Hard ground makes too great resistance, as air makes too little resistance, to the surfaces of roots.—JETHRO TULL, 1733
Soil loss during agricultural production is mainly caused by water, wind, and tillage. Additionally, landslides (gravitational erosion) may occur on very steep slopes. While water erosion and landslides occur under extremely wet soil conditions, wind erosion is a concern with very dry soil. Tillage erosion occurs on fields that are either steep or have undulating topography and is not affected by soil moisture conditions, because the soil movement downslope is caused by the action of farm implements. SOIL AND WATER CONSERVATION IN HISTORICAL TIMES Some ancient farming civilizations recognized soil erosion as a problem and developed effective methods for runoff and erosion control. Ancient terracing practices are apparent in various parts of the world, notably in the Andean region of South America and in Southeast Asia. Other cultures effectively controlled erosion using mulching and intercropping that protected the soil surface. Some ancient desert civilizations, such as the Anasazi in the southwestern U.S. (A.D. 600 to 1200), held back and distributed runoff water with check dams to grow crops in downhill depressions (see the picture of a now forested site). Their methods, however, were specific to very dry conditions. For most agricultural areas of the world today, erosion still causes extensive damage (including the spread of deserts) and remains the greatest threat to agricultural sustainability and water quality.
Erosion is the result of the combination of an erosive force (water, wind, or gravity), a susceptible soil, and several other managementor landscape-related factors. A soil’s inherent susceptibility to erosion (its erodibility) is primarily a function of its texture (generally, silts more than sands and clays), its aggregation (the strength and size of aggregates, which are related to the amount of organic matter), and soil water conditions. Many management practices can reduce soil erosion, although different types of erosion have different solutions.
Water erosion occurs on bare, sloping land when intense rainfall rates exceed a soil’s infiltration capacity and runoff begins. The water concentrates into tiny streamlets, which detach the saturated soil and transport the particles downhill. Runoff water gains more energy as it moves down the slope, scouring away more soil and also carrying more agricultural chemicals and nutrients, which end up in streams, lakes, and estuaries (figure 6.1). Reduced soil health in many of our agricultural and urban watersheds has resulted in increased runoff during intense rainfall and increased problems with flooding. Also, the lower infiltration capacity of degraded soils reduces the amount of water that is available to plants, as well as the amount that percolates through the soil into underground aquifers. This reduction in underground water recharge results in streams drying up during drought periods. Watersheds with degraded soils thus experience lower stream flow during dry seasons and increased flooding during times of high rainfall.
Soil erosion is of greatest concern when the surface is unprotected and directly exposed to the destructive energy of raindrops and wind (figure 6.1). While degraded soils tend to promote erosion, the process of erosion in turn leads to a decrease in soil quality. Thus, a vicious cycle is begun in which erosion degrades soils, which then leads to further susceptibility to erosion, and so on. Soil is degraded because the best soil material— the surface layer enriched in organic matter—is removed by erosion. Erosion also selectively removes the more easily transported finer soil particles. Severely eroded soils, therefore, become low in organic matter and have less favorable physical, chemical, and biological characteristics, leading to a reduced ability to sustain crops and increased potential for harmful environmental impacts.
The picture of wind erosion from the Dust Bowl era (figure 5.12) provides a graphic illustration of land degradation. Wind erosion can occur when soil is dry and loose, the surface is bare and smooth, and the landscape has few physical barriers to wind. The wind tends to roll and sweep larger soil particles along the soil surface, which will dislodge other soil particles and increase overall soil detachment. The smaller soil particles (very fine sand and silt) are lighter and will go into suspension. They can be transported over great distances, sometimes across continents and oceans. Wind erosion affects soil quality through the loss of topsoil rich in organic matter and can cause crop damage from abrasion (figure 6.2). In addition, wind erosion affects air quality, which is a serious concern for nearby communities.
The ability of wind to erode a soil depends on how that soil has been managed, because strong aggregation makes it less susceptible to dispersion and transportation. In addition, many soil-building practices like mulching and the use of cover crops protect the soil surface from both wind and water erosion.
Landslides occur on steep slopes when the soils have become supersaturated from prolonged rains. They are especially of concern in places where high population pressure has resulted in farming of steep hillsides (figure 6.3). The sustained rains saturate the soil (especially in landscape positions that receive water from upslope areas). This has two effects: It increases the weight of the soil mass (all pores are filled with water), and it decreases the cohesion of the soil (see the compaction of wet soil in figure 6.10) and thereby its ability to resist the force of gravity. Agricultural areas are more susceptible than forests because they lack large, deep tree roots that can hold soil material together. Pastures on steep lands, common in many mountainous areas, typically have shallow-rooted grasses and may also experience slumping. With certain soil types, landslides may becomes liquefied and turn into mudslides.
Tillage degrades land even beyond promoting water and wind erosion by breaking down aggregates and exposing soil to the elements. It can also cause erosion by directly moving soil down the slope to lower areas of the field. In complex topographies—such as seen in figure 6.4—tillage erosion ultimately removes surface soil from knolls and deposits it in depressions (swales) at the bottom of slopes. What causes tillage erosion? Gravity causes more soil to be moved by the plow or harrow downslope than upslope. Soil is thrown farther downslope when tilling in the downslope direction than is thrown uphill when tilling in the upslope direction (figure 6.5a).
Downslope tillage typically occurs at greater speed than when traveling uphill, making the situation even worse. Tillage along the contour also results in downslope soil movement. Soil lifted by a tillage tool comes to rest at a slightly lower position on the slope (figure 6.5b). A more serious situation occurs when using a moldboard plow along the contour. Moldboard plowing is typically performed by throwing the soil down the slope, as better inversion is thus obtained than by trying to turn the furrow up the slope (figure 6.5c). One unique feature of tillage erosion compared to wind, water, and gravitational erosion is that it is unrelated to extreme weather events and occurs gradually with every tillage operation.
Soil loss from slopes due to tillage erosion enhances the potential for further soil losses from water or wind erosion. On the other hand, tillage erosion does not generally result in off-site damage, because the soil is merely moved from higher to lower positions within a field. However, it is another reason to reduce tillage on sloping fields.