Well, I learned the hard way that some commercial composts are not particularly good for the garden. They may not be all the way finished (they often will still smell of ammonia), and they may be high in salts. It's not a a bad idea to ask for test results or to test it yourself before applying large quantities.
My particular problem was that the compost was hydrophobic. I ordered a large dump truck full of compost to use as a top dressing/mulch around established perennials. Usually, this is an excellent idea -- it provides nutrients, keeps down weeds, improves soil moisture, and will improve the soil as it breaks down.
So, after spreading it around the plants, I watered it in a bit. The water just beaded up on the surface and ran off.
|Water beading off hydrophobic compost. You can also see that it is not very "finished", since it still has recognizable wood chips in it|
That was my error in judgement -- even though I had tried hard to moisten it before the winter, our very dry fall last year created a hydrophobic layer at the soil/compost interface that kept the moisture from the snow from ever getting down into the ground. Over the winter, everything died in the bed that had taken pains to "baby" with extra compost. What a shocker. What a bummer. All those established and beautiful plants, gone! And here I thought I was helping them by giving them a boost, but instead they died of drought.
The moral of this story is that hydrophobic compost needs to be turned into the soil (it doesn't seem to have the hydrophobicity once it has been mixed with soil) It can't be used as a top-dressing. If you intend to use commercial bulk compost as a top-dressing, water it before you spread it. If the water runs off, learn from me and DON'T use it as a mulch.
I don't think this same problem would happen with home compost, since it rarely gets hydrophobic. Unfinished home compost could potentially burn roots due to the ammonia, so give it a sniff before applying to your soil.