Solid Acidity

Solid Acidity is measured using the PH scale, which runs from 0 extremely acidic through 7 neutral to 14 strongly alkaline. The PH of most soils ranges from 4 to 8, but some soils are outside this range. The soil of the Pygmy Forest in Mendocino Country, California is extremely acidic (pH 2.8 to 1.9). At the other extreme, certain saline soils in Death Valley, California, have a pH of 8.5.

Soil pH affects the plants and other organisms living in the soil and is, in turn, influenced by those organisms. Piano and affected by soil pH partly be­cause the solubility of certain minerals varies with differences in pH, and soluble mineral elements.

Soil Erosion

Water, wind, ice, and other agents promote soil erosion, the wearing away or removal of soil from the land. Water and wind are particularly effective in removing soil. Rainfall loosens soil particles, which can then he transported away by moving water (Figure 14-12). Wind loosens soil and blows it away, particularly if the soil is barren and dry. Because erosion reduces the amount of soil in an area, it limits the growth of plants. Erosion also causes a loss of soil fertility, because essential min­erals and organic matter that are part of the soil are also removed. As a result: of these losses, the pro­ductivity of eroded agricultural soils drops, and more fertilizer must be used to replace the nutrients lost to erosion. Problem is that rarely makes the headlines. To get a feeling for how serious the problem is, consider that approximately 2.7 billion metric tons (3.0 billion tons) of topsoil are lost each year from U.S. farm­lands as a result of soil erosion. The U.S. Depart­ment of Agriculture estimates that approximately one-fifth of U.S. cropland is vulnerable to soil ero­sion damage.

Humans often accelerate soil erosion with poor soil management practices. Here we consider soil erosion caused by agriculture in some detail, bur it is important to realize that agriculture is not the only culprit. Removal of natural plant communities during the construction of roads and buildings also accelerates erosion. Unsound logging practices, such as clear cutting large forested areas, cause se­vere erosion (Figure 14-13).

Soil erosion has an impact on other resources as well. Sediment that gets into streams, rivers, and lakes affects water quality and fish habitats. If the sediment contains pesticide and fertilizer residues, they further pollute the water. Also, when forests within the watershed for a hydroelectric power fa­cility are removed, accelerated soil erosion can cause the reservoir behind the dam to fill in with sediment much faster than usual. This process re­sults in a reduction of electricity production at that facility.

Sufficient plant cover limits the amount of soil erosion: leaves and stems cushion the impact of

rainfall, and roots help to hold the soil in place. Although soil erosion is a natural process, abun­dant plant cover makes it negligible in many natural ecosystems.

Wind Erosion in Grasslands Semiarid lands, such as the Great Plains of North America, have low annual precipitation and are subject to periodic draughts. Prairie and steppe grasses, the plants that grow best in semi arid lands, are adapted to survive droughts. Although the aboveground portions of the plant may die, the root system can several years of drought. When the rains return, the root systems send up new leaves. Soil erosion is minimal because the dormant but living root sys­tems hold the soil in place and resist the assault by water and wind.

The soils of semiarid lands are of very hi»h quality, due largely to the accumulation of a thick, rich humus over many centuries. These lands are excellent for grazing and for growing crops on a small scale. Problems arise, however, when large areas of land are cleared for crops or when the land is overgrazed by animals. The removal of the natural plant cover opens the way for climatic conditions to "attack" the soil, and it gradually deteriorates from the onslaught of hot summer sun, occasional violent rainstorms, and wind. It a prolonged drought occurs under such conditions, disaster can strike.

The American Dust Bowl:

The effects of wind on soil erosion were vividly experienced throughout several western states during the 1930s. Throughout the late 19th and early 20th cen­turies, much of the native grasses had been re­moved to plant wheat. Then, between 1910 and 1937, the semiarid lands stretching from Oklahoma and Texas into Canada received 65 percent less annual precipitation than was normal. The rugged prairie and steppe grasses that had been replaced by crops could have survived these conditions, but not the wheat. The prolonged drought caused crop fail­ures, which left fields barren and particularly vul­nerable to wind erosion.

Winds from the west swept across the barren, exposed soil, causing dust storms of incredible mag­nitude. Topsoil from Colorado and Oklahoma was blown eastward for hundreds of miles. Women hanging out clean laundry in Geor­gia went outside later to find it dust-covered. Bak­ers in New York City had to keep freshly baked bread away from open windows so it wouldn't get dirty. The dust even discolored the Atlantic Ocean several hundred miles off the coast.

The Dust Bowl occurred during the Great De­pression, and ranchers and farmers quickly went bankrupt. Many abandoned their dust-choked land and dead livestock and migrated west to the prom­ise of California; their plight is movingly portrayed in the novel The Grapes of Wrath by John Steinbeck. Although the Midwest is no longer a dust bowl, soil erosion is still a major problem there and else­where.

Mineral Depletion of the Soil

In a natural ecosystem, essential minerals cycle from the soil to live organisms, and back again to the soil when those organisms, die and decay. An agricultural system disrupts this pattern when the crops are harvested. Much of the plant material, containing minerals, is removed from the cycle, so it fails to decay and release its nutrients back to the

soil. Thus, over time, soil that is fanned inevitably loses its fertility.

Mineral Depletion in Tropical Rain Forest Soils

In tropical rain forests, the climate, the typical soil type, and the removal by humans of the natural forest community result in a particularly severe type of mineral depletion. Recall that oxisols, soils found in tropical rain forests, are nutrient poor because the nutrients are stored in the vegetation. Any minerals that are released as dead organisms decay in the soil and are promptly reabsorbed by plant roots and their mutual is tic fungi. If this did nut occur the heavy rainfall would quickly leach the nutrients away. Nutrient reabsorption by vege­tation is so effective that oxisols can support luxuri­ant rain forest growth despite the relative infertility of the soil, as long as the forests remain intact.

