Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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.