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definitely cal mag deficiency but the causes can be many, be careful with this wood you have underneath, may be impregnated with chemicals, I had a problem like yours for 6 months and I could not find the problem, finally it was a plastic floor that I had put and it released ethane and it killed my plants

Most certainly, 3 litres is way to small to grow healthy cannabis plants, let alone flowering plants. Autos should be in 10-15 litre pots for best results and photoperiod plants in anything from 20-45 litre pots. Not only do small pots limit root growth, which limits plant growth, they also run out of water and nutrients very quickly and also increases the risk of mineral salts from the fertilizers building up to toxic levels quickly. So, by having small pots you will need to feed often and feeding often will lead to the salts building up to toxic levels much faster. You can continue doing all sorts of things to try and keep pH in the right zone, but that small amount of coir is going to fluctuate very often and very simply due to being so small in volume. In other words, you will have a constant battle on your hands. It seems you are already running into plant health issues and this will only get worse/more frequent if you continue with the 3 litre pots. Transplanting into much larger pots will provide the plants with a much better environment for happy and healthy growth and you won't have to pull your hair out by constantly fighting a losing battle with plants growing in exhausted and unforgiving substrate that will be building up in salty residues harmful to your plants and a somewhat common problem in coir. Transplanting asap is best for your plants, leaving them in the small pots will stress them further and it is something that can not be fixed with constant applications of cal/mag, which in the long run can be more harmful than beneficial in those small pots. Keeping your plants in those small 3 litre pots and expecting them to produce a bountiful harvest of sticky prime flowers is not going to work, it would be a bit like raising an elephant in your bedroom, it will fit to a certain degree, but no one is going to be happy, least of all, the elephant. So my advice would be to transplant ASAP, the longer you leave it, the more stressed your plants will become. Constantly adding this or that and then constantly flushing and battling pH issues is not the answer for optimum growth and flowering. Sorry if my answer may seem a bit blunt, but there is no point saying "yeah, everything is fine and dandy, the world is wonderful, everything is so nice, blah blah blah, your plants are experiencing some sub prime situational awareness......" Your plants need rescuing and if you want decent yields of quality flowers, much bigger pots is the only answer. Hope this helps,................ Organoman.