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.
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