Lettuce will thank you for elevated nutrient solution concentration, especially in the cooler months!
Lettuce is one of the world's foremost commercial hydroponic crops and NFT (nutrient film technique) is certainly the dominant growing technique. The most common types of lettuce grown using this method include looseleaf, butterhead and romaine.
The most common NFT-style commercial growing technique is 'gulley growing' - this to place plants through holes along a plastic pipe (e.g. PVC pipe), tube, or an inverted trough (gutter). Nutrient solution is pumped into the head of each gulley and it flows along a slight gradient nourishing all the plant roots as it goes. At the lower end the solution is collected and channeled back to the nutrient reservoir where it is recirculated.
Pictured below is a hobby-friendly NFT growing system. The Nutriculture Mulit-Duct NFT system (2 channel, 6ft) has small 5mm deep grooves running down the profile diagonally. The flow rate on this grow was about 800ml per min down each channel and the pumps run continuously 24/7.
These lettuces were grown inside a greenhouse at Aquaculture Hydroponics in the UK in November. Most guides will tell you that an electrical conductivity (nutrient solution strength) of around 1.2 is ideal for lettuce. However some growers prefer a stronger nutrient solution of an EC of 1.8-2.2.
The lettuces pictured above were grown with a nutrient solution at EC 2.2. Lower ECs of 1.0 - 1.2 can cause tip burn issues (calcium deficiency) and this, in turn, invites fungal diseases such as powdery mildew. Many commercial lettuce, herb and micro green growers (all using NFT) attempt to avoid these issues by using elevated E.C. levels of around 1.8 - 2.2 and enjoy healthier, more resistant plants as a result.
In low light conditions (in the cooler months) nutrient uptake is lower (particularly calcium) so higher ECs can really help - as high as 2.2! During summer EC levels can be reduced to 1.8, sometimes 1.6 as temperatures are higher and relative humidity is lower - causing high transpiration rates. In these conditions it is advisable to run a lower EC in an attempt to prevent mineral salt accumulation in the reservoir.
Has anyone out there successfully grown hydro crisp lettuce like cos, little gem or iceberg at low ECs? (Lower than 1.8?)
With thanks to Gareth Hopcroft, Aquaculture Hydroponics, and Nutriculture.