autoflower flower bed for top yield

What would be the best soil or soilless base for a flower bed? what spacing to use for a 10x6 flowerbed? No drainage for the bed possible. I am looking to use 2000watts and want to get the most out of the grow. All help and advice welcome.

Well, what I would do (not necessarily what someone else would) is use some FFOF soil as you can grow most autos in it with minimal supplemental nutrients. Some say its too hot for autos, but I have had great success using it in the past and will continue to recommend it. *just read the drainage detail, that might change things

Spacing? Well I would probably put two rows of 3 in that bed, with respective spacing.

i thought maybe 20 autos

You could be able to do 20 since they don't get very tall and thus won't need as much space for root growth. I (personally) wouldn't overload any more than one plant per square foot of light. Autoflowers need a constant 18/6 or 20/4 light cycle. I wouldn't suggest 24/0. They don't need much nutrients, standard potting soil would work just fine.

you wont need 2000 watts for 20