Re : garlic
If you're not organic, the instructions that came with some garlic that I received did mention that by February the garlic would benefit from an application of sulphate of potash (3oz/sq yd).
This I've done for the first time so whether it will produce a noticeable difference I don't know.
It's a hard neck variety, whether you would treat soft neck varieties the same I'm not sure.
And as said before, keep the alliums as weed free as possible, they don't like competition. I found out the hard way once.