How Soil Is Formed - Soil: Reflections on the Basis of our Existence - page 31



Earthy hues. Soil gets its reddish tint from iron and aluminium oxides. Known as ochre, these
compounds have been used as pigments since ancient times. Aboriginal Australians still use ochre
to colour their faces and for their traditional bark paintings. During the 1800s it was popular to
extract ochre for textile dyeing. The southern French village of Roussillon was an ochre centre and
the warm colours of the buildings there still bear witness to that period. Nowadays, the village is
a tourist destination where visitors can wander round the old open quarry and enjoy the sunlight
playing on the colours in the soil and vegetation

Coniferous forest litter promotes podzolisation and the phenomenon is most
strongly observed in New Zealand in areas with high precipitation, where the kauri
tree grows on acidic ground. The tree sheds large fragments of bark, which inten-
sifies podzolisation in the area of soil directly beneath the tree, resulting in egg-
cup-shaped deposits of bleached soil in podzolised areas.
Time is a key factor in soil formation. At ￅkerb￤ck we observed how both
bleached and rusty soil layers grew thinner the nearer to the coast we did our sam-
pling. A few hundred metres from the shore, it was hard to identify any bleached
layer at all. On the shoreline itself we found a cordon of alder trees, whose nitro-
gen-fixing root bacteria had helped bind nitrogen in the soil—a critical factor ena-
bling spruce to colonise.

Plants Accelerate Soil Formation

How, one might well ask, would the soil formation process work if plants were
absent? Rainwater washes away soil on an unvegetated hillside but penetrates flat-
ter ground, possibly allowing eluviated soil layers to form because atmospheric
carbon dioxide dissolves in rainwater and makes it acidic. But it would doubtless
take time.
In summer I often stop at a place called Kastad, near Gjövik in Norway, on
my way to my cabin in the Norwegian fells. There is a clearing in the spruce for-
est where the ground is completely bare. Clumps of grass grow around the edges
amid a string of low, stressed spruces. The soil here is highly polluted by lead—
not from human activity but from an underground source of galena, the natural
mineral source of lead sulphide. On top of the galena lies a metre-thick layer of
moraine. The area is interesting because the barren space allows one to study the
significance of plants for soil formation. Eluviated soil layers always occur in
soil where spruces have grown—soil where apatite has leached from the topsoil.
Apatite is a phosphate mineral and important source of nutrition for the spruce,
and the fungi that live in symbiosis with the tree's roots are effective at utilising
it. Apatite dissolves from the topsoil due to the action of rainwater on the bare
ground, though our models indicate that the rate of weathering is one hundred
times faster when spruce grows there. Clearly, ground vegetation plays a key role
in soil formation, which helps explain why soil in the Antarctic, where vegetation
cannot grow, is so thin and underdeveloped.