Magnesium and cannabis plants
Just like calcium and potassium, magnesium is one of the essential elements you will need to keep an eye on when growing weed.
Magnesium moves around a plant with ease. When there’s a shortage of it, the chloroplasts in medium-aged leaves (under the blooming bud tip) become damaged and magnesium is transported from the young parts of the plant. This degradation of the chlorophyll shows as red-brown blotches and/or vague, cloudy yellow flecks (chlorosis) between the veins.
Magnesium is extracted with difficulty from the oldest leaves; it is obviously permanently attached to the organic substances. A light magnesium shortage has little effect on the bloom, even though magnesium deficiency does retard the fruiting mechanism.
Progression of magnesium deficiency
- For the first 3-4 weeks there are no observable symptoms, the plant grows well, is dark green in colour and looks healthy.
- Deficiency symptoms first show up in cannabis around the 4-6th week; then small, rust-brown patches of necrosis appear and/or cloudy chlorosis in the medium-aged leaves (those under the blooming buds). The colour of the young leaves and the fruit development are normal.
- The size and quantity of the rust-brown patches are extended over the leaf, the chlorosis increases too and becomes more yellow.
- The symptoms increase over the plant, which looks battered.
- With a serious shortage the young leaves also become chlorotic and their production falls
Possible causes of magnesium deficiency
Magnesium shortages occur more frequently than other deficiency illnesses. Magnesium deficiency symptoms can show up at normal and high concentrations of the element as well as from deficiencies. This is because its uptake can be hindered by other x-factors.
Some of these circumstances are:
- A very wet, cold and/or acid root environment.
- High levels of potassium, ammonia and/or calcium (perhaps from too much lime in the mains water, or chalk-rich clay soils), compared to the level of magnesium.
- Limited root system and heavy stress on plant.
- High EC in the medium, reduced evaporation.
What to do
- Prevent a deficiency in the first place by using magnesium-rich fertilizers. If it occurs, spray with Epsom salts
- Check the temperature, humidity, EC and pH of the soil or substrate.
- If you confirm a deficiency the best thing to do is spray with 2% solution of Epsom salts
- Fertilize via the roots: inorganic: Epsom salts (with hydro), Kieserite (magnesium sulphate, use with soil). Organic: old, rotted stable or turkey manure.
- Correct any causes: with too low a pH (< 5) when growing on soil), use magnesium-containing lime fertilizer. When growing on hydro: introduce a feed-water with a temporarily raised pH (6.5). When the EC is too high: flush and/or temporarily use pure mains water. Keep the root temperature, if growing indoors, above 19 ºC (20-25 ºC).
A little extra magnesium can’t do any harm. An excess of magnesium does not happen often when growing in soil. If it does occur, the uptake of calcium will be retarded and a picture will emerge of too much salt, stunting growth and colouring the plant dark green.
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