David, compost can actually be toxic to plants if it has stayed too wet and not been aerated. The difference is aerobic vs. anaerobic decomposition and anaerobic decomp makes a nasty smell and toxic stuff.
@RickCorey can explain in detail.
If you detect a strong smell, not earthy but sour and stinky, when you dig in the old soggy compost, I would try to dig it up, turn it over and let it dry out somewhat before trying to plant in it. Things like squash and melons typically grow wonderfully on old compost heaps.
Aerobic decomp makes the lovely, healthy fluffy "garden gold" that is the target we are going for in composting. That's why you must keep on turning or otherwise aerating your pile.
