420 Magazine Background

Magnesium issue?


New Member
I am guessing I am seeing a magnesium deficiency here. Can anyone tell me if I am on the right track?

First photo is an indica, second a sativa. Both seem to be showing signs.

Both are about 2 1/2 months old and were stunted by a nasty fungus gnat infestation that was treated with BTI and a spray with "pyrethins".

They are in an organic mix soil under LED lighting. I am watering with GH's flora series using the label's directions.

Temps are a bit warm but have been for their whole lives. 78-84 with the occasional temporary trip up to the high 80's on hotter days.

I was going to pick up some cal-mag tomorrow until it dawned on me that on 7/4 the hydro store is probably closed. Any suggestions or ideas would be appreciated!



Well-Known Member
Good morning and HAPPY 4TH!
Yep your right looks like Mag deficiency.
Most times with Mag Defs a sufficient amount of Mag is in the soil but the PH of the soil is too low for the plant to absorb it. Check the PH of the run off after watering. Mag is best absorbed by soil planted Cannabis when the Ph is between 6.0 and 7.0. If you suspect your growing cannabis plant has a magnesium deficiency, flush your system with clean, pH'd water that contains a regular dose of cannabis-friendly nutrients that includes magnesium. This will remove any nutrient salts that may be affected the uptake of magnesium and help restore pH to the proper levels.
Most times the leaves affected will not recover but you will notice that the new growth looks great in which case you won the battle!
Good luck!


Well-Known Member
2 tablespoons per gallon of water - first mixing the salts with a bit of warm water to thoroughly dissolve them. This can be given to the plants every four to six weeks, if necessary.

To apply a foliar spray, dissolve up to one tablespoon per gallon of water and spray Cannabis plants at the beginning of each cycle or when needed, focusing the majority of the fine mist upon the underside of the leaves.


New Member
5.5 _6.5 is optimal for hydro grows :)

Alas, I cant go back in time to fix my number confusion. All I can do is correct it now and in the future. I am hoping this prevents any more yellowing. Time will tell!

As an additional update I am still spraying the surface of the soil with the safe garden pyrethrin spray and will continue to do so for a while after I stop seeing dead bugs. There are still a few dead ones showing up in the water trays under the pots.

I will probably keep treating up until I switch to 12/12. I wonder when that will be. After 3 months of veg? Maybe 4 months?

Fuzzy Duck

Well-Known Member


Well-Known Member
If you read the charts carefully they will indicate the appropriate PH for most growing mediums.


The BOLD number is the one to aim for.
Top Bottom