If you want to manage your garden soil wisely, keep foremost in mind that the rate of humus loss is far more important than the amount of humus present. However, natural processes remove humus without our aid or attention while the gardener's task is to add organic matter. So there is a very understandable tendency to focus on addition, not subtraction. But, can we add too much? And if so, what happens when we do?
How Much Humus is Soil Supposed to Have?
If you measured the organic matter contents of various soils around the United States there would be wide differences. Some variations on crop land are due to great losses that have been caused by mismanagement. But even if you could measure virgin soils never used by humans there still would be great differences. Hans Jenny, a soil scientist at the University of Missouri during the 1940s, noticed patterns in soil humus levels and explained how and why this occurs in a wonderfully readable book, Factors in Soil Formation. These days, academic agricultural scientists conceal the basic simplicity of their knowledge by unnecessarily expressing their data with exotic verbiage and higher mathematics. In Jenny's time it was not considered demeaning if an intelligent layman could read and understand the writings of a scientist or scholar. Any serious gardener who wants to understand the wide differences in soil should become familiar with Factors in Soil Formation. About organic matter in virgin soils, Jenny said:
"Within regions of similar moisture conditions, the organic matter content of soil . . . decreases from north to south. For each fall of 10 degree C (18 degree F) in annual temperature the average organic matter content of soil increases two or three times, provided that [soil moisture] is kept constant."
Moist soil during the growing season encourages plant growth and thus organic matter production. Where the soil becomes dry during the growing season, plant growth slows or stops. So, all things being equal, wet soils contain more organic matter than dry ones. All organic matter eventually rots, even in soil too dry to grow plants. The higher the soil temperature the faster the decomposition. But chilly (not frozen) soils can still grow a lot of biomass. So, all things being equal, hot soils have less humus in them than cold ones. Cool, wet soils will have the highest levels; hot, dry soils will be lowest in humus.
This model checks out in practice. If we were to measure organic matter in soils along the Mississippi River where soil moisture conditions remain pretty similar from south to north, we might find 2 percent in sultry Arkansas, 3 percent in Missouri and over 4 percent in Wisconsin, where soil temperatures are much lower. In Arizona, unirrigated desert soils have virtually no organic matter. In central and southern California where skimpy and undependable winter rains peter out by March, it is hard to find an unirrigated soil containing as much as 1 percent organic matter while in the cool Maritime northwest, reliable winter rains keep the soil damp into June and the more fertile farm pastures or natural prairies may develop as much as 5 percent organic matter.
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.