Ever thought about fertilizing the foliage?
 

Generally speaking, the plants root system is the best way of absorbing nutrients for the plant. But some conditions can prevent the uptake of some nutrients. When plants are stressed and/or have suffered from root damage or loss, they can suffer from nutrient deficiency’s as well. Even environmental conditions like temperature, insufficient oxygen levels in [...]

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you can buy ferts made for that, usually micro nutrients,

but you must make sure you get a wetting agent because as you know plants don't feed in the dark so they must be sprayed in the light, the wetting agent lets the fert. absorb into the leaves quickly with out burning,

i have had good luck with coco wet and the fert that goes with it

side note: don't spray seedlings, i learned the hard way....