My Issues Came From VERY Tiny Root Aphids
The damage aphids do to your roots will cause magnesium 'looking' deficiencies. They slow growth, generally prevent clones from rooting in soil, and open the plant up to additional infections through the roots.
Most root aphids are rather easy to see. Unfortunately, I currently have the tiny ones referred to as Micro Root Aphids (yeah, really technical term. lol). I used a 60x and 100x scope and was unable to find these buggers. I read a reference to using potato slices to check for root aphids. Wow... that sure works!
Not only do the root aphids love potato, holding a slice and watching it for a minute makes it really easy to see aphids. No digging through dirt or camping out a puff of roots, waiting for an aphid to put on a show for you. Once I tracked down the movement (they're fast...), I knew what I was looking for. They're all over these potato slices.
Make sure the sliced potato is at least 1" thick. You want it to stay moist for 5 days or so, and I left my slices in for a week. Watering helps, but a thinner slice will dry out and possibly not be as desirable to the aphids.
Three Photos of Root Aphids on a Slice of Potato
Can you see them? If not, scroll down to see the animated gif. You'll see them right away. :D
Everything Slowed Down a Couple Months Ago
I haven't been running hydro like normal, and I've been growing in soil for at least 6 months now. I do many things well, but tracking time is not one of them. lol A few months ago the growth in the veg cabinet slowed down. I only have a few mums in there, so it wasn't something I really noticed right away.
I Thought My Soil Was Too Hot
I have a Lemon Cello plant I sprouted recently, and it seemed to not like the strength of the soil. I diluted the soil with some seedling mix the next transplant, and the problems continued. I know this strain is a NLD type and should want less nutrients, but not 'that' little. Besides, there was something familiar about all this, as I went through it over a year ago.
I've been growing indoor for 15+ years, but my experience with pests other than mites are limited. I'm still gaining awareness of what each pest looks like, and how to treat them. :)
Animated Gif of Root Aphids on a Potato Slice
A thorough root drench using pyrethrins. I'm using Pyganic 1.4 at 70ml/gallon of water, pH'd to 6.5. It's a bit expensive for larger operations, so I'd recommend Pyganic 5% at 20ml/gal. Again, be sure to pH to 6.5 or appropriate.
Thoroughly, drenched is the ticket. Along with repeat treatments. Aphids are born pregnant, so they're extremely tough to get eliminate. The Pyrethrins need to be around for the entire time pregnant aphids are hatching. This means completely drenching the soil with the solution, with lots of run-off.
I use the run-off from my cannabis plants to treat house plants.