size wins on price per pound of calcium.
How the numbers in this guide were calculated
A US gallon is 3.785 liters. Parts per million from a soluble product = grams per gallon × the guaranteed-analysis percentage ÷ 3.785 (so 1 g of 19% calcium nitrate per gallon ≈ 50 ppm calcium). Cal-Mag Plus figures assume the product page's 2.5 lb per gallon stock, about 300 g per liter, so 10 ml of stock ≈ 3 g of product. Pounds per area = product-page rate × analysis percentage, normalized to 1,000 sq ft where stated. All figures are calculated nutrient contributions from label analyses, not plant-performance trials.
Mistakes that waste the purchase
Mixing calcium with phosphate or sulfate concentrates. Calcium nitrate and Cal-Mag Plus react with concentrated MAP, MKP, potassium sulfate, Epsom salt, or gypsum to form insoluble calcium phosphate and sulfate, which takes the calcium out of solution and clogs emitters. Separate stock tanks; dilute before they meet; jar-test unfamiliar combinations.
Liming without a test. Dolomite on a soil already near neutral, or already high in magnesium, buys a micronutrient lockout. Gypsum is the calcium source for a pH that should not move.
Buying gypsum to loosen clay. The structural benefit is documented for sodic soils and some heavy clays, not for ordinary compaction, and gypsum can leach magnesium and potassium[4]. Coarse mulch and organic matter do more for compacted garden soil.
Treating Cal-Mag Plus as the whole calcium supply. At 10 ml per gallon it adds about 25 ppm calcium. A fruiting crop on RO water targets 150–200 ppm; the rest comes from calcium nitrate.
Eggshells and Epsom salt for blossom end rot. Coarse eggshell is calcium carbonate that takes years to dissolve; Epsom salt supplies no calcium at all. The fix for blossom end rot in calcium-adequate soil is even watering[1][3].
Key takeaways
- Test first. A garden that tests adequate for calcium at a workable pH needs even watering and moderate nitrogen, not a calcium product.
- Soluble salts for speed and soilless systems: Calcium Nitrate (19% Ca, with nitrogen) for reservoirs and tested mid-season shortfalls; Cal-Mag Plus (3.2% Ca, with magnesium and iron) as the supplement for RO water and coco.
- Minerals and meals for building soil before planting: gypsum where pH should not move, dolomite where pH and magnesium are both low, bone meal where phosphorus is also low, crustacean meal where nitrogen is also wanted without a pH shift.
- Concentration and speed run opposite ways; compare on price per pound of calcium at the bag size you would actually buy.
- Tomato 4-18-38, Azomite, and Epsom salt are not calcium fertilizers. Never combine calcium concentrates with phosphate or sulfate concentrates.
Shop
Calcium sources, matched to the job
Calcium Nitrate 15.5-0-0
19% calcium plus nitrate nitrogen, fully soluble. Reservoirs, fertigation, and tested mid-season shortfalls.
RO water & cocoCal-Mag Plus 2-0-0
Calcium, magnesium, and EDTA iron in one supplement. The first pour on soft water.
No pH shiftGypsum
22.5% calcium and 18% sulfur. Tested calcium shortfalls at adequate pH, and sodic soil.
Acid soilDolomite Lime
22.7% calcium and 11.8% magnesium, raising pH as it releases. Three to four weeks before planting, on a test.
Organic beds: Bone Meal 3-15-0 (24% Ca, where phosphorus is low) · Crustacean Meal 4-0-0 (12% Ca, with slow nitrogen). Browse the full calcium & magnesium collection, hydroponic nutrients, or organic fertilizers and meals.
FAQ
Calcium fertilizer questions
What is the best calcium fertilizer for tomatoes?
It depends on where the tomatoes grow. In hydroponics or containers on soft water, Calcium Nitrate is the standard, with Cal-Mag Plus for magnesium. In raised beds and in-ground soil, gypsum worked in before planting supplies calcium without changing pH, and dolomite does the same job where the soil is acid. Blossom end rot on tomatoes is usually a watering problem rather than a calcium shortage, so even moisture and moderate nitrogen matter more than the product.
What is the best calcium source for hydroponics?
Calcium Nitrate 15.5-0-0. It dissolves completely, supplies 19% calcium with nitrate nitrogen, and reaches the 150–200 ppm calcium target for fruiting crops at roughly 3–4 g per gallon in zero-calcium water. Add it to the reservoir first, before phosphate or sulfate nutrients. The same 3–4 g brings roughly 125–165 ppm nitrogen, which comes off the base feed. Cal-Mag Plus supplements magnesium and iron alongside it; at 10 ml per gallon it adds only about 25 ppm calcium, so it is not a stand-alone calcium source.
