How Often Should I Feed CalMag? A Definitive Grower's Guide

November 21, 2025 •

4 min read

Over 75% of plant nutrient deficiencies are often misdiagnosed, with calcium and magnesium issues being particularly common, especially in hydroponic or coco coir setups. This guide explains how often should I feed CalMag to prevent these critical nutrient imbalances and support robust plant development from seedling to harvest.

Quick Summary

The ideal CalMag feeding frequency is highly dependent on your grow medium and water source. Soilless systems and reverse osmosis water generally require consistent supplementation with every feeding, while soil growers can often supplement less frequently or as needed. Always monitor plant health and water quality to determine the correct schedule.

Key Points

Medium Matters: Feed CalMag with every watering in hydroponics or coco coir, but less frequently in soil-based grows.
RO Water Requires CalMag: If using reverse osmosis or soft water, consistent CalMag supplementation is essential to prevent deficiencies from the start.
Watch for Symptoms: Spot calcium deficiency in new, distorted leaves and blossom end rot, while magnesium issues show as yellowing on older leaves.
Proper Mixing Order: Always add CalMag to your water and mix thoroughly before adding any other nutrients to avoid precipitation and nutrient lockout.
Monitor and Adjust: Regularly check your water's pH and monitor plant health. Adjust your CalMag dosage and frequency based on visual cues and the plant's growth stage.
Consider Water Hardness: Test your tap water for existing calcium and magnesium levels. If it's hard, you may not need to supplement CalMag as often, or at all.
Foliar for Fast Fixes: Use a foliar spray for a quick, corrective boost when you see signs of deficiency, applying it at night to prevent leaf burn.

What is CalMag and Why is it Necessary?

CalMag is a nutrient supplement containing calcium (Ca) and magnesium (Mg), two secondary macronutrients vital for healthy plant growth. Calcium strengthens cell walls, aiding in overall plant structure, root development, and resistance to disease. Magnesium is the central atom in chlorophyll, making it essential for photosynthesis—the process of converting light into energy. Incomplete or unbalanced nutrient formulas, combined with specific water sources or growing media, can lead to deficiencies requiring CalMag supplementation.

The Critical Role of Water and Medium

Your feeding frequency is primarily dictated by your water source and growing medium. Plants grown in inert media, like coco coir or perlite, or those watered with soft or reverse osmosis (RO) water, lack sufficient background levels of calcium and magnesium. This is a key reason many commercial nutrients and water treatment systems often recommend consistent CalMag use.

CalMag Feeding Schedule: Soil vs. Soilless Media

Your feeding schedule will differ significantly based on whether you are using soil or a soilless medium like coco or hydroponics. Getting this right is crucial for preventing nutrient lockout and ensuring optimal plant health.

Feeding in Hydroponic and Coco Coir Systems

For growers using hydroponics or coco coir, regular CalMag supplementation is standard practice. These inert media do not naturally contain enough calcium and magnesium to support plant growth.

Frequency: Add CalMag with every feeding throughout the plant's life cycle.
Dosage: Follow the manufacturer's recommended dose. For example, some suggest 5 mL per gallon of water. The optimal ratio is typically 3:1 to 4:1 (calcium to magnesium).
Water Source: If using RO or distilled water, consistent CalMag is non-negotiable to prevent deficiencies.

Feeding in Soil-Based Systems

Healthy soil often contains some calcium and magnesium, especially if amended with products like dolomitic lime. Therefore, the need for CalMag is less frequent.

Frequency: For preventative measures, apply CalMag every other feeding or once per week. Alternatively, you can use it only when signs of a deficiency appear.
Considerations: If your tap water is hard, it may already contain adequate levels of Ca and Mg, so check your water report to avoid over-supplementation.

Recognizing Deficiency Symptoms

Knowing when your plants need a boost is key. Here are the tell-tale signs to look for:

Calcium Deficiency Signs

Stunted growth and poor root development.
New leaves are distorted, twisted, or curled, and may have tip burn.
Blossom end rot in fruits like tomatoes, peppers, and squash.
Rotting or stunted growth at growing points (apical meristems).

Magnesium Deficiency Signs

Interveinal chlorosis, which is the yellowing of leaf tissue between the green veins.
This typically starts on older, lower leaves and progresses upwards.
Leaves may show reddish-brown spots or tints in some cases.

Application Methods: Root Drench vs. Foliar Spray

CalMag can be applied in two primary ways, each with its own advantages.

