Should I add Cal-Mag to every watering?

Calcium and magnesium are essential nutrients for healthy plant growth. Cal-Mag supplements provide these nutrients in an easily absorbed form to prevent deficiencies. While adding Cal-Mag to every watering may seem beneficial, it is generally not necessary and can even cause problems if overdone. Here is a look at when and how often Cal-Mag should be used for optimal results.

What is Cal-Mag?

Cal-Mag refers to a blend of calcium and magnesium, two macronutrients that plants need in relatively large amounts. Calcium strengthens cell walls and improves plant structure and resistance to diseases. Magnesium is a central component of chlorophyll and so aids photosynthesis. Cal-Mag supplements are soluble mineral powders that dissolve in watering solutions. They provide calcium and magnesium in immediately bioavailable forms to prevent or correct deficiencies.

The role of calcium and magnesium

Calcium plays several crucial roles in plants:

  • Strengthens cell walls – Calcium forms cross bridges between cell wall pectins that stabilize and strengthen cells.
  • Improves plant structure – Adequate calcium promotes strong stems, roots and thick foliage growth.
  • Enhances disease resistance – Calcium strengthens cells walls against fungal infections.
  • Essential for cell signaling – Calcium ions are secondary messengers that relay signals within cells.
  • Regulates nutrient uptake – Calcium helps regulate selective absorption of nutrients.
  • Reduces physiological disorders – Calcium corrects issues like blossom end rot in tomatoes.

Magnesium has the following key roles:

  • Central component of chlorophyll – Chlorophyll molecules contain a magnesium ion at their center.
  • Activates enzymes – Magnesium is required to activate many plant metabolic enzymes.
  • Contributes to sugar and oil production – Magnesium aids in carbohydrate and fat formation.
  • Increases phosphorous utilization – Magnesium improves plants’ ability to uptake and utilize phosphorous.
  • Enhances fruit, seed, and flower production – Magnesium improves reproductive growth and development.

Causes of calcium and magnesium deficiencies

Despite their critical importance, calcium and magnesium deficiencies are very common. Some potential causes include:

  • Insufficient levels in tap water – Hard tap water contains abundant calcium and magnesium, while soft water is often lacking.
  • Poor soil cation exchange capacity – Sandy soils with low organic matter readily leach calcium and magnesium.
  • Low pH – Calcium and magnesium availability decrease as soil pH drops below 6.0.
  • Excess potassium, ammonium, or sodium – These positively charged ions interfere with Ca and Mg uptake.
  • Excessive rain or overwatering – Calcium and magnesium dissolve and leach from soils in heavy rains.
  • High fertilizer rates – Phosphorous and potassium in high fertilizer doses depress magnesium availability.
  • Fast growth – Periods of rapid vegetative growth increase calcium and magnesium demand.
  • Advanced plant age – Older, senescing leaves often exhibit deficiencies first.

Identifying calcium and magnesium deficiencies

Plants display distinct symptoms as calcium and magnesium deficiencies develop. Calcium deficient plants exhibit:

  • Stunted shoot growth
  • Poor root development
  • Weak stems and structural collapse
  • Wilting of young leaves
  • Leaf tip and margin burn
  • Failure of young leaves and buds to develop
  • Blossom end rot on tomatoes

Magnesium deficiencies lead to:

  • Interveinal chlorosis starting on lower leaves
  • Leaves turn reddish purple then yellow between veins
  • Necrotic leaf margins on severely deficient plants
  • Premature leaf drop
  • Reduced fruit and seed development

Soil and tissue testing

While visual symptoms can indicate low calcium or magnesium, soil and tissue testing are more definitive ways to diagnose deficiencies. Soil tests measure extractable calcium and magnesium levels in ppm. Tissue tests reveal the percentage of each nutrient within the plant. Sufficiency ranges are:

Nutrient Soil Range (ppm) Tissue Range (%)
Calcium 500-2000 1.0-4.0
Magnesium 50-500 0.25-1.0

Testing confirms deficiencies if levels fall below these adequate ranges. It also guides proper calibration of fertilizer applications to supply deficient nutrients.

When to apply Cal-Mag

Targeted Cal-Mag applications are recommended:

  • At planting -adding soluble Ca and Mg encourages early root growth.
  • During rapid growth – periods of vigorous vegetative or fruit expansion increase demand.
  • After heavy fruit loads – large fruit yields deplete calcium reserves.
  • Following heavy rains – downpours can leach mobile nutrients like Mg2+ from soils.
  • In cases of observed deficiencies – foliar sprays swiftly alleviate symptoms.
  • With alkaline water – high pH above 7.0 reduces calcium solubility.
  • For salt-sensitive plants – Cal-Mag enhances tolerance to saline conditions.

