Organic Gardener's Composting by Steve Solomon

So any compost is good compost. But will it be good fertilizer? Answering that question is a lot harder: it depends on so many factors. The growth response you'll get from compost depends on what went into the heap, on how much nitrate nitrogen was lost as ammonia during decomposition, on how completely decomposition was allowed to proceed, and how much nitrate nitrogen was created by microbes during ripening.

The growth response from compost also depends on the soil's temperature. Just like every other biological process, the nutrients in compost only GROW the plant when they decompose in the soil and are released. Where summer is hot, where the average of day and night temperatures are high, where soil temperatures reach 80 degree for much of the frost-free season, organic matter rots really fast and a little compost of average quality makes a huge increase in plant growth. Where summer is cool and soil organic matter decomposes slowly, poorer grades of compost have little immediate effect, or worse, may temporarily interfere with plant growth. Hotter soils are probably more desperate for organic matter and may give you a marked growth response from even poor quality compost; soils in cool climates naturally contain higher quantities of humus and need to be stoked with more potent materials if high levels of nutrients are to be released.

Compost is also reputed to make enormous improvements in the workability, or tilth of the soil. This aspect of gardening is so important and so widely misunderstood, especially by organic gardeners, that most of Chapter Seven is devoted to considering the roles of humus in the soil.