How to distinguish between too much light versus too little mag?

[Rama777]

Yo! I’ve got some plants that I’ve never been able to bring the light intensity up to even the lowest levels of what’s typically recommended. I’m sure it’s due to my own error, but I’m not quite sure what I’ve done wrong.

I use RO water and all through 12 or so weeks of veg I had given them about 160ppm of calmag.

Every time I tried to bring the light levels up by 5% or so, the leaves would just get mag deficient, or bleached edges. So then I turn the lights back down and all’s well, but I get nowhere with my levels.

In retrospect I probably should have just upped my calmag probably..

But for future reference, is there a way to be able to read any subtle nuance that would inform me as to wether or not I’m dealing with

A) Too much light for the plants to withstand
B) True magnesium deficiency
C) Perhaps a theoretically reasonable level of light intensity, but not enough magnesium in my water for the plants to be able to withstand it

Hope that makes sense..

 

[LoveGrowingIt]

Have you tried increasing magnesium without increasing calcium? That could be done by foliar feeding Epsom salt. Try doing foliar feed a day or two before raising the light intensity.

 

[ArtfulCodger]

Too much light and magnesium deficiency are two sides of the same coin. Mg is the central atom in chlorophyll, so more light increases the demand for Mg. How much Mg a plant can take up has a lot to do with root development and root health. Two keys to that are proper watering and proper feeding. Watering too often restricts and damages the roots. Feeding too much can restrict how much nutrient the roots can take up. If you've been giving 160 ppms of calmag every watering for the entire grow, it's possible that the issue isn't "not enough mag in the pot."

 

[Rama777]

Too much light and magnesium deficiency are two sides of the same coin. Mg is the central atom in chlorophyll, so more light increases the demand for Mg. How much Mg a plant can take up has a lot to do with root development and root health. Two keys to that are proper watering and proper feeding. Watering too often restricts and damages the roots. Feeding too much can restrict how much nutrient the roots can take up. If you've been giving 160 ppms of calmag every watering for the entire grow, it's possible that the issue isn't "not enough mag in the pot."

Good points, good points..

I don’t overwater as a practice. This is only my second grow but I make my living as an organic farmer: I have started 10’s upon 10’s of thousands of plants in flats and pots. True I never had hardly any experience with pots as big as 7 gallons, but basically I just don’t overwater and am very confident about that.

As for feeding, I made the mistake of starting these plants in soil that was too hot. I will never make that mistake again. They were somewhat stunted from too much N in the soil for about 6 weeks before they totally grew out of it. I suspect they would have grown out of it faster if I could have managed more light and photosynthesis.

In the 7 gallon pots for the last 7 weeks or so I have only given them nutes twice. First time at 25%, second time at 35% which causes light overfeeding symptoms. I never gave them any N again and they are in about the third week of flowering. They still complain a little bit about the N in the soil unfortunately.

So there’s a sensitivity to N, a very low light level (flowering at around 250ppfd right now), and a consistent 160ppm (give or take a hair) of calmag in RO with bang on water ph.

I have long suspected a relationship between the N sensitivity coupled with the low light levels surrounding the mag deficiency symptoms that always surface when I bump the light levels up..

The plants are healthy overall. It’s not an emergency, I just really want to attempt to discover what the issue is for my future knowledge.

 

[ArtfulCodger]

As for feeding, I made the mistake of starting these plants in soil that was too hot. I will never make that mistake again. They were somewhat stunted from too much N in the soil for about 6 weeks before they totally grew out of it. I suspect they would have grown out of it faster if I could have managed more light and photosynthesis.

In the 7 gallon pots for the last 7 weeks or so I have only given them nutes twice. First time at 25%, second time at 35% which causes light overfeeding symptoms. I never gave them any N again and they are in about the third week of flowering. They still complain a little bit about the N in the soil unfortunately.

If measurement appeals to you, and it sounds like it might, seeing the PPFD reference, you might check the TDS/EC of your runoff next time you water. If nothing else, it might help you rule out over-feeding as a cause. Just for context, if I'm growing in amended soils and transplanting as the plant grows in veg, I might not feed until well into flower.

 

[Rama777]

I do like measurements, but only initially. Once I understand my process and have more experience I cast the details aside :)

I don’t actually water to runoff ever. I let the pots dry almost all the way and then I water them just to the point of saturation. I use a pump sprayer every time.

 

[BuffNuts]

There is no such thing as too much light.

It's probably zinc deficiency, not magnesium. Since Calimag restricts Zn.

Number one reason Cannabis growers struggle to produce quality Cannabis:

https://www.sciencedirect.com/science/article/abs/pii/S0098847216301666

Cannabis needs lots of B and Zn. Grower tries raising all trace minerals. Overdoses on Mn. Goes broke on calimag. Ends up B Zn deficient anyway.

 

[Rama777]

Hmmm, I don’t think that applies in my context.

I have one grow under my belt. I used all the same stuff and didn’t have any of the issues you speak of. I had zero deficiencies in veg, though I was using MH and HPS. I knew there would be a learning curve with reading and responding to the plants under LED..

 

[Grower1974]

My indoor plants suck up Cal-mag (TPS) like no tomorrow. I mix in 4ml (around 75ppm) every watering. (Usually every 2 days)

If I don't use enough, my plants will lighten, and form yellow/brown rust spots randomly, and by mid/end of flower, not look great at all. (yield and quality is still great)

My grow room and photoperiod tent, sits at around 650-750ppm of CO2 (naturally from human breathing downstairs with a very air tight home), and my plants see an AVG of 500-600PPFD during veg, and 850-1050 mid/late flower.

 

[Newty]

What light brand/model are you using?
Height above canopy?
Dimmer setting?

I see it may be a light intensity issue but no mention of how the light is setup currently lol.