1.5: 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 United States and around the world have become degraded: they have become “worn out.” Degradation most commonly begins with tillage—plowing and harrowing the soil—causing soil aggregates to break apart, which then causes more rapid loss of soil organic matter as organisms have greater access to residues. This accelerates erosion, because soils with lower organic matter content and less aggregation are more prone to accelerated erosion. And erosion, which takes away topsoil enriched with organic matter, initiates a downward spiral resulting in poor crop production. Soils become compact, making it hard for water to infiltrate and for roots to develop properly. Erosion continues 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.)

Figure 1.2. Agricultural soil (left) and natural soil (grassland; right) from adjacent sites in the U.S. Great Plains. Agricultural soil has lower soil organic matter and higher density. Photos by Kirsten Kurtz.

Soil degradation caused significant harm to many early civilizations, including the drastic loss of productivity resulting from soil erosion in many locations in the Middle East (such as present day Israel, Jordan, Iraq and Lebanon) and southern Europe. This led either to colonial ventures to help feed the citizenry—like the Romans invading the Egyptian breadbasket—or to the decline of the civilization. The only exceptions were the convergence zones in the landscapes, valleys and deltas where the nutrients and sediments flow together and fertility can be maintained for many centuries (more about this in Chapter 7).

Tropical rainforest conditions (high temperature and rainfall, with most of the organic matter near the soil surface) may lead to 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 (the readily decomposed organic matter) 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 stimulated by rapid soil degradation in the East, originally a zone of temperate forest. Under the environmental conditions 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 (Figure 1.2).

… What now remains of the formerly rich land is like the skeleton of a sick man, with all the fat and soft earth having wasted away and only the bare framework remaining. Formerly, many of the mountains were arable. The plains that were full of rich soil are now marshes. Hills that were once covered with forests and produced abundant pasture now produce only food for bees. Once the land was enriched by yearly rains, which were not lost, as they are now, by flowing from the bare land into the sea. The soil was deep, it absorbed and kept the water in the loamy soil, and the water that soaked into the hills fed springs and running streams everywhere. Now the abandoned shrines at spots where formerly there were springs attest that our description of the land is true.

—Plato, 4th century B.C.

The extent of deteriorating soil on a worldwide basis is staggering: Soil degradation has progressed so far as to decrease yields on about 20% of all the world’s cropland and on 19–27% of the grasslands and rangelands. The majority of agricultural soils are in only fair, poor or very poor condition. Erosion remains a major global problem, robbing people of food and each year continuing to reduce the productivity of the land. Each year some 30–40 billion tons of topsoil are eroded from the croplands of the world.