Confirm This Deficiency For Me

[Sirius]

Okay, I have some White Satin, Arjan's Haze #3, and Critically Smashed seedlings that are two weeks old. They were planted directly into rock-wool cubes that were immersed in tap-water adjusted down to a pH of 5.8, and they have been watered with bottled water adjusted down to 5.8 as well. The temperatures underneath their fluorescents (6500 lumens a few inches above, 4x18watt grow-lux) have been pretty constant at 30°C/86°F.

Getting closer to their second week, a few of the seedlings across the board have shown some signs of a deficiency. It looks like magnesium, unless the magnesium has just been locked out by the pH, but more likely is that its been a few weeks and they are starting to feel hungry for some nutrients. They were transplanted into 3.4 liter pots of coco that was adjusted down to a run-off of 5.5 and given a quarter dosage of coco-specific nutrients, Atami Coco A&B, which includes micro-nutrients. The problem hasn't gotten worse since this time, but it still looks the same. Since the time of transplanting, the temperatures in the cabinet have been reduced to around 26°C/78°F.

The problem has been some reddish/brown spots, mostly occurring on the tips of lower leaves. There is some interveinal chlorosis as well at the tips. One of the seedlings has little brown spots all over its set of true leaves. I've taken pictures of that one and one with the colors occurring at the tips. It was kind of hard to work with the lighting to really show what is happening, so bear with these...

What does everyone think? Since they just got transplanted and it might take a little bit before the roots grow out into the coco and start bringing in those nutrients, and also considering that its possible they needed more than a quarter dose (starting low and working my way up), and the possibility that the pH might need adjusted more, I'm thinking about making a foliar spray to help get the problem solved in the meantime. Sound like a good idea? Is this magnesium, or another micronutrient deficiency?

 

[knna]

I believe the problem is more related to the rockwool used than anything else.

Is it horticultural grade rockwool? Meaning it had ph already corrected. Did you checked what was the ph of the initially adjusted to ph 5.8 water where you soaked the rockwool? Rockwool needing adjusting often need more than a bath in 5,8 to neutralize its alkalinity.

If its the case, it would explain that plant symthoms, wich seems related to high ph. Probably once roots colonize the coco, the problem will dissapear. Try to add a root estimulator.

You can use higher dosages than a quarter at first feedings in coco, due it may store large amounts of nutes (after that, lower it and adjust EC according to plants signs). Or you can give a very soft foliar feed ( use 1/10 of max watering dosage as max) to help plants until their roots are larger.

 

[Sirius]

I couldn't say what grade the rock-wool is, bought it from an online hydro store, but it didn't come with any real information beyond that it was rock-wool. I believe it is made by Atami, so one would assume it should be intended for growing, since they are in the nutrient business. Didn't think to test the run-off of the rock-wool; next time I'll know better. The problem looks like it might be getting a little bit better; at the very least, it hasn't gotten worse.

I made a foliar spray to try out in the meantime and have tested it out on one of the leaves of one of the seedlings. It was a diluted mix of some MgO and Ca, the Ca coming with a little bit of N, Fe, and Cu. Our intention is to bump watering the coco to half strength nutes tomorrow if it doesn't look like they have responded negatively to the first quarter dose.

Thanks for the reply!

 

[knna]

If its atami rockwool, it should be already corrected.

In that case, its possible the cause its the rockwool taking too much water and drowing roots (similar to overwatering but due to the different water/air ratio between the rockwool and the coco). Ive never used rockwool into coco (i prefer to root directly on coco, as its too an excelent rooting media) so i cant say for sure. Most coco setups using clones rooted on rockwool ive seen uses the cube over the coco, not into it. Surely its possible to use it that way, but until the plants develop enough large roots, it may be a problem.

What i know for sure is rockwool dont like to be too wet.

 

[Sirius]

Well, its definitely possible we overwatered them, I guess. The problems had shown up before the transplant was made, so it couldn't be from the difference between the two substrates. The roots had grown to the bottom of the rock-wool cubes a couple days before the transplant, so is it possible that it happened from them sitting there underneath like that?

Next time we'll germinate in coco, which is probably the best way to go, since we're planning on going to DWC next time.

Thanks for the input, I'll let you know how it goes.

 

[PharmaCan]

One of your problems is adjusting your coco run-off to 5.5. You really can't tell a whole lot from coco run-off. you just have to go by what you put into the coco. You see, the coco is going to naturally buffer anything in the pot up to around 6.0. Trying to fight this buffering effect is counter productive. If you look at a nutrient absorption chart, you'll see that different nutes are better absorbed at different pH's. What you want to do is use the natural buffering effects of coco to your advantage. If you start with a 5.5 pH nute solution (same pH I use in veg), while the nute solution is in the pot its pH will gradually increase from 5.5 to around 6.0, thus giving different nutes their optimum pH absorption levels.

Somewhere on the Canna site they have a discussion of coco run-off and the basic conclusion is that measuring coco run off is a waste of time because it just isn't a good indicator.

When I put r/w cubes in coco, I leave about 1/2 the cube above ground, it'll stay a lot drier that way.

PC

 

[Sirius]

Hey, thanks for the response. We were aware to a small extent the nature of the pH drift with coco, and were only initially bringing the pH of the run-off to around 5.5 before we transplanted. After that, we just used 5.5 water and that was the plan after that. Thanks for the tip on the rock-wool cubes too. We started considering using water adjusted to 5.8 to help make sure magnesium could be absorbed, but do you think it is a better idea to just stick with 5.5?

 

[Sirius]

Also, thanks for the mention of canna coco's site, I'll go look for that in a little bit. :wink:

 

[PharmaCan]

You can vary your pH on a daily basis if you think it will help. You just have to find the sweet spot for your particular strain. Coco is like hydro in that the plants will respond almost immediately to what you do, so you can try one pH for a couple days, see what happens, and just play it by ear from there.

PC