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Title:
THE EVOLUTION OF SOIL.
Presenter & Author:
Ward Chesworth,
University of Guelph,
Guelph, Ontario,
CANADA N1G 2W1.
Extended Title:
THE EVOLUTION OF SOIL AND ITS USE IN AGRICULTURE.
Abstract:
Soils form most readily in the most active weathering environments on the land
surface. These are found under humid climatic conditions, and a soil will normally
achieve a profile with A, B and C horizons within an order of hundreds or
thousands of years. Chemically, the process involves the replacement of cations
on mineral surfaces by protons, the latter deriving ultimately from acids produced
by the breakdown of organic matter, or from volcanic sources. The land surface
is inevitably overtitrated in this process so that the clay minerals produced
progress through stages where 2:1 sheet silicates dominate, to late stages in
which 1:1 clays and Fe and/or Al hydroxides are characteristic. Materials
produced in this way may be subjected to climate-change, or may be transported
by water, wind or ice into different climatic zones and geomorphological
situations. Further changes are thereby induced, producing soils that evolve
towards saline end points (e.g. in arid climates) or low redox states (e.g. in watersaturated topographic lows). Since the mastery of fire by Homo erectus about 1.5
million years ago, human activities have modified soils and soil-forming
processes, with the modifications becoming pandemic after the Neolithic
invention of agriculture by Homo sapiens. At the present stage, we have
commandeered virtually all the soils of the grassland and temperate forest
biomes, and in so doing have become a new and dominant geological force on
the land surface. The more pessimistic ecologists believe that the clash between
human economy and biosphere has currently reached a critical state, and that
civilization itself is under threat.
Summary:
All the success of human beings in creating the great cultural artifacts of
civilisation comes about because we live on the products of a dung heap, the
soil. This seething mass of (mostly microbial) organisms and their waste
products, is the source of over 99% of our food, and for about the last 10 to 12
thousand years we have co-opted more and more of it for our own uses. All of
the written history of the human species is little more than a footnote to the
Neolithic invention of agriculture that brought this situation about.
In effect, the origin and major paths of evolution of the soils of the earth are
controlled by two chemical pumps and two chemical sinks. First, protons are
pumped into the system in the form of acids from atmospheric precipitations and
from organic matter. Bases, represented by the minerals (principally
aluminosilicates and carbonates) of the earth’s crust constitute the
complementary sink for these hydrogen ions. Acid and base interact in the
process of hydrolysis, by which the minerals are stripped of their cations, which
are replaced by protons. This makes a number of important nutrient elements
available to sustain the biosphere, and produces clay minerals, which act as a
reservoir to help store the nutrients in a readily released form.
A second significant reservoir of nutrients is the dead and decaying detritus left
by soil organisms. This organic material constitutes the second, or electron
pump, the principal complemetary sink being atmospheric oxygen. Redox
reactions between pump and oxygen-sink are a major factor in the control of
overall oxidation-reduction conditions in the soil, though on a smaller scale local
electron acceptors are important. Micro-organisms are crucial in the mediation of
these processes.
Soil formation is most active in humid climatic zones, on landscapes and in
materials which allow a ready flow-through of water. As a consequence of the
hydrolytic breakdown of primary minerals and a gradual loss of released cations
by leaching, soils in humid climates become more acid and less fertile as they
evolve. Given enough time and geological stability, soils evolve towards relatively
infertile end points in which silica, iron and aluminum predominate. The fact that
we still have fertile soils on the planet is due fundamentally to the processes of
plate tectonics, particularly at spreading centres and subduction zones, where
pristine material from the mantle and lower crust provides fertilizer in the form of
fresh, nutrient-rich rocks to be distributed over the earth’s surface by the
erosional cycle. Without an active tectonic system, backed by the erosional
forces of water, wind and ice, the biosphere could not be maintained. It is likely
that if there ever was life on Mars for example, it died because of the failure of
the tectonic system needed to sustain it.
Two further evolutionary trends occur in soils. Where soil parent material has
been formed or transported into dry climatic regions, the evolution is towards
increasing alkalinity, with saline soils as an endpoint. A third trend is found in
very reduced environments (for example, under conditions of water saturation),
wherein iron, normally oxidized and relatively immobile in the ferric state at the
earth’s surface, becomes mobilised in the reduced, ferrous form.
