Soils and irrigation

Nothing has been more fundamental to the success of human societies than their relationships with the earth’s soils. Unique to the earth, as far as is known, soils are a vital natural resource that holds the nutrients, moisture and microorganisms essential for global food production. Since the emergence of agriculture in the Fertile Crescent of the Middle East some 11,000 years ago, the dual challenge facing farmers everywhere has been how to sustain soil fertility and prevent soil erosion. Agricultural activity has the potential to enrich or deplete soils, and with careful land management crops flourish and livestock thrive from generation to generation. But poor farming practices can soon strip a region of a crucial resource that is nonrenewable over human timescales. Soils are formed slowly over centuries or millennia, accumulating as a ‘living skin’ on the planet’s surface that is rarely more than a metre or two thick (and often much less). Agriculture makes soils vulnerable to erosion and exhaustion at faster rates than the processes that created them. Estimates for erosion vary widely, but soils are currently being lost at somewhere between two and ten times natural rates (and often more rapidly in tropical environments). Across the globe, human action has seriously degraded about 2 billion hectares of agricultural land, an area the size of the USA and Canada combined. Since the dawn of agriculture, human-induced soil erosion has surged three

times: first, around 2000 BCE as agriculture spread from the river valleys of China, theMiddle East and South Asia; second, with the worldwide expansion of European settler societies from the fifteenth century; and third, after the Second World War, with the advance of commercialised agriculture onto the marginal lands of the tropics and the widespread adoption of heavy farm machinery. This chapter is chiefly concerned with investigating the impacts of the second and third surges, which greatly accelerated soil erosion on a global scale. The environmental changes caused by the use of irrigation systems and chemical fertilisers in producing crops and combating soil exhaustion are discussed. This chapter will also examine the evolution, implementation and success of soil management and conservation strategies. We will begin, however, by briefly examining the key role played by the world’s soils in sustaining life.