14.1: Introduction

So long! It’s been good to know you. This dusty old dust is a gettin’ my home. And I’ve got to be drifting along.

—Woody Guthrie, 1940

The dust storms that hit the Great Plains of the United States during the 1930s, centered in parts of Oklahoma, Kansas and northern Texas, were responsible for one of the great migrations in our history. As Woody Guthrie pointed out in his songs, soil erosion was so bad that people saw little alternative to abandoning their farms. They moved to other parts of the country in search of work. Although changed climatic conditions and agricultural practices improved the situation for a time, there was another period of accelerated wind and water erosion during the 1970s and 1980s. (Ironically, some of the worst-struck areas during the Dust Bowl are now producing crops again when the Ogallala aquifer was tapped for irrigated agriculture, although the water will run out in a few decades).

In many other areas land degradation has forced families off the farm to urban areas or caused them to seek out new lands by developing natural areas like rainforests. Fertile soils on slopes in southern Honduras are now severely eroded (Figure 14.1) after years of slash-and-burn agriculture. Much of the land has turned to pasture or been abandoned, and the area has become depopulated.

Figure 14.1. Erosion on steep lands in Central America. Removal of the fine topsoil left mostly boulders behind. Sorghum plants show drought stress due to lack of rain and low waterholding capacity in soil.

Erosion Is A Natural Process, But ...

Erosion of rock and soil is a natural process that over the eons has caused the lowering of mountains and the formation of river valleys and deltas. And natural erosion is going on all the time as water, ice and wind have their effects on rock and soil. One dramatic example of such erosion is the dust picked up by winds just south of Africa’s Sahara Desert—the Sahel region of transition from desert to savannah—traveling some 3,000 miles to South America and the Caribbean, and occasionally reaching the southeastern United States. This dust is thought to be a major source of phosphorus for the Amazon River basin, balancing losses that occur there. The problem in agricultural soils is greatly accelerated erosion that is especially severe when the soil is bare, unprotected by living plants, their roots or residue mulch. Also, breaking up of soil aggregates with tillage lessens rainfall infiltration into soil, which worsens runoff and erosion.

Climate and soil type are important factors affecting erosion. Intense or prolonged rainstorms are major causes of water erosion and landslides, while drought and strong winds are critical factors in wind erosion. More extreme weather conditions as a result of climate change are therefore adding to the concerns of both water and wind erosion. Soil type is important because it influences the susceptibility to erosion as well as the amount that can occur without loss of productivity. In Chapter 6 we discussed how some soils (especially silts) with poor aggregation are more susceptible than other soils, especially those with good aggregation. This is reflected in the soil erodibility ratings, which soil conservationists use to plan control practices.