HOW DO SOILS BECOME DEGRADED?
HOW DO SOILS BECOME DEGRADED? 
Although we want to emphasize healthy, high-quality soils because of their ability to produce high yields of crops, it is also crucial to recognize that many soils in the U.S. and around the world have become degraded— they have become what many used to call “worn-out” soils. Degradation most commonly occurs when ero-sion and decreased soil organic matter levels initiate a downward spiral resulting in poor crop production (figure 1.1). Soils become compact, making it hard for water to infiltrate and roots to develop properly. Erosion contin-ues, and nutrients decline to levels too low for good crop growth. The development of saline (too salty) soils under irrigation in arid regions is another cause of reduced soil health. (Salts added in the irrigation water need to be leached beneath the root zone to avoid the problem.) Historically, soil degradation caused significant harm to many early civilizations, including the drastic loss of productivity resulting from soil erosion in Greece and many locations in the Middle East (such as present-day Israel, Jordan, Iraq, and Lebanon). This led either to colonial ventures to help feed the citizenry or to the decline of the culture. Tropical rainforest conditions (high temperature and rainfall, with most of the organic matter near the soil surface) may cause significant soil degradation within two or three years of conversion to cropland. This is the reason the “slash and burn” system, with people moving to a new patch of forest every few years, developed in the tropics. After farmers depleted the soils in a field, they would cut down and burn the trees in the new patch, allowing the forest and soil to regenerate in previously cropped areas. The westward push of U.S. agriculture was stimu-lated by rapid soil degradation in the East, originally a zone of temperate forest. Under the conditions of the humid portion of the Great Plains (moderate rainfall and temperature, with organic matter distributed deeper in the soil), it took many decades for the effects of soil degradation to become evident. The extent of erosion on a worldwide basis is staggering—it is estimated that erosion has progressed far enough to decrease yields on an estimated 16% of all the world’s agricultural soils. The value of annual crop loss due to soil degradation by erosion is around $1 bil-lion. And erosion is still a major global problem, robbing people of food and each year continuing to reduce the productivity of the land.
Although we want to emphasize healthy, high-quality soils because of their ability to produce high yields of crops, it is also crucial to recognize that many soils in the U.S. and around the world have become degraded— they have become what many used to call “worn-out” soils. Degradation most commonly occurs when ero-sion and decreased soil organic matter levels initiate a downward spiral resulting in poor crop production (figure 1.1). Soils become compact, making it hard for water to infiltrate and roots to develop properly. Erosion contin-ues, and nutrients decline to levels too low for good crop growth. The development of saline (too salty) soils under irrigation in arid regions is another cause of reduced soil health. (Salts added in the irrigation water need to be leached beneath the root zone to avoid the problem.) Historically, soil degradation caused significant harm to many early civilizations, including the drastic loss of productivity resulting from soil erosion in Greece and many locations in the Middle East (such as present-day Israel, Jordan, Iraq, and Lebanon). This led either to colonial ventures to help feed the citizenry or to the decline of the culture. Tropical rainforest conditions (high temperature and rainfall, with most of the organic matter near the soil surface) may cause significant soil degradation within two or three years of conversion to cropland. This is the reason the “slash and burn” system, with people moving to a new patch of forest every few years, developed in the tropics. After farmers depleted the soils in a field, they would cut down and burn the trees in the new patch, allowing the forest and soil to regenerate in previously cropped areas. The westward push of U.S. agriculture was stimu-lated by rapid soil degradation in the East, originally a zone of temperate forest. Under the conditions of the humid portion of the Great Plains (moderate rainfall and temperature, with organic matter distributed deeper in the soil), it took many decades for the effects of soil degradation to become evident. The extent of erosion on a worldwide basis is staggering—it is estimated that erosion has progressed far enough to decrease yields on an estimated 16% of all the world’s agricultural soils. The value of annual crop loss due to soil degradation by erosion is around $1 bil-lion. And erosion is still a major global problem, robbing people of food and each year continuing to reduce the productivity of the land.
