It's a lot more involved than focusing on the generic ppm. Assuming you are using the NaCl scale, you actually added 2 teaspoons of Flora Nova Grow of the one part 7-4-10 fertilizer liquid to your 5 gallon culture. If that confirms what you did, that was the equivalent of putting about

53 ppm of nitrogen, and those are real ppm of actual nitrogen vs. the generic ppm measurements being thrown around. Seedlings do fine on half strength, which for lettuce is commonly around 140 ppm of actual nitrogen, so you had about a 40% strength. The little plants don't eat much and you are adding weekly so far. Other people put in around 90-100 ppm of nitrogen and leave it for a couple of weeks at the beginning. If you change more frequently, your plants are just as well off if not better. The formula you are using is well endowed with nitrogen, the principal nutrient the plants need to kick off their growth, together with calcium from your water.

But as they get bigger they will eat more so you will need to increase the strength. 650 ppm on your scale is a good number for lettuce, but if you change weekly you can throttle it down to around 500-600 ppm of fertilizer since the plants don't have time to exhaust it. Your water is somewhat hard so it sounds like there is enough calcium to go around.

In the last two weeks the lettuce plant does most of its weight gain, so it will exhaust weak solutions much more quickly. As an added benefit, if there is no deficit of micronutrients towards the end, you will probably have lettuce with more nutritional value.

FYI, You get quite close to an optimal mature lettuce formula with this extremely concentrated ammonium-rich product using exactly 1 teaspoon per gallon as long as you do have somewhat hard water (the label doesn't mention strength by crop, only by generic terms that are not as specific as some people would prefer). Hope this helps

This post was edited by PupillaCharites on Mon, Oct 28, 13 at 23:47