With the coming of agriculture, our species began to assume a dominant role in
the evolution of soil. Agriculture appears to have begun in at least eight separate
places, not all independent of each other. It is generally accepted that the oldest
centres are in the Fertile Crescent of SW Asia and NE Africa, sites of the great
hydraulic civilizations of Mesopotamia and Egypt - civilizations which were totally
dependent on irrigation. A necessary precursor was the development of rainfed
agriculture in the highlands bordering this region, notably at Catal Huyuk in
Turkey, and similar localities. It was in such places that a number of common
farm animals were first domesticated, and a number of annual plants were
adopted as crops – notably the annual grasses that our ancestors genetically
modified into wheat, barley and oats by selective breeding.
Tilling the soils in the highlands around the Fertile Crescent involved
deforestation and increased soil erosion. Consequently, the valleys of the Tigris
and Euphrates have been considerably modified by the heavy sedimentary load
carried all the way to the Persian Gulf. The inherent fertility to the lower river
valleys depended on this process, which also modified and extended the
coastline. When farmers moved down from the highlands into the valleys, this
annually renewed fertility provided a relatively long period of sustainability to the
agricultural system, while the rivers themselves provided irrigation water to
replace the atmospheric precipitations relied upon in the rainfed regions. Starting
with the Sumerians, the system in lower Mesopotamia seems to have lasted for
about 2500 years before salinisation and climate change, brought about its
demise. The roughly contemporaneous system in the Nile Valley continued into
the 20th Century, possibly because the annual Nile floods are more dependable
both as a source of water and a source of fertility. The Aswan High Dam and
modern water-diversionary schemes, now threaten the future sustainability of
agriculture in this area.
Damage to the soil, especially in terms of erosion and salinisation, is clearly as
old as agriculture, and the damage has increased with the growth in human
population that a settled agriculture allows. As a result we have had to
commandeer an ever greater area of soil to produce the food necessary to avoid
the kind of Malthusian misery that overtook Easter Island. Currently, we have
appropriated most of the grassland soils, much of the temperate forest biome,
and many of the wetlands of the planet. Since WWII we have been rapidly
expanding into arid and semi-arid regions, thereby making ever greater demands
on the earth’s water resources for irrigation. Consequently, the old
Mesopotamian problem of salinisation has resurfaced in soils as diverse as those
exploited on the horticultural farms of California and in the cotton fields of the
Aral Sea region.
Agriculture has always expanded at the expense of the biosphere. Currently,
farming has produced an agricultural scar on the planet which affects about 1.5
billion hectares - a third of all suitable soils. In taking over so vast an area for our
own purposes, we have initiated an anthropic stage in the evolution of soil, which
is continually displacing other species from their niches. Chemically, the overall
tendency has been to emphasise natural trends, for example acidification in
humid climates, salinisation in arid and semi arid zones. At the same time several
important biospheric cycles have been modified – notably the erosional cycle and
the biogeochemical cycles of the plant nutrients. Important amongst the latter,
are our modifications to the cycles of the major nutrients nitrogen and
phosphorus. As a direct consequence, nutrients from the soil are leaching into
ground and surface waters at rates high enough to cause such problems as
eutrophication in water bodies on land (Lake Erie for example), and even in the
marine environment (for example the 500km long “dead zone” offshore from the
Mississippi delta).
Because our agricultural manipulation of the soil is not a self-contained activity
capable of unlimited expansion as the human demand for food increases, we
continually threaten the biosphere in which we are embedded. Soil represents
one of the major points of impact. It functions as a kind of placenta, by means of
which the land biota is maintained. For an agricultural system to be sustained
over the long term, it must safeguard this function, and on the whole, farming has
not done so. For more than 10,000 years we have conducted a long agricultural
experiment with the biosphere by using modified annual grasses to mine the soil
for plant nutrients. Like all mining operations, this one will eventually fail unless
we can find a way of conserving the fertility of soil that does not depend on the
diminishing natural resources that we currently use for that purpose. We have
undoubtedly accumulated a great deal of the knowledge of soil processes that
would enable us to develop a sustainable agriculture if the socio-political situation
were to allow it, but it is a sobering thought that in the two areas where
continuous agriculture has persisted the longest – namely in Egypt and in
Northern China – success was brought about not because human beings are
smart, but because a non-human agency – the local erosional system – has
provided the water and soil-fertility to maintain the farmer. Clearly, such selfmaintaining systems are scarce on the planet, and unless we can marshall all our
ingenuity, we will eventually fail to achieve a sustainable agriculture on a global
scale. The natural consequence would be that we would fail to sustain civilization
itself.