yep... I can get the cauliflower leaves free from my supermarket. The girls eat the greens and some of the stalks from those leaves (dividing them into greens and stalks for simplicity). I also add in the stalk from the actual cauliflower floret itself when we buy them. So the soup is the stalks from the leaves and the stalks from the head of cauliflower I have at home but mostly from the stalks from the eaves. It is no different to using the stalk of a broccoli either. In fact most of the nutrients in both cauliflower and broccoli can be found in the stalks/stems not the actual flower buds (which is what they both are).
It wasn't the fact that you were making soup from stalks that I was curious about (I do the same myself). It was the idea of putting them out for the chickens and then collecting the remnants up to make soup that I found slightly odd.