Don't quote me on it, by I've read that neem does work systemically and can be absorbed through the roots & foliage.
I'm sure someone on here has done this and has better advice.
The azadirachtin (active compound of neem oil) is both a local and systemic product. But what is the use of using it in the res ? As it go systemic after the foliar spray, it will protect the whole plant ....
So, you'd better put some trichoderma and mycorhizes in the res, it would be more beneficial
He would want to use it in the res if he was having fungus problems in his growing medium. Such as in my case, where there seems to be a couple sorts of mold or fungus growing on rockwool cubes.
I just made a mix of about .5 oz. Neem in 3/4 gal. water and added about half of that water into my res. We'll see what happens... Fuck mold. I'm currently on the fence about rockwool, slowly falling towards the anti side of the fence