you'll likely get tip burn on the lettuce leaves. and the high nitrogen levels may deform your lettuce plants.
Also, on another thread, someone mentioned lettuce liking a higher pH than is normally used for tomatoes.

Hi gmcd!
"High PPM" as commonly said for tomatoes is a relative matter: while tomatoes do not need more than 150 PPM of nitrogen, - 200 are classically recommended for lettuce. High N, induces too much vegetative growth with tomatoes (and may interfere or prevent blooming and fruiting), while lettuce is a purely vegetative (leafy) specie. Some 280-350 PPM of K are kinda standard for tomatoes, while a classical lettuce formula got only about 210 PPM. Hence both ideal nutrient requirements (and formulas) are obviously not compatible.

The other reason why tomatoes and lettuce do actually not fit very well in the same system, is that lettuce does not need any support to grow big, while tomatoes (except some true dwarf varieties) badly need support. Although this is not a major issue (where one could compromise), it's one more reason to think it over.

I dropped a link from my Web site showing the EC and pH of most commonly grown stuff. You'll see right off the bat that the nutrient requirements for lettuce an tomatoes are vastly different.

:O)

Here is a link that might be useful: EC & pH Chart

According to THAT chart, the PH (even though much different from my knowledge) should be pretty compatible and could still be settled at 6 as I proposed earlier. The EC of 2.0-2.5 for tomatoes given at this chart is leaned on a high feeding strategy at final stage. But I wouldn't recommend that concentration for smaller plants. 1. Tomatoes grow very well with much lower EC. 2. Especially with tomatoes, there are good nutrient formulas and plans with much lower EC! In fact strength wise compatible with lettuce until first cluster and further. Theoretically, with such a strategy, there 'd be time enough to grow lettuce until EC is due for a raise. From my perspective, the ACTUAL key problem of the compatibility remains the clearly different elemental requirements of both plants (as in N and P).

Thanks for the answers, so everyone runs separate pumps and reservoirs for individual plants or those with roughly the same requirements? Damn my wife is going to kill me now that I need to buy more pumps, reservoirs and air pumps :D

Hi again gmcd,

Just a little add here to avoid that you get a wrong idea at the end!
Actually lettuce and tomatoes are quite an extreme, when talking about compatibility (growing in the same system).

Many other plants can be (and are actually) grown together - even in symbiosis, so to speak. Some nutrient formulas are what one can call "general purpose" and many plants can be grown under a common PH (mostly around 6) , obviously in the same setup or system.

Since you're new to hydroponics (which I'm assuming since you don't already have multiple pumps and such), you may want to start with a single plant. It generally makes the first go a less bumpy ride as you're not trying to appease or trouble shoot multiple plants at the same time. and you won't need additional equipment.

i agree with grizz... i have a real desire to grow a number of plants hydroponically... but until i get things going consistantly well with basil I will be keeping them at that point... soil for the rest :)
Lucas out of curiousity what plants grown together do you prefer... say your top 6? (top 3 pairs or tris ect.) not a loaded question just impressed with many of your suggestions thus far.

thanks!

it does! actually both posts here and in the weak basil stem post have been great ... I run DWC and want to do some toms this winter... do you have a prefered variety of tomatoes? I have only gathered seeds from my local garden shop which i don't care for.. any help is AGAIN appreciated! gracias!

Thanks for all the answers, decided I will grow my toms in DWC.