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The Fertilizing Value of Compost

It is not possible for me to tell you how well your own homemade compost will fertilize plants. Like home-brewed beer and home-baked bread you can be certain that your compost may be the equal of or superior to almost any commercially made product and certainly will be better fertilizer than the high carbon result of municipal solid waste composting. But first, let's consider two semi-philosophical questions, "good for what?" and "poor as what?"

Any compost is a "social good" if it conserves energy, saves space in landfills and returns some nutrients and organic matter to the soil, whether for lawns, ornamental plantings, or vegetable gardens. Compared to the fertilizer you would have purchased in its place, any homemade compost will be a financial gain unless you buy expensive motor-powered grinding equipment to produce only small quantities.

Making compost is also a "personal good." For a few hours a year, composting gets you outside with a manure fork in your hand, working up a sweat. You intentionally participate in a natural cycle: the endless rotation of carbon from air to organic matter in the form of plants, to animals, and finally all of it back into soil. You can observe the miraculous increase in plant and soil health that happens when you intensify and enrich that cycle of carbon on land under your control.

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.