Page 616 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

580 part IV Soils, Ecosystems, and Biomes
▲Figure18.8 Soildegradation.Anexampleofsoillossthroughsheet and gully erosion on a northwest iowa farm. One millimetre of soil lost from a hectare weighs about 11 tonnes. [USDA–NRCS, National Soil Survey Center.]
An expanding practice for slowing soil erosion is no-till agriculture (also called no-till farming). In this approach, farmers no longer till, or plough, the soil after a harvest. Instead, they leave crop residue on the field between plantings, thus preventing soil erosion
by wind and water. Seeds are then inserted into the ground without disturbing the soil. Planting the new crop on top of the old crop also preserves moisture. The photos in Figure 18.9 compare roots of wheat planted using no-till practices with those of wheat planted using conventional agriculture. Because the soil is less compacted, roots in the untilled fields can grow longer, reaching moisture and nutrients farther below the surface.
The Dust Bowl Large-scale removal of native vegetation associated with the expansion of farming into the North American Great Plains in the late 1800s and early 1900s led to a catastrophic soil erosion event known as the Dust Bowl. Intensive agriculture and overgrazing combined with reduced precipitation and above-normal tempera- tures to trigger a multiyear period of severe wind erosion and loss of farmlands.
The deflation of soil—in some places as much as 10 cm over several years—extended from southern Canada, through the United States and into north- ern Mexico. The transported dust darkened the skies of cities (requiring streetlights to stay on through- out the day) and drifted over farmland, accumulat-
▼Figure 18.9 Roots of wheat plants in tilled and untilled soils. [NRCS.]
ing in depths that covered failing crops. The recent prolonged drought through 2012 in the U.S. Southwest and into west and central Texas and Oklahoma is similar climatically to the conditions that led up to the Dust Bowl (see www .pbs.org/wgbh/americanexperience/films/ dustbowl/ for more information).
Erosion Rates and Costs Soil erosion can be compensated for in the short run by using more fertilizer, increasing irrigation, and planting higher-yielding crops. How- ever, agricultural fertilizers pollute runoff, with devastating effects on streams, rivers, and river deltas. The “dead zone” created by nitrogen and other agricultural ferti- lizers in the coastal waters of the Missis- sippi River Delta is reaching its largest size ever (see discussion and illustrations in Chapter 19).
(a) No-till farming leads to greater moisture retention and looser soil, allowing wheat to develop longer root systems that access moisture and nutrients farther below the surface.
(b) Conventional farming leads to soil compaction and shorter plant root systems.
Georeport 18.2 Slipping through Our Fingers
About half of all cropland in Canada and the United States is experiencing excessive rates of soil erosion—these countries are two of the few that monitor loss of topsoil. The U.S. General Accounting Office estimates that from 1.2 to 2.0 million hectares of prime farmland are lost each year in the United States through mismanagement or conversion to nonagricultural uses.
Worldwide, about one-third of potentially farmable land has been lost to erosion, much of that in the past 40 years. The Canadian En- vironmental Advisory Council estimated that the organic content of cultivated prairie soils has declined by as much as 40% compared with noncultivated native soils. in Ontario and Québec, losses of organic content increased to as much as 50%, and losses are even higher in the Atlantic Provinces, which were naturally low in organic content before cultivation. The causes for degraded soils, in order of severity, include: overgrazing, vegetation removal, agricultural activities, overexploitation, and industrial and bioindustrial use.