I have used malted barley for years now.
I grind it to a powder/flour.
I give my larger vegetative plants and flowering plants, 2-4 heaped tablespoons each, applied every month.
For seedlings, reduce this to 1-2 teaspoonfuls.
Small plants, 1-3 flat tablespoons, every 3 weeks or so.
Always water in with plain water after application.
I have also tried adding whole malted barley kernels to the soil mix, but don't think this is as effective as the finely ground up flour/meal/powder.