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Calcium and cannabis plants

CalciumIn both plant and animal life calcium is an essential element; it is found among other paces in bones, eggshells and seashells. Muscles and nerves do not function without calcium.


The name calcium comes from the Latin calx, calcis = stone. In its pure form it is a silver-looking metal, a bit harder than lead and easy to work. The metal ignites on heating and reacts violently with water and acids. Calcium is found all over the world; 3.64% of the Earth’s crust consists of calcium. The hardness of mains water is mostly caused by (the amount of) dissolved calcium salts.

Calcium in the plant

Calcium is found throughout the plant and is needed for many processes in it, but the most important is for the growth process. It has a regulation effect in the cells and contributes to the stability of the plant.

Plant anatomy

Plants have two transport systems: the xylem and the phloem. Most nutrients can be moved around by either system, but not calcium. Since calcium is pretty much only transported via the xylem, in the plant it is a not very mobile element. This is why it is important that there is always sufficient calcium present is in the root environment, so that it is continuously available for uptake by the plant.

Calcium deficiency

Sad cannabis leafCalcium is moved around in the plant by the upward sap stream. In cases of calcium deficiency it is the older leaves that show the first symptoms. It is usually not the very lowest leaves but those just above them (as in a magnesium deficiency, see cAnna’s info on Magnesium Deficiency). A calcium shortage is recognisable by the presence of yellow/ brown flecks on the leaves, often surrounded by a sharply defined brown edge. Also symptomatic are a stunted growth rate, and in the serious cases smaller, loose-growing buds. One can guess the result: a strikingly reduced yield.

The progression of calcium deficiency in chronological order.

  • The main visible characteristic of a calcium shortage is the sharply defined yellow/brown flecks with a brown edge to the leaf. The symptoms of a calcium deficiency often come on rapidly; within a week to two weeks the first flecks will be visible on the older leaves. These flecks mostly begin as small, light-brown spots that later become bigger.
  • After 2 weeks there are more and more flecks on the older leaves, often these appear on the edge of the leaf, as they do in a potassium deficiency or as a symptom of burning. Yet the differences from a potassium deficiency are mostly easy to differentiate; the flecks produced by a calcium deficiency are Calcium deficiencywell-defined, and do not only appear from the edge of the leaf. The retardation in growth is usually visible within a week.
  • In some cases the growth calyxes begin to shrivel and the calyxes have skinny, slim leaves, without flecks.
  • Older leaves slowly die-off, around the necrotic spots there may be yellowish cloud-like stains. The older the leaf, the more serious the symptoms (see photos 1-3).
  • The bloom is also inhibited and delayed and the number of flower heads is sharply reduced. The buds do not grow tight and compact and remain small.
  • The Pistils turn brown more quickly.

Causes for shortage of calcium

A shortage of calcium can come about due to:

  • Too little or the wrong fertilizer.
  • Cultivation on calcium-fixing soils.
  • An excess of ammonia, potassium, magnesium and / or sodium in the root environment. Ammonia inhibits the uptake the most, sodium the least.
  • Problems with transpiration; by having too high an EC, too low or too high a relative air humidity.

What to do

  • If the EC in the substrate or the soil is too high, you can best rinse the whole grow, perhaps with slightly acidic water.
  • If the soil contains too little calcium then this can be supplemented by applying dolomite, gypsum, lime, potassium pebble lime, magnesium pebble lime, slag, triple super phosphate or Limecalcium nitrate. Be careful with chloride-containing fertilizers
  • You can add extra calcium via the nutrient feed by mixing in some liquid lime fertilizer, calcium nitrate solution for example. With too acid a soil you can use milk of lime to raise the pH.
  • Use good soil that is not too acidic. An acid soil usually contains too little lime. Good compost and coco is enriched with lime.

Lime in a free state is quickly taken up by the plant. Because lime is not very mobile in the plant the symptoms in the older leaves will not disappear. But normal development will be reassumed without any signs that there was a deficiency.

Calcium in the nutrient base

In addition to problems caused by a calcium shortage in the plant, there can also be trouble as a result of a shortage of calcium in the soil. Calcium shortages in the soil often accompany acidification of the soil. In an acidic root environment, among other things phosphate becomes less freely available to the plant and a number of heavy metals become more available for it, which can lead to poisoning of the plant (by for instance aluminium, manganese, nickel).

Heavy metal cannabis danger

Too much calcium is damaging to the plant and usually has a retarding effect on the growth and leads to a darker coloured plant as a result. An excess of calcium can also lead to boron, manganese, zinc, magnesium, and sometimes copper deficiencies.

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