That's a great idea. Wool / cotton jumpers will break down over time, but slowly.
You could investigate lasagna gardening. You can use it for raised beds. You can use layers of grass clippings, newspapers, seedweed, chopped leaves, manure, twigs, coffee grounds etc - green and brown layers. Friends have put kitchen waste as a layer (uncomposted) - no cooked, dairy or meat.
It's like making your own compost in a bed; works down over time. You'd need smaller layers but the principle will work the same. The link I gave suggests layering up, topping the whole with straw and leaving it to decompose.
It's a slower process than whacking in soil and planting, but it can be cheaper and make for great, rich planting medium, esp if you have the cheap/free ingredients to hand.
Depending on the depths of the beds, you could perhaps investigate doing a mixed system, putting some layers beneath a soil/manure mix.
Your planting sounds permanent, the plants will do best over time with a good quantity of good quality soil and compost. Polystyrene beads will take up space but in that case why not use smaller planters. You have large planters beacause the plants will do better in them. A mix of quality top soil, potting compost and garden compost will work. Potting composts degrade over time so it's important to include some loam. Make sure you have plenty of drainage holes and cover them to stop them blocking, broken clay pots are often recommended. you can buy metal cattle drinking troughs which are not cheap but make excellent large containers. Expensive to buy and fill though.
Do you plan on moving them? Do they really need to be light weight? I've used empty water bottles, with caps tight on, stacked up to fill in the bottom of a barrel before.. but I was only planting flowering annuals. Otherwise, if you really don't actually plan to move them around, I agree with Firefly that lasagna method would be better. Fill up the bottom with chunks of wood, a mix of grass clippings and chopped leaves or straw (browns and greens), phone books, etc. Look up Hugelkultur. Last years used potting soil from other containers works well too, mix it up with fresh soil based compost before dumping it in.
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You could investigate lasagna gardening. You can use it for raised beds. You can use layers of grass clippings, newspapers, seedweed, chopped leaves, manure, twigs, coffee grounds etc - green and brown layers. Friends have put kitchen waste as a layer (uncomposted) - no cooked, dairy or meat.
It's like making your own compost in a bed; works down over time. You'd need smaller layers but the principle will work the same. The link I gave suggests layering up, topping the whole with straw and leaving it to decompose.
It's a slower process than whacking in soil and planting, but it can be cheaper and make for great, rich planting medium, esp if you have the cheap/free ingredients to hand.
Depending on the depths of the beds, you could perhaps investigate doing a mixed system, putting some layers beneath a soil/manure mix.
Make sure you have plenty of drainage holes and cover them to stop them blocking, broken clay pots are often recommended.
you can buy metal cattle drinking troughs which are not cheap but make excellent large containers. Expensive to buy and fill though.
Otherwise, if you really don't actually plan to move them around, I agree with Firefly that lasagna method would be better. Fill up the bottom with chunks of wood, a mix of grass clippings and chopped leaves or straw (browns and greens), phone books, etc. Look up Hugelkultur.
Last years used potting soil from other containers works well too, mix it up with fresh soil based compost before dumping it in.