2nd grow questions hydro purple stem and soil.

Having smoked my last to plants grown in soil with better than I expected results, I've decided to try and grow hydroponicly. So me and my mate built a home made flood and drain system with a 700lph pump. I use 'amsterdam indoors' nutrients at about 3ml per litre. the lable says 6ml, so i decided to go for half strength until the seedlings are bigger. They seem to be growing the same speed as the soil plants I have, although the hydroponic plants have purple stems. Is this a problem? I also took a cutting off a mates plant and bunged it in the hydro kit, the stem has turned purple within a few days. is this a good sign (eg the plant is absorbing the nutrients, thus grown roots), or is the cutting suffering the same way the seedlings are? The plants grown in soil, planted at pretty much the same time, same strain of seed, do not have purple stems.
also one of the grapefruit seedlings in soil is growing a hell of a lot faster than the others, even though it's in the same soil, same amount of nutrients, water etc.

I'm using 2 tropic sun 3ft 40w flourecent tubes, and having the 400w hps light on for 2 or so hours a day. When should I switch fully to the hps?

The 2 seedlings in the hydro are grapefruit. the cutting is white widow.

3 of the seedlings in soil are grapefruit, on seedling is white widow.

here are some pics...

some strains have purple stems but it is usually caused by a nutreint deficiency either nitrogen phosphorus or Sulphur (S) deficiency if you do a search on this forum you will find a old thread of mine called plant problems that may help you out.the postman.

thanks postman dave, i'll try some methods out. i thought that the amsterdam indoors was complete nutrients designed specificaly for cannabis, but obviously not.

I had a question on this.. if the stems are purple.. and they are young then what? if its a N def.. then what? wait till they are older to give em the nutes they so need...?