You will, of course, want to add compost, but there are other ingredients to consider, as well. If you have only a small amount of compost to add, you probably need something else to lighten the soil further. Peat moss comes in handy for this purpose.
But for raised beds as deep as the one featured in this project, another issue arises when it's time to fill the raised bed. You'll probably ask yourself, "How the heck am I going to come up with enough soil, compost, peat moss, etc. to fill in such a large area?" In other words, the issue of using "fillers" arises.
While you may very well end up using fillers (I did!), there are three caveats to keep in mind, concerning:
- Drainage
- Settling
- Nitrogen depletion
Drainage for raised beds is a double-edged sword. On the one hand, you need good drainage; on the other hand, good drainage means you'll need to water more often. Crushed rock (if you happen to have some lying around) or similar material provides excellent drainage. However, many of us aren't crazy about dumping rocks into planting areas; we typically think more in terms of organic matter when we consider our options for fillers.
But organic matter, too, can have its drawbacks. Relatively fresh material (e.g., recently cut brush) will settle as it decomposes. You'll have to add more soil at some point to bring the soil level of the raised bed back up where you want it.
There's another problem regarding the use of organic material that has yet to decompose: it uses nitrogen as it breaks down, thereby depleting the soil of nitrogen. So you'll end up having to add nitrogen to the raised bed's soil to make up for this depletion. But if the compost you've added to the pile has plenty of nitrogen in it (and it should, if it contains ingredients such as coffee grounds or manure), you may be able to compensate sufficiently.
