Other factors, like the basic mineral content of the soil or its texture, also influence the amount of organic matter a spot will create and will somewhat increase or decrease the humus content compared to neighboring locations experiencing the same climate. But the most powerfully controlling influences are moisture and temperature.
On all virgin soils the organic matter content naturally sustains itself at the highest possible level. And, average annual additions exactly match the average annual amount of decomposition. Think about that for a moment. Imagine that we start out with a plot of finely-ground rock particles containing no life and no organic matter. As the rock dust is colonized by life forms that gradually build in numbers it becomes soil. The organic matter created there increases nutrient availability and accelerates the breakdown of rock particles, further increasing the creation of organic matter. Soil humus steadily increases. Eventually a climax is sustained where there is as much humus in the soil as there can be.
The peak plant and soil ecology that naturally lives on any site is usually very healthy and is inevitably just as abundant as there is moisture and soil minerals to support it. To me this suggests how much organic matter it takes to grow a great vegetable garden. My theory is that in terms of soil organic matter, vegetables grow quite well at the humus level that would peak naturally on a virgin site. In semi-arid areas I'd modify the theory to include an increase as a result of necessary irrigation. Expressed as a rough rule of thumb, a mere 2 percent organic matter in hot climates increasing to 5 percent in cool ones will supply sufficient biological soil activities to grow healthy vegetables if the mineral nutrient levels are high enough too.
Recall my assertion that what is most important about organic matter is not how much is present, but how much is lost each year through decomposition. For only by decomposing does organic matter release the nutrients it contains so plants can uptake them; only by being consumed does humus support the microecology that so markedly contributes phytamins to plant nutrition, aggressively breaks down rock particles and releases the plant nutrients they contain; only by being eaten does soil organic matter support bacteria and earthworms that improve productivity and create better tilth.
Here's something I find very interesting. Temperate climates having seasons and winter, vary greatly in average temperature. Comparing annual decomposition loss from a hot soil carrying 2 percent humus with annual decomposition loss from a cooler soil carrying 5 percent, roughly the same amount of organic matter will decay out of each soil during the growing season. This means that in temperate regions we have to replace about the same amount of organic matter no matter what the location.
Like other substantial colleges of agriculture, the University of Missouri ran some very valuable long-term studies in soil management. In 1888, a never-farmed field of native prairie grasses was converted into test plots. For fifty succeeding years each plot was managed in a different but consistent manner. The series of experiments that I find the most helpful recorded what happens to soil organic matter as a consequence of farming practices. The virgin prairie had sustained an organic matter content of about 3.5 percent. The lines on the graph show what happened to that organic matter over time.