Avoid overapplying Cal-Mag during slower growth or with adequate soil levels, as excess calcium and magnesium can also create nutrient imbalances reducing plant growth and quality.

How often to use Cal-Mag

There are no fixed rules dictating ideal Cal-Mag application frequency. However, some good general guidelines include:

  • Add every 1-2 weeks during the active growing season.
  • Increase applications to every week for fast growing vegetable crops.
  • Use 1-2 times per month for ornamentals and fruiting plants.
  • Reduce to once a month during slower winter growth.
  • Allow 2-3 applications after initial deficiency correction.
  • Suspend applications if soil Ca and Mg levels are already sufficient.

More frequent Cal-Mag doses often provide no added benefit and may raise soil pH excessively. Target the times of peak calcium and magnesium demand to optimize plant growth and quality.

What concentration of Cal-Mag should I use?

Follow manufacturer’s instructions when mixing Cal-Mag solutions. Typical dosages are:

  • 1-2 tsp per gallon of water for preventative soil applications.
  • 2-3 tsp per gallon for mild deficiency correction.
  • Up to 5 tsp per gallon to treat severe deficiencies.
  • 0.5-2 tsp per gallon for maintenance foliar feeding.

Consider factors like water alkalinity and application method when fine tuning rates. For hydroponics, target 60-150 ppm calcium and 30-60 ppm magnesium. Exceeding 300 ppm combined Ca and Mg can suppress other nutrients. Build dilution rates gradually when correcting diagnosed deficiencies.

Foliar vs. soil Cal-Mag applications

Cal-Mag can be supplied to plants through both foliar sprays and soil applications:

  • Foliar sprays provide immediate relief from deficiency symptoms as leaves rapidly absorb calcium and magnesium. Use foliar methods to treat emerging deficiencies.
  • Soil applications increase available Ca and Mg reserves for sustained correction. However add Cal-Mag to soils primarily as a preventative measure.

Absorption and resulting impacts differ between these two routes:

Foliar Sprays Soil Applications
Speed of Response Hours to Days Days to Weeks
Total Nutrients Supplied Lower Higher
Application Timing Correctional Preventative
Target Tissues Older Leaves New Leaves and Roots

For well-rounded nutrition, use foliar sprays to rapidly alleviate deficiency symptoms while building soil calcium and magnesium levels through regular soil applications.

Organic vs. synthetic sources

The calcium and magnesium in Cal-Mag products may derive from organic or synthetic sources:

  • Organic sources – Mined minerals like dolomite, gypsum, calcified seaweed.
  • Synthetic sources – Inorganic salts like calcium nitrate, magnesium sulfate.

Natural and synthetic Cal-Mag forms are equally bioavailable to plants. However, key differences include:

  • Organic Cal-Mag releases nutrients more slowly over time.
  • Synthetic Cal-Mag is immediately soluble in watering solutions.
  • Organic forms like dolomite lime also raise pH as they dissolve.
  • Synthetic forms allow precise nutrient targeting without altering pH.

For rapid correction of deficiencies, synthetic Cal-Mag provides faster results. But for sustained preventative applications, slower release organic sources are ideal.

Should I apply calcium and magnesium separately?

While it is possible to supply calcium and magnesium individually using compounds like gypsum and Epsom salts, combined Ca-Mg blends offer several advantages:

  • Pre-mixed Cal-Mag simplifies nutrient programs.
  • Cal-Mag solutions prevent and correct both deficiencies.
  • Balanced Ca-Mg prevents skewed ratios between the nutrients.
  • Foliar Cal-Mag maximizes combined absorption and mobility within plants.
  • Less risk of overapplying individual Ca or Mg sources.

However, applying dedicated calcium or magnesium supplements may benefit certain situations:

  • Gypsum to rebut high sodium soils.
  • Dolomite lime where pH correction is also needed.
  • Epsom salts for magnesium-specific deficiencies.

But for broadly preventing and treating calcium and magnesium deficiencies, blended Cal-Mag supplements provide the greatest benefit and flexibility.


Cal-Mag fertilizer provides supplemental calcium and magnesium that are commonly deficient but essential for optimal plant growth. While adding Cal-Mag to every watering may appear beneficial, doing so excessively can lead to nutrient imbalances. Target key growth stages, deficiency symptoms, and periods of enhanced calcium and magnesium demand. Soil and tissue testing offer the best guides to proper Cal-Mag rates and timing for your conditions. Apply calcium and magnesium regularly but judiciously to sustain vigorous plants and quality yields.

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