Hey mate, I'm a bit late to the party and missed this earlier but is that PPM measurement using the PPM 0.50 scale? Would that be EC1.1-1.3? If so, that's quite a bit lower than what we normally run with these lights. If your EC is higher than that then please correct me, but I think you can go a bit higher next round. Stick with me and I'll explain why . . .
One of the more common issues we've been dealing with when growers switch to High Lights is that they tend to stick to their old nutrient regimes and the plants start to develop red stems and light green to yellow leaves early in growth. I made the same mistake myself when I first developed these lights and it took a while to figure out what was going on, but over time I narrowed it down to a general NPK deficiency, as well as magnesium and calcium – especially magnesium.
At first I interpretted this as light stress, but that was only half of the story. When plants lose chlorophyll their ability to photosynthesise diminishes. That means they are more susceptible to light stress which in turn can hurt an already existing nutrient condition and on it goes. Don't get me wrong – your plants look good and the bud development is there, but that's primarily because they are not missing out on potassium as much as they might be missing out on other nutrients.
VPD can be a challenge with LEDs – lower temperatures will generally lead to less transpiration (especially at higher humidities) and that affects calcium uptake. Calcium regulates other nutrient uptake, including magnesium and nitrogen, and that's primarily where red stems and yellow leaves can come from (as well as phosphorous, which is also locked out by lower temps). The only major nutrient that is not as affected is potassium.
Unfortunately, coco releases potassium as it binds magnesium and calcium via cation exchange. Ca and Mg are double positive ions, K is singular, so the coco – which is negatively charged – has a stronger attraction to Ca and Mg, and K is released in exchange. This can get worse the longer you grow, because as coco breaks down into smaller pieces it has a larger surface area which attracts more Ca and Mg and releases more K, which also locks out Ca and Mg. The only counter to this is the plant starts to use up more K as it flowers, so excess K lockout (of other nutrients) appears to get better as the plant flowers, but the underlying deficiencies are still there.
I believe there are reasons why this happens under our lights. The first is obviously lower temps compared to other types of light (the giveaway is reddish-purple/brown streaks on leaves which indicated a P deficiency). But the main reason is that the full spectrum – and we are talking about a proper full PAR spectrum here from 400-700+nm (not what every other company calls "full spectrum" which has no deep blue/violet etc, and certainly no UV and minimal far red, if any) – targets more chloroplasts with the potential for faster growth . . . but only if the amount of nutrient is increased.
A nitrogen deficiency can mask a magnesium deficiency so that it does not look like the typical inter-veinal chlorosis (yellow stripes between green veins) that growers normally associate with a magnesium deficiency. Instead, the leaf takes on an alround light green to yellow hue and it starts with the lower to middle leaves. You may see the odd rust spot or "burnt" tip, which is an accompanying calcium deficiency. Some parts of the leaf may be greener than others, but will eventually turn yellow (with or without Ca spots).
Nitrogen facilitates magnesium uptake, so a nitrogen deficiency will accelerate a magnesium deficiency whilst masking it at the same time – which can be hard to diagnose.
If the nutrient issue is not cleared up, then the plant starts to lose colour – chlorophyll – which makes it susceptible to heat stress (a build-up of leaf temperatures from unphotosynthesised photons). This burns the leaves and further yellows them and of course this is rightly diagnosed as light burn but is not the primary cause – the nutrient deficiency was the primary cause. The only thing you can do is then reduce the amount of light until the nutrient issue is resolved, and once the plant greens up again you can hit it with more light.
pH is important – especially if runoff is overly acidic indicating an excess of K. The irony is that to get the pH up again, most growers will typically use pH Up which is potassium hydroxide! This adds even MORE K to the root zone. And on it goes.
The solution in most cases is to foliar feed a balanced NPK (emphasis on the N and P) solution at about half strength with a full-strength cal-mag supplement which usually contains extra N. Flushing with warm water can remove any build-up of excess nutrients, but you then need to feed with a strong nutrient soloution to make up for the fact the plant was under-fed in the first place. I will typically be running my run-to-waste tanks at EC2.2-2.4. Sometimes higher depending on the strain.
I am writing all this not to be a smart-arse but to try to help other growers as I have not only seen this first-hand, but have done it myself! I just want you to get the most out of those lights because all it takes is a little dialling in to get even better results.
Apart from that, you're in for a bumper harvest!
