First of all, congratulations for all the information present in this forum and the always helpful attitude.

I managed to assemble a NFT system from scratch which holds 12 adult plants + 12 seedlings. This is not a highly controlled environment. Only an hack to play with hydroponics and to see what i can grow. It seems to work reasonable well.

I've read most of the information in this forum regarding hydroponics, however, after successfully growing two crops, I have some questions that maybe you can help.

The system is composed by three PVC pipes with holes on top (see pictures 5 and 6 ). Plants grow from the holes directly without any medium. Seeds are started in a pot with rock wool and then moved to the NFT. One of the pumps moves water to the pipes while the other circulates the water in the reservoir and aerates the nutrient solution. The nutrient solution is a 5 part element which I add every time I change the solution (every two weeks). I acquire the nutrients in powder from a professional nutrient supplier.

Nutrients are set at 1.5-1.7mS, PH adjusted between 6 and 6.5 with Sodium Bisulfite. I know Sodium is not so good, but the nutrient solution is buffered and I only have to adjust PH 1-2 times before the solution is changed. Pumps run in intervals of 30m on, 15m off during the day and every hour during the night. The reservoir has 40 liters.

Currently i'm growing Wattercress and Lettuce (Grand Rapids) and I manage to get a crop of Lettuces in 50-60 days (weather varying).

My questions:

Why some lettuces get some strange marks? It is a Magnesium deficit? They grow normally but always have some leaves with those strange marks (See picture 8).

Why plants starting dying when at the sun? The air temperature is 12C. At night they look healthy again. This reminds my they are unable to absorb enough water. Nutrients running too high?

Why even small lettuces start developing long >10cm, 2mm wide roots with absence of little "hair" alike roots? (See picture 15)

Do you know any resource with information specific for Lettuces?

The photos are here: http://www.dropbox.com/gallery/2768489/1/hidroponics?h=27b359

Thanks in advance.