Factors of Pedogenesis - Major Soil Groups of the World - page 68

can intercept within the first few metres of the soil all the bases carried
downwards. In sum, biological recycling of bases hinders and sometimes
puts a cap on depletion (Vedy 1973).
As years pass, there comes a time when additions of litter will barely
compensate for the 'losses' by incorporation. We can walk in the forest
without having to swim through needles or dead leaves. But it takes
long for the equilibrium to be attained. In the Douglas fir plantations
on crystalline rocks, considered for constructing Table 2.8, equilibrium
has still not been attained after 40 or 60 years (Marques 1996). At
equilibrium, we will be able to calculate the turnover time by dividing
the values of the humus stock (line 2) by the corresponding values of
annual restitutions (line 7).

2.6 SOIL FAUNA AND MICROORGANISMS

We do not study the soil fauna because they have been examined in
great detail in a high-quality work (Gobat et al . 2004). We only shall
summarize some essential points.

Termites represent 100 kg of living organic matter per hectare in the
tropical zone, or 1000 individuals per m 2 . They completely modify
the soil to a depth of one metre and dig galleries up to 10-20 m.
They produce pellets smaller than 0.8 mm in diameter. Termites play a
considerable role in the genesis of tropical soils (Chap. 5).

Termites

Earthworms belong to several species (Bouché 1972). They have many
roles: they incorporate the surface organic materials into the soil, promote
aggregation of soil particles and drill very numerous galleries in the soil.
They have the reputation of achieving in thirty years the equivalent
of one ploughing. Furthermore, just like trout in water, they are good
indicators of the quality of the environment. In the vineyards of the
Midi in France, earthworms had totally disappeared following chemical
weeding for twenty years. The soil, compacted by the repeated passage
of machines, could not be loosened by ploughing and by worm activity.
Water did not infiltrate any more and yields dropped… The farmers
finally understood that their cultural method had to be reappraised.

Earthworms

Microorganisms are involved in two particular areas: (i) mineralization
and transformation of organic matter, partly utilizing it for achieving

Microorganisms