“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.