purple stems of original skunk #1

hi fellas

i am growing 5 fem. original skunk #1 and 5 fem. white widow from seedsman genetics.

WW is ok,but stems of skunk 1s are purple. all of them is in same medium and they are being fed same nutirents .
medium :70%coco,10% peatmoss,20% Worm casts ( i will add some perlite as soon as i transplant )
nutrients: biobizz grow and root juice.. i'll give them voodoo piranha and tarantula in my next water.
they are 15 days old .

what is the problem ?why my skunk 1s are purple ?

i have heard of this its mostly genetics but i wouldnt rule out a phosporous or magnesium deficiency either so..

my white widows are turning purple in the stems and i suspect temps low and pH too high

but then again i cant find hard evidence on this subject. Anyone KNOW about stems going purple?

Add some gardenlime to the mix:http://www.amazon.com/Espoma-Organic-Traditions-Garden-Lime/dp/B000BWZAQQ.Purple stems could be nute prob. ph,cold soil,or just genetics of the plant.

Thanks for the timely suggestion. My soil is organicaire, supposedly neutral but pH keeps climbing above 7 and plants stressssed