We need to take a short diversion from our focus on soils and briefly discuss climate. Various characteristics of precipitation affect the potential for crop production and the losses of water, sediment, and contaminants to the environment. These include the annual amount of precipitation (for example, arid vs. humid climate); the seasonal distribution and relation to the growing season (can rainfall supply the crops, or is irrigation routinely needed?); and the intensity, duration, and frequency of rain (regular gentle showers are better than infrequent intense storms that may cause runoff and erosion).
So climate affects how soils function and the processes occurring in soils. What is perhaps less understood is that good soil management and healthy soils are important to reducing susceptibility to climatic vagaries and making the soil more resilient to weather extremes. The Great Plains area of the United States learned this during the Dust Bowl era of the 1930s (figure 5.12), when a decade of drought and unsustainable soil management practices resulted in excessive wind and water erosion, crop failures, the collapse of the agricultural industry, and massive human migrations out of the region. That devastating experience gave birth to the soil conservation movement, which has achieved much; but most soils, even in the U.S., are still in need of protection from erosion.Precipitation patterns are hardly ever ideal, and most agricultural systems have to deal with shortages of water at some time during the growing season, which remains the most significant yield-limiting factor worldwide. Water excess can also be a big problem, especially in humid regions or monsoonal tropics. The main problem, however, is not the excess water itself, but the lack of air exchange and oxygen. Many management practices focus on limiting the effects of these climatic deficiencies. Subsurface drainage and raised beds remove excess water and facilitate aeration; irrigation overcomes inadequate rainfall; aquatic crops like rice allow for grain production in poorly drained soil; and so forth. (See chapter 17 for a discussion of irrigation and drainage.)