My initial posting noted that I felt that the discussion (up to
that point) had perhaps been biased toward temperate zone soils,
which exist in quite a different context than tropical soils. I
still feel this bias has not been addressed.
Perhaps one way of evaluating both temperate and tropical soils
using a level playing field is the amount of external energy inputs
required to produce a sustainable yield of a given crop.
The advantage to looking at soil quality from this angle is that
for temperate soils, there is a lot of energy stored in the organic
matter (in the form of chemical bonds) of the soil, but temperate
soils do not receive the same flux of solar energy that tropical
soils receive. Tropical soils do not store their enery as soil
organic matter, but as living plant material and associated soil
organisms.
Temperate soils, with their generally higher organic matter may
or may not need fewer inputs to provide a sustainable yield of a given crop.
However, tropical soils have the advantage of a high solar flux
which can be converted into plant material (in many cases, legumes,
which are much more common in the tropics than in temperate
areas).
This is a simplistic model, but is perhaps a starting point. Rex
Dufour
rexd2ncatfyv.uark.edu