What is the best organic calcium fertilizer?
Three options, chosen by what else the soil needs. Gypsum supplies calcium and sulfur without moving pH and is generally allowed in organic systems subject to certifier review. Bone Meal 3-15-0, repackaged from OMRI Listed® material, carries the most calcium per pound (24%) with 15% available phosphate, so it fits only where phosphorus tests low to moderate. Crustacean Meal 4-0-0 adds calcium with slow nitrogen and chitin at a neutral pH. All three take weeks to release, so apply them before planting on a soil test.
Calcium nitrate vs. gypsum vs. lime — which should I use?
Calcium nitrate is a soluble salt: use it when you need calcium within days, in a reservoir, or in a crop that also wants nitrogen. Gypsum is a slow mineral that supplies calcium and sulfur without changing pH: use it in soil that tests low in calcium at a pH that should stay put, or to displace sodium. Lime (dolomite or calcitic) supplies calcium and raises pH: use it only where a soil test shows the pH is too low, three to four weeks before planting. The soil test decides between gypsum and lime; the calendar decides between the minerals and calcium nitrate.
Is Cal-Mag Plus enough calcium by itself?
Only when the water and the base feed supply the rest of the calcium. At 10 ml of stock per gallon it adds about 25 ppm calcium against a 100–150 ppm target for leafy crops and 150–200 ppm for fruiting crops, so on RO, distilled, or rainwater it is not a complete calcium source by itself at any stage. Its job is to cover the calcium, magnesium, and iron that low-mineral water and coco coir leave out, alongside a base feed or calcium nitrate that carries the bulk.
What is the best calcium fertilizer for containers and potting mix?
Start with the potting-mix label and the analysis of your complete feed: many mixes already contain lime or gypsum and a starter charge, and some feeds carry calcium. Add only the deficit. Where the mix has none, one tablespoon of gypsum per gallon at planting builds a small reserve without changing pH (bone meal or crustacean meal in an organic mix), and a soluble source in the feed carries fruiting crops, because soilless mix holds little calcium and containers dry unevenly. Coco coir needs Cal-Mag Plus at every watering and a pre-buffer soak before planting.
Do eggshells, oyster shell, or antacid tablets work as calcium fertilizer?
Eggshell and oyster shell are calcium carbonate, which dissolves slowly and only in acid soil: coarse eggshell pieces take years to break down and add almost nothing in the season you apply them, while finely ground shell behaves like a slow, mild lime. Some antacids are calcium carbonate too, but ingredients vary — magnesium, aluminum, and sodium compounds are common — and tablets are not formulated or labeled as fertilizer. None of them helps blossom end rot in calcium-adequate soil, where the cause is water delivery, not calcium supply.
Review & sources
Reviewed and sourced
Reviewed by Amir Tajer, B.S.M.E., QAL — Co-Owner & Technical Director, Greenway Biotech. Reviewed against Oregon State University Extension, Iowa State University Extension, University of Georgia Extension, Washington State University Extension, UF/IFAS Extension, and University of Minnesota Extension guidance. Last updated August 24, 2026; originally published October 15, 2021.
Disclosure: Greenway Biotech sells the calcium nitrate, Cal-Mag Plus, gypsum, dolomite lime, bone meal, and crustacean meal discussed here. Calcium chloride, calcitic lime, oyster shell, and eggshell are named where they are the appropriate alternative. Analyses and rates are taken from the live Greenway Biotech product pages as retrieved on August 24, 2026; the label on your product governs. Each fertilizer discussed here is CDFA-registered and third-party tested for heavy metals; results are posted on our heavy metal analysis page.
- Blossom-end Rot of Tomatoes (FS 139) — Oregon State University Extension
- Managing Blossom End Rot in Tomatoes and Peppers — Iowa State University Extension
- Blossom End Rot — University of Georgia Cooperative Extension (2024)
- Gypsum Use in Home Gardens and Landscapes (FS307E) — Washington State University Extension
- Soil pH Range for Optimum Commercial Vegetable Production (HS1207) — UF/IFAS Extension
- Calcium (Ca) and Sulfur (S) for Citrus Trees (SL382) — UF/IFAS Extension
- The Albrecht Method: How Important Are Nutrient Ratios? — University of Minnesota Extension
Greenway Biotech · Madera, California, since 1989
