Are my rocks killing my plants?

[sticky]

Hey everyone,

I set up some pipes for a DWC mostly to grow lettuce and maybe some vining plants at some point. I started  two different types of lettuce, spinach, and celery. Unfortuantely these plants have not be doing to well as of late. At first, based on the poor coloration of my spinach, i thought it was iron and i needed to supplament. But yesterday i noticed a perplexing trend.

The few plants that are doing well in the DWC are the ones with larger media. In an attempt to provide the small little seedling with extra support, i went out and bought smaller rock from a pet store. i didn't have quite enough so i ended up using the 1/2 inch river rock from the flood and drain beds.

So my question is, can anyone tell me what is wrong. The only thing that comes to mind is that the smaller rock is keeping the air from getting to the roots. But, other than that, i am really at a loss for what is killing my plants.

so this is an overall view of my DWC set up

so, as you can see the pots with the little rocks aren't doing so well, but the one with the big rocks are doing better.

And now for the kicker,

These lettuce plants were sprouted at the same time and transplanted into my AP systems within a couple of days of each of the DWC plants.

Some of these are 2-3x bigger than the DWC plants. Again the only thing i can think of is that the smaller rocks are keeping the air away from the roots.

Any advice would be much appreciated.

Thanks

[Keith Rowan]

are you filtering the water before it goes into your nft tubes?

how deep is the flow?  how are you wicking water up to the cups?

[sticky]

the pipes are 4" pvc, and they are set up to drain from one pipe down to the next. so i will run water into the top one and then it will drain to the next and then the next and finally back into my tanks. when the water is running the pipes will fill up within a 1/4" on the top of the pipe and then when the water is off will drain down an inch or so, this keeps water in the pipes at all times. i am using 2" netted pots that allows the water in just fine. the idea is to run water into the pipes and fill them up so when i have small seedlings the water can get to even short roots as the plants are developing. at the same time, when the water is not running, the water drains down far enough to expose the roots to air. Once the plant grows enough it will have roots that reach the water and will have access all the time, but again when the water is off and the pipe isn't full there will be
some access to air.

[Jeff S]

I do all my growing in grow beds, large and small. In the small beds I grow my seeds. I use expanded clay balls that are probably smaller than the stones in your pics. I just toss in the seeds and they grow within 4-7 days. Unfortunately I don't have as much success with mature plants producing and the are in larger grow stones.