Calcium (Ca) in Cannabis – Deficiency and Requirements

The shoot tip stays stunted, and new leaves grow crooked and pale at the base. That is how calcium deficiency begins in vegetative growth. Calcium is one of the six macronutrients and strengthens cell walls and cell membranes. The plant can barely withdraw it from old leaves. It reaches young leaves almost only with the water the leaves transpire. That is why it can run short in humid, still air even when the fertiliser contains enough.

How do I recognise calcium deficiency?

In vegetative growth calcium deficiency shows first at shoot tips and young leaves. The new leaflets are narrower and paler at the base than at the tip. Later the tissue between the veins yellows, the margins die, and leaves grow twisted. Finally the shoot tip dies, and many side shoots grow below it.

In a flowering trial, by contrast, the deficiency began on the lower fan leaves. Fan leaves are the large fingered leaves on the stem and branches. Spots with a yellow ring around a brown centre appeared between the veins. The solution in this trial contained a lot of sodium, which according to the researchers may have contributed.

Every stage with its look-alikes is described in the article on nutrient deficiency and overdose.
In vegetative growth, check the top: there calcium deficiency starts on new growth while the lower leaves stay green.
Rule out boron deficiency. In the middle stage the whole leaf curls down with boron, only the margin with calcium.
In vegetative growth, rule out potassium deficiency. It scorches margins at the bottom, calcium deficiency at the top.
Rule out the fungal disease Septoria leaf spot. It starts on damp inner leaves and later shows grey centres with tiny black dots.

How much calcium does cannabis need?

No trial with graded calcium rates on cannabis was found. Only the base solutions of trials testing other nutrients are known. They contained between about 45 and 260 mg/L calcium by calculation. The unit mg/L means milligrams per litre of feed solution.

Calcium is still needed. In a flowering trial, solutions without calcium gave 30 to 50 % less plant and bud mass. Solutions without potassium, magnesium or sulphur did the same. Across 6119 hemp samples, the middle half of leaf values lay at 1.5 to 2.9 % calcium in the dry matter.

Count the calcium in your tap water. It is listed on the water supplier’s analysis.
Alkalinity describes how much dissolved lime the water carries. Water with moderate alkalinity often already brings 40 to 60 mg/L calcium.
Reverse osmosis water and soft water bring hardly any calcium. The fertiliser must then supply it.
Check whether the fertiliser contains calcium at all. Some complete fertilisers such as 20-10-20 contain none.
Compare a lab leaf sample with 1.5 to 2.9 %, together with an analysis of substrate or solution.

Why is calcium short although I feed enough?

Calcium travels upwards almost only with the transpiration stream, and humid and still air slow that stream. How strongly the air draws water from the leaf is described by the vapour pressure deficit, VPD for short. If it is low, the leaf transpires little and receives little calcium.

Antagonists also suppress uptake, such as a lot of potassium, magnesium and manganese. Ammonium, one of the two nitrogen forms in fertiliser, belongs here too. In soil, calcium becomes scarce below pH 6.5. The pH value describes how acidic a solution is. According to manufacturer information, unbuffered coco binds calcium and magnesium from the nutrient solution.

Measure humidity and VPD, and avoid still air.
Check the fertiliser for high potassium, magnesium and ammonium. How much potassium is enough is in the article on potassium.
In soil, measure the pH.
In coco, use buffered material. This advice comes from manufacturer information.
Only then raise calcium and watch the new growth. Deformed leaves do not recover.

What does too much calcium do?

No damage pattern of its own is described for cannabis. In tomatoes given a lot of calcium and geraniums given lime, too much calcium led to magnesium deficiency. This happened even when enough magnesium was given.

In ornamentals a lot of calcium also counts as an antagonist of boron. In greenhouse cucumbers a lot of calcium and magnesium from fertiliser and water suppressed potassium. Raising pH with lime adds calcium at the same time. As pH rose, calcium in the hemp leaf rose from 1.03 to 2.86 %.

Add up hard tap water, lime or dolomite in the substrate, and calcium-magnesium additives.
If magnesium deficiency appears despite magnesium feeding, consider too much calcium.
Match the calcium in the fertiliser to the water. New growth shows whether it works.

How do I give calcium correctly?

Usually as calcium nitrate, and that always brings nitrogen along. It is mostly sold as a double salt with ammonium nitrate. This contains about 18.5 % calcium and 15.6 % nitrogen by calculation, of which 1.3 percentage points are ammonium. One gram of it per litre therefore supplies about 185 mg/L calcium and 156 mg/L nitrogen by calculation.

Pure calcium nitrate tetrahydrate contains about 17 % calcium and 11.9 % nitrogen by calculation. The data sheet tells you which form is in the bag. On EU fertiliser labels calcium is usually listed as CaO, and that value times 0.71 gives the calcium.

The full calculation including tap water is in the article on calculating a nutrient solution.
Count the nitrogen from calcium nitrate towards the nitrogen value of the solution.
Multiply the CaO value on the label by 0.71.
Mix calcium fertilisers separately from sulphate and phosphate fertilisers, otherwise a precipitate forms in the concentrate.
Keep stock solutions below pH 5.