What filled them to begin with? Truckloads of purchased clay soil? Native sandy soil? compost? Other OM?

When you speak of replacing it with compost, that OM will disappear too as it is consumed by the biota. Did you fill them with compost initially, or with soil?

Karen

I usually start with a mix of native clay soil and our own compost. When I ran out of compost and need to raise the soil level I add store bought garden soil - few bags usually per bed. As compost becomes available I add it - in fall and early spring depending on what is growing in the beds. But I just cant create enough compost to refill all beds at this speed - every year, 3 beds. And I am just curious how it can disappear so fast so much.

The compost is creating a lot of 'fluff' so part of the volume you see initially is air. As the compost further decomposes and disappears, its own volume goes away as well as some of the fluff. What you need is more of the mineral fraction of soil. Keep adding those bags of topsoil or soil from some other source, along with compost, and eventually you'll have a stable system with full beds.

If your soil is healthy the compost and other organic material will be consumed and disappear very quickly. It will pretty much need to be replaced yearly.

If you want to add loads of organic material yearly, fall leaves are probably the easiest to come by. You can mound them up several feet deep. They will improve the soil but disappear by summer, then need to be replaced.

The best way to keep it full of soil is to add soil. It's cheaper to buy by the truck load than in bags. Chances are that will be pretty crappy soil that will need plenty of organic matter added to improve it anyway. BUT the soil component that you add won't disappear. The organic matter will disappear.

Karen

Well, I guess I really need to get on my husband to fix my shredder to create more shredded bark and leaves.

Thanks

If you wish to mitigate the cycle of decomposition, you'll need to add something besides sources of organic matter. Granite grit, turface, very coarse sand, and similar substances will add permanent substance to your beds as well as great porosity. Planting beds cannot be composed of mostly organic compounds....or they will 'disappear' pretty rapidly.

The mineral portion of a soil is very important. Don't try to replace it completely with organic matter (leaves, clippings, chips, etc.)

For a longer lasting organic component, think in terms of conifer BARK, rather than wood chips. The outer bark, with all of the lignin, suberin, and other such compounds is very slow to decompose.

ok, so what falls under "mineral portion" ?

That would be actual soil. Think of the dirt that was in your yard when you moved there.

Compost and organic matter will add humus, decayed organic matter, which is one component of soil. But soil is also made of sand, clay, silt, minerals. Lots of stuff.

If you want to raise the level of soil in the beds, add actual soil as opposed to organic matter like compost. Add your compost or organic mulch to that new soil to make it better soil.

Karen

Well, my clay soil is still there in the garden. So do I just add it into the garden beds with bark and leaves?

Soil is composed of the mineral portion, the sand, silt, and clay particles that comprise most of soils and the organic matter that is added either by you or Ma Nature and that organic matter portion is what "disappears", or rather is digested by the soil batcteria and fungi that make the nutrients your plants feed on.
Compost provides most of the bacteria that your garden needs to digest the organic matter you add and it does not need to be all of the organic matter. Use whatever waste vegetative material you can for the major part of the organic matter you add and add about a 1/2 inch layer of compost to that mix and that should be enough to start to increase the amount of organic matter, humus, in your soil.