I would say calcium ,,it tends to be the most common deficiency these type of plants see,,I use hydrated lime 1 tea spoon per gallon of water once a month ,,does the trick for me keeps my soil PH a perfect 6-7 and takes care of the calcium ,,I use milk as a source of calcium as well is a old school trick I have never owned or purchased cal mag,,either lime it or get a cal mag supplement ,,treat it,,go with the less is mor approach,,I often mix my feed weaker then it suggest,,and increase the amount slowly,,, ,,The first 2-3 weeks of flower,,, the plant will still take a bit of nitrogen,,,,, as it will do a bit of up right growing during this time,,I use a 7-5-5 in veg and a 3-10-10 in flower ,,when I start flower I give my normal veg nutrients,,for the first 2-3 weeks,,,, then start with my flowering nutrients,,,I mix it at half strength and each feeding it gets more until it can handle the full dose,,,,,,I personally would do hydrated lime at least once,,,I use 1 teaspoon per gallon of water this will ensure booth your soil and PH are a good ,,Lime will amend your soil and allow your plant to take up more nutrients that you are giving,,,it makes the soil better,, for the microbes and bacteria,,, that live in the soil thus improving it,,, the more they flurious the better the soil,,,,,Lock up or stunted growth may be a result of deficiency ,,it will often keep them from taking up the right nutrients even if they are present,,this will correct itself if you lime it,,,, I would then pull the effected leaf's off as they will not repair them selves,,and watch for any more signs over the next 2 weeks in new and old leaves ,,,,I will add Lime is not a nutrient but as with anything there can be to much of a good thing so ,,I would stick with the 1 tea spoon per gallon of water,,,I only do that once a month ,,,, also here is a link with a plant problems / deficiency's with pictures ,,will give you the cause and the correction needed,,cheers mate
http://www.thenug.com/galleries/diagnose-your-sick-marijuana-plants
