Wicked Organics · Fertilizer Guide

Magnesium Sulfate

OrganicOMRI Listed

Magnesium sulfate — commonly known as Epsom salt — is a fast-acting, water-soluble source of two essential secondary nutrients: magnesium (10%) and sulfur (13%). While it carries an NPK of 0-0-0, its role in plant health is critical. Magnesium is the central atom in chlorophyll, the molecule that drives photosynthesis, and sulfur is es...

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Overview

What It Is

Magnesium sulfate is a naturally occurring mineral compound with the chemical formula MgSO4. In its hydrated form (Epsom salt), it contains approximately 10% magnesium and 13% sulfur by weight. It is one of the most water-soluble magnesium sources available, which makes it ideal for both soil application and foliar feeding.

Magnesium plays a central role in photosynthesis as the core element of the chlorophyll molecule. Without adequate magnesium, plants cannot efficiently convert sunlight into energy, leading to reduced growth and pale foliage. Sulfur supports amino acid and protein production, influences flavor in crops like onions and garlic, and helps plants access other nutrients.

Unlike many fertilizers, magnesium sulfate does not significantly alter soil pH, making it safe to use across a wide range of soil types. It is especially beneficial on sandy or acidic soils where magnesium leaches easily.
Crop Fit

Best For

TomatoesPeppersRosesPotatoesBlueberriesEvergreensLawnsHerbs
Tomatoes and peppers are heavy magnesium users and commonly develop deficiency symptoms mid-season — Epsom salt applied as a foliar spray or side-dress corrects this quickly. Roses respond with deeper green foliage and more prolific blooms. Potatoes benefit from the sulfur component, which improves tuber quality. Blueberries and evergreens growing in acidic, sandy soils are prone to magnesium leaching and benefit from regular supplementation.
Application Rates

How Much to Use

Garden & beds1-2 lbs per 100 sq ft
Lawn3-5 lbs per 1,000 sq ft
Trees & shrubs2 tbsp per gallon of water, drench root zone
Containers1 tbsp per gallon of soil or dissolved in water
FrequencyEvery 4-6 weeks during the growing season
Step by Step

How to Apply

1

Identify Deficiency

Identify magnesium deficiency symptoms: interveinal yellowing on older leaves (veins stay green, tissue between yellows).

2

Dissolve

Dissolve magnesium sulfate in water for foliar spray or soil drench application.

3

Foliar Spray

Apply as a foliar spray in early morning or evening to avoid leaf scorch from midday sun.

4

Dry Application

For dry application, broadcast granules evenly and water in thoroughly to dissolve.

5

Reapply

Repeat every 4–6 weeks during the growing season until deficiency symptoms resolve.

For foliar spray, dissolve 1-2 tablespoons per gallon of water and spray directly onto foliage, covering both upper and lower leaf surfaces. Apply in early morning or evening to avoid leaf burn. For soil application, broadcast 1-2 lbs per 100 sq ft and water in. For individual plants, dissolve 1 tablespoon in a gallon of water and drench the root zone. For transplanting, add 1 tablespoon to the planting hole.
Handling & Timing

Safety & Season

Safety & Storage

Magnesium sulfate is non-toxic and food-grade safe. No special protective equipment is required, though gloves are recommended for extended handling. Store in a dry location — Epsom salt absorbs moisture readily and can clump if exposed to humidity. Keep in a sealed container.

Seasonal Notes

Apply magnesium sulfate from early spring through late summer during active growth. Foliar sprays are most effective during rapid vegetative growth and early fruiting. Avoid late fall application as plants enter dormancy. For evergreens, an early spring application helps prevent winter damage.

In Context

How It Compares

Magnesium sulfate vs. Dolomitic Lime: Both supply magnesium, but dolomitic lime also raises soil pH significantly. Use Epsom salt when you need magnesium without altering pH. Magnesium sulfate vs. Sul-Po-Mag: Sul-Po-Mag (0-0-22) provides potassium, magnesium, and sulfur — use it when potassium is also needed. Use Epsom salt for targeted magnesium/sulfur supplementation.

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