Root Drench: The most common method, mixing CalMag into your regular nutrient solution and watering your plants as usual. This is ideal for continuous feeding and preventative care.
Foliar Spray: For a rapid, corrective fix when deficiencies are already visible, foliar feeding provides a quick boost. Mix and spray the solution directly onto the leaves during nighttime or when lights are off to avoid leaf burn. Calcium is immobile in the plant, so foliar sprays only affect the leaves directly treated, while magnesium is mobile.

Comparison Table: CalMag Feeding by Medium


Feature	Soil Grow	Coco Coir	Hydroponics
Water Source	Use cautiously with hard tap water; essential with soft/RO water	Essential with all water sources, particularly RO	Essential with all water sources
Frequency	As needed or preventatively every 1-2 weeks	Every watering, from early growth	Every feeding, throughout the life cycle
Application	Root drench or occasional foliar spray	Root drench for daily feeding; foliar for correction	Added directly to the nutrient reservoir
pH Range	Optimal 6.0–7.0 for uptake	Optimal 5.5–6.5 for uptake	Optimal 5.5–6.5 for uptake
Deficiency Risk	Lower, depending on water and soil quality	High, as it's an inert medium	High, requires full nutrient control

CalMag Best Practices and Common Mistakes

To maximize the benefits of CalMag, avoid these common pitfalls:

Improper Mixing Order: Always add CalMag to your water first and mix thoroughly before adding other nutrients. This prevents nutrient reactions that can cause precipitation and lockouts.
Ignoring Water Quality: Blindly following a generic feeding schedule without testing your tap water can lead to over or under-supplementation. Use a simple TDS meter or check your municipality's water report.
Over-application: Using too much CalMag can cause nutrient lockout, as excess calcium can block the uptake of other elements like potassium or iron.
Neglecting pH: Maintaining the correct pH is paramount for nutrient availability. CalMag won't be effective if the pH is outside the optimal range for your medium.
Not Tapering Off: Towards the end of the flowering stage, it's wise to taper off or stop CalMag entirely. This helps flush the plant and avoid harsh flavors in the final product.

Conclusion: Tailoring Your CalMag Regimen

Determining how often you should feed CalMag is not a one-size-fits-all answer but rather a tailored approach based on your specific growing conditions. The key is consistency in soilless systems, and careful observation in soil. By understanding your water quality, monitoring your plants for deficiency signs, and adhering to proper application techniques, you can ensure a balanced feeding program that promotes vigorous growth and a healthy, high-quality yield. Remember that CalMag is a supplement, not a cure-all, and is most effective when integrated into a well-managed nutrient strategy. Always reference the manufacturer's specific instructions for the product you are using, as different formulations may have varying dosage requirements.

Learn more about nutrient deficiencies and plant health from the Royal Horticultural Society.

Frequently Asked Questions

You should begin feeding CalMag preventatively, especially if using RO water or coco coir, from the seedling or early vegetative stage. For soil, you can wait until a deficiency is observed or start a preventative schedule during flowering.

Yes, but with caution. Hard water already contains calcium and magnesium, so you may need to use a reduced dose or no supplement at all. Test your water first to determine its mineral content and avoid nutrient excess or lockout.

Overfeeding can cause nutrient lockout, specifically hindering the uptake of other elements like potassium or iron. Symptoms can include leaf tip burn, brown spots, or stunted growth. Always follow recommended dosages and monitor your plants.

Root drenching is best for consistent, preventative feeding, ensuring the roots and entire plant receive nutrients. Foliar feeding is for rapid, corrective action when a deficiency is visually identified, but it is less effective for long-term supply, especially for immobile nutrients like calcium.

Yes, CalMag is important during flowering to support fruit and bud development. Continue using it, but some growers choose to taper off supplementation a couple of weeks before harvest to ensure a clean final product.

Yes, CalMag can slightly alter the pH of your nutrient solution. It is crucial to check and adjust the pH of your solution after adding CalMag and other nutrients to ensure they remain within the optimal range for your medium.

Most coco coir sold is pre-buffered, but if it is not, a soak with CalMag solution is necessary before use. If you see deficiency signs early on with pre-buffered coco, it could indicate insufficient initial buffering.

Yes, pH levels outside the optimal range can prevent your plants from absorbing calcium and magnesium, even if they are present in the solution. Always check your pH to ensure the nutrients are accessible to the plant.