[mikegreen]

By being able to control the nutrients and how fast the lettuce grows. Potassium intake varies with how fast the plant grows, so adjusting the light and temperature (along with other nutrients, like nitrogen, salt, etc) allows the lettuce to grow quickly (possibly under lights 24hrs a day) without taking up excess potassium.

With a clean environment, potassium levels can be reduced as well, as high potassium soil (fertilized that way) will allow the plant to uptake more potassium and increase its resilience to disease and bug damage.