<< Do you think I should put the well rotted kitchen compost into the onion beds aswell?>>
The more the merrier
Kitchen compost will not overfeed the onions but will provide gentle feeding and bulk. Maybe you don't really need FYM in addition to compost, as plants grow slowly over the winter anyway. Sounds like these are going to be the most pampered onions in TAS - post photos in the summer.
You are very organised - I'm still looking in horror at all the weeds that sneaked into the veggie patch when I turned my back. I know I need to get rid of them now, but on the other hand I know it will be easier when we have had a good hard frost to kill off the top growth - too much procrastination (I like that for 'word of the week'
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Wood ash is good around the root run of fruit trees too - the kind with stones, like plums. Also on pasture.