Our Indispensable Soil - Soil: Reflections on the Basis of our Existence - page 8

soil in the Midwest originated as silt in dry climates south of the Ice Age glacial
zone. Known as loess, the silt is finer than sand and easily blown on the wind.
Once deposited, it formed thick layers that over time developed into fertile soil.
However, ploughing weakens the loess's aggregates, making them prone to col-
lapse under pressure from wind and rain. Thus, loess is suitable primarily for
growing grass since this does not expose the ground directly to the elements.
A number of great civilisations, including Genghis Khan's Mongol Empire,
have been founded on livestock farming on steppe-covered loess. This soil is com-
mon in northern China, where it can form a layer several hundred metres thick, but
erodes quickly if used for cultivation—a problem that has escalated in China in
recent years due to population growth and rising economic prosperity.
Soil aggregates become increasingly stable in areas of unbroken grassland, where
they can lie undisturbed in the ground. Anaerobic conditions eventually take hold
in the inner part of an aggregate as mineral particles become packed on the outside
and prevent oxygen from entering. Organic decomposition ceases when the micro-
organisms inside the aggregate are deprived of oxygen, and this process stabilises
the aggregate. By contrast, regular tilling or ploughing halts this chain of events and
makes the aggregates vulnerable to attrition from rain. If drought follows rainfall,
soil particles may be swept up by the wind and blown away. Tilling also damages
fungal hyphae in the soil, further weakening the bonds between aggregates.
The lesser of two evils? Iceland has major problems with erosion. The combination of a harsh cli-
mate and intensive sheep grazing has left the protective vegetation sparse. Recent years have seen
the introduction of American lupin, a plant that makes the soil more fertile by living in symbiosis
with bacteria that capture nitrogen from the air. More nitrogen means more plant biomass, which
breaks down into organic matter when the plants die. The lupins prevent erosion but outcompete
the natural vegetation. Thus the advantages of decreased erosion must be weighed against the
drawbacks of reduced biodiversity