When the rain forest is cleared, whether to sell the wood or to make way for crops or rangeland, its efficient nutrient recycling is disrupted. Removal of the vegetation that so effectively stores the forest's nutrients allows minerals to leach out of the sys-

Crops can be grown on these soils for only a few years before the small mineral reserves in the soil are depleted. When cultivation is abandoned, a secondary forest develops, but it is never as luxuri­ant or biologically diverse as the primary forest, because most of the original nutrients have left the system. If the secondary forest is later cleared for cultivation, the soil becomes even more impover­ished. Eventually, only a very few species of plants are capable of growing on the compacted, exposed soil. (Chapter 17 discusses aspects of deforestation other than soil degradation.)

Laterization of Tropical Soils When a forest is eliminated in tropical regions, Laterization, a soil process that produces a rock-hard soil, may occur. (The term "Laterization" comes from the Latin word for brick, later.) Laterized soil is so hard that in tropical areas it has been cut into bricks, allowed to dry, and used to construct temples and shrines. Although the removal of the forest causes most minerals to be washed away, iron and aluminum compounds, which don't leach readily, can he pres­ent in high concentrations, giving laterize soil a red or yellow color. As the remaining humus decays, the soil hardens in the sun.

Large areas of Laterized soils, which are often called "red deserts," are common in parts of India and Southeast Asia. Some scientists initially ex­pressed concern that South America's recent tropi­cal deforestation would lead to widespread laterite

The Perils of Cotton

Most people think of cotton as a natural

Not so. In addition to requiring large doses of pesticides and chemical fertilizers, cotton is commonly grown in hot, dry American states such as Arizona, California, and Texas, where it needs plenty of irrigation. The irri­gating depletes groundwater reservoirs and causes serious soil erosion. It is estimated that 15 tons of topsoil are lost each year from Texas farmland that yields only a quar­ter of a ton of cotton fiber.

Formation. However, there is little current evidence that Laterization on a large scale there.

Soil Problems in the United States

In spite of 50 years of government-supported soil conservation programs, erosion is still a serious threat to cultivated soils in many regions through the United States. One contributor to the problem is the fact that federal agricultural policies are inconsistent with one another id with the goal of soil conservation. In many federal policies, for example, increased food production is a greater priority than is the protection of soil and other resources. Some farm programs (price support programs, for example) offer incentives for farmers to produce crops at the expense of fragile lands and soils. The problems caused or exacerbated by government policies, however, can be mitigated by reforming the policies. The Food Se­curity Act of 1985 is a good exam­ple because it contains provisions that eliminate inconsistencies.

The plains and deserts are particularly vulner­able to wind erosion. When this land is irrigated, crops can be grown without danger of failure, hut without irrigation the frequent and prolonged droughts increase the likelihood of crop failures, which result in hare, easily eroded soil. Because of persistent water shortages—particularly in the Southwest—rainy farmers are abandoning farming altogether, it may take centuries for the abandoned barren land to return to its natural state; until then, it will be susceptible to erosion, especially by wind.

Erosion of soil by water is particularly severe along the Mississippi and Missouri rivers, as well as the central valley of California and the hilly Palouse River region of Washington State. The Soil Conservation Service estimates that about 25 percent of agricultural land in the United States is losing topsoil faster than it can be regenerated by natural soil-forming processes. This loss is often so gradual that even farmers fail to notice it. For ex­ample, a big rainstorm may wash away 1 mm (0.04 inch) of soil, which seems insignificant until the cumulative effects of many storms are taken into account. Twenty years of soil erosion to the loss of about 2.5 cm of soil, an amount that would take 500 years to replace by natural soil forming processes.

Worldwide Soil Problems

Soil erosion and mineral depletion are significant problems worldwide. More than 1 billion people depend upon agricultural lands that are not produc­tive enough to adequately support them. A combination of factors has created this situation including unsound farming methods, extensive soil erosion, and expanding deserts. Along with these factors, the needs of a rapidly expanding population exacerbate soil problems worldwide.

Local and regional soil problems have been reported for many years. The first global assessment of soil conditions, released in 1992, was the sum­mary of Three-year study of global soil degradation sponsored by the United Nations Environment Program. It reported that 1.96 billion hectares (4.84 billion acres) of soil—an area equal to 17 percent of the Earth's total vegetated surface area— have been degraded since World War II. Eleven percent of the Earth's vegetated surface—an area the size of China and India—has been degraded so badly that it will be very costly (or in some cases impossible) to reclaim it. The main causes of soil degradation are farming, overgrazing, and deforest­ation.

Asia and Africa have the largest land areas with extensive soil damage, and in both places the problem is compounded by rapid population growth. The Sahelians in Africa, for example, must use [heir land to grow crops and animals for food or they will starve, but the soil is so overexploited that it is able to support fewer and fewer people. The day is approaching when the Sahel will be utterly unproductive desert. To reclaim the land would require restricting its use for many years so it could recover; but if these measures were taken, the Sahelians would have no means of ob­taining food.

Attempts to develop highly productive, sustainable "temperate" agriculture in tropical areas have often failed, particularly in humid areas. When a rain forest is cur down mid burned to pre­pare the land for crops or pastures, all the nutrients tied up in the vegetation are released at one time instead of being released slowly and reabsorbed quickly by the plants. The nutrients are then rapidly leached, and the soil quickly becomes, unproductive.