Municipal composting schemes usually must process huge volumes of material on very valuable land close to cities. Economics mean the heaps are made as large as possible, run as fast as possible, and gotten off the field without concern for developing their highest qualities. Since it takes a long time to reduce large proportions of carbon, especially when they are in very decomposition-resistant forms like paper, and since the use of soil in the compost heap is essential to prevent nitrate loss, municipal composts tend to be low in nitrogen and high in carbon. By comparison, the poorest home garden compost I could find test results for was about equal to the best municipal compost. The best garden sample ("B") is pretty fine stuff. I could not discover the ingredients that went into either garden compost but my supposition is that gardener "A" incorporated large quantities of high C/N materials like straw, sawdust and the like while gardener "B" used manure, fresh vegetation, grass clippings and other similar low C/N materials. The next chapter will evaluate the suitability of materials commonly used to make compost.
Analyses of Various Composts
Source N% P% K% Ca% C/N
Vegetable trimmings & paper 1.57 0.40 0.40 24:1
Municipal refuse 0.97 0.16 0.21 24:1
Johnson City refuse 0.91 0.22 0.91 1.91 36:1
Gainsville, FL refuse 0.57 0.26 0.22 1.88 ?
Garden compost "A" 1.40 0.30 0.40 25:1
Garden compost "B" 3.50 1.00 2.00 10:1
To interpret this chart, let's make as our standard of comparison the actual gardening results from some very potent organic material I and probably many of my readers have probably used: bagged chicken manure compost. The most potent I've ever purchased is inexpensively sold in one-cubic-foot plastic sacks stacked up in front of my local supermarket every spring. The sacks are labeled 4-3-2. I've successfully grown quite a few huge, handsome, and healthy vegetables with this product. I've also tried other similar sorts also labeled "chicken manure compost" that are about half as potent.
From many years of successful use I know that 15 to 20 sacks (about 300-400 dry-weight pounds) of 4-3-2 chicken compost spread and tilled into one thousand square feet will grow a magnificent garden. Most certainly a similar amount of the high analysis Garden "B" compost would do about the same job. Would three times as much less potent compost from Garden "A" or five times as much even poorer stuff from the Johnson City municipal composting operation do as well? Not at all! Neither would three times as many sacks of dried steer manure. Here's why.
If composted organic matter is spread like mulch atop the ground on lawns or around ornamentals and allowed to remain there its nitrogen content and C/N are not especially important. Even if the C/N is still high soil animals will continue the job of decomposition much as happens on the forest floor. Eventually their excrement will be transported into the soil by earthworms. By that time the C/N will equal that of other soil humus and no disruption will occur to the soil's process.
Growing vegetables is much more demanding than growing most perennial ornamentals or lawns. Excuse me, flower gardeners, but I've observed that even most flowers will thrive if only slight improvements are made in their soil. The same is true for most herbs. Difficulties with ornamentals or herbs are usually caused by attempting to grow a species that is not particularly well-adapted to the site or climate. Fertilized with sacked steer manure or mulched with average-to-poor compost, most ornamentals will grow adequately.
But vegetables are delicate, pampered critters that must grow as rapidly as they can grow if they are to be succulent, tasty, and yield heavily. Most of them demand very high levels of available nutrients as well as soft, friable soil containing reasonable levels of organic matter. So it is extremely important that a vegetable gardener understand the inevitable disruption occurring when organic matter that has a C/N is much above 12:1 is tilled into soil.
Organic matter that has been in soil for a while has been altered into a much studied substance, humus. We know for example that humus always has a carbon to nitrogen ratio of from 10:1 to about 12:1, just like compost from Garden "B." Garden writers call great compost like this, "stable humus," because it is slow to decompose. Its presence in soil steadily feeds a healthy ecology of microorganisms important to plant health, and whose activity accelerates release of plant nutrients from undecomposed rock particles. Humus is also fertilizer because its gradual decomposition provides mineral nutrients that make plants grow. The most important of these nutrients is nitrate nitrogen, thus soil scientists may call humus decomposition "nitrification."