StonedSurfer54 Bonz is right, looks like nutrient overload - you are killing them with kindness. Possibly, watering to often & using too much fertilizer. The soil you are using if very good stuff, it should carry your plants for the first month - with no additional fertilizers. If you have used any fertilizer before that 4 week mark - they are probably overfed. If not - it is possible, the soil you got came with too much fertilizer in it. It happens, just not often. To repair the damage, do the following:
Mix up a quarter strength fertilizer - pour through the affected pots - thoroughly -to flush out the toxic condition. (You have to use some fertilizer to bond with and drag out the buildup). Make the flush, gallon for gallon; if you have a 3 gallon pot flush with that amount, minimum. Next, mix up 1 cup of hydrogen peroxide 3 % (like you disinfect a cut with) with two gallons of water. Flush again. Make as much as you need. Flush gallon for gallon. Then Wait. The extra oxygen in the HP will bath the roots with much needed oxygen. If you are watering too often you are drowning your roots. If not, the HP will not hurt a thing.
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Three days later, feed a quarter strength mix of grow fertilizer (higher N). Do not water every other day. Wait until soil is dry at least two inches down from the top.
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Only feed your plants once a week, water as needed. If the soil in your pots is drying out any faster than 3 days, the area you have them in is too hot. Or you have too much air flow across the pots, in effect - air drying them. So, reduce temp. or reduce air flow (that blow directly on your plants). I suspect that big fan, may be a little to much, considering their size. If your grow area is so hot the big fan is necessary, move your plants.
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Hope this helps... Keep it Real.... Organic..