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Accelerated erosion has made an impact, at one time or another, virtually everywhere in the temperate and tropical zones of the world that have been cultivated or heavily grazed or logged-over. The process has been slowed down or even halted in most of Europe, Canada, the United States, Australia, South Africa, South Korea, the Soviet Union, China, and Chile, and a few other countries. Erosion continues unabated in most of Africa, the Middle East, the Andes Mountains, and the Mediterranean countries. It is worsening rapidly in the hilly and heavily populated regions of southern Asia.
The time required for soil development is intermediate between that required for nonrenewable mineral resources, metals, coal, gas, petroleum, and for renewable resources, forests, water, crops. For example, soils form naturally at rates of 0.5 to 0.02 mm a year. Natural erosion is highly variable in space and time ranging from essentially zero to an average maximum of 1.0 mm per year. Average man-induced erosion is 2.0 mm per year, which is far greater than natural rates of soil formation and natural erosion. Therefore, agricultural soils are being depleted, and they must be considered to be a nonrenewable resource.
During the last 100 to 200 years man has greatly accelerated erosion on many agricultural fields. One result of this erosion has been the loss of an appreciable part of the rooting zone in soils having restrictive subsurface horizons. We believe that we should emphasize the maintenance of a favorable rooting zone. Obviously in soils shallow to rock or with other limiting layers that are not modified quickly, the erosion rate that is permissible should be lower than that in soils without restrictive horizons or layers. If we place emphasis on how rapidly we can develop a favorable rooting zone, we may be able to keep pace with the natural events that will take place on our landscapes in the next 2,000 to 5,000 years.
Soil erosion always increases the cost of crop production and causes potential environmental hazards as well as human suffering. Erosion of soils by water reduces crop yields principally through the loss of nutrients and available water. Exposed subsoils caused by severe soil erosion also exhibit many adverse properties with respect to soil management for economic crop production.
Agronomic implications of soil erosion by water in the United States have been derived mainly from limited research on Mollisols, Alfisols, and Ultisols. Because cultivated Ultisols of the southeastern USA are thinner and suffer problems associated with subsoil acidity, crop yield reductions appear more permanent and difficult to restore. The permanency of soil erosion on crop yield reductions on many Mollisols soils appears ephemeral, because only additional quantities of N, occasionally P, and micronutrients are required to restore crop yields.
Additional research is urgently needed to quantify crop yield losses associated with soil erosion and reduce the cost of restoring crop production to an economic competitive level on eroded landscapes. Research of this nature would also provide insights for controlling unacceptable soil erosion levels.
The many factors influencing the sensitivty of forest and range soils to erosion have been extensively studied. These studies have not, however, produced an accepted method to evaluate potential erosion effects on forest site productivity. A bioassay technique is suggested and its use demonstrated to make this evaluation. The general expectation is that forest soils are intolerant to erosion. Although the bioassay technique does not provide a soil loss tolerance T-value for productivity, it does show some forest soils may be less sensitive to nutrient loss through erosion.
Rangelands, which occupy about 40% of the earth's surface, have been increasingly recognized as an important and often mismanaged natural resource. Accelerated erosion is recognized as a major problem associated with the management of this vast resource. Attempts to extend the concept of soil loss tolerance (T value), as developed for cropland, to range ecosystems is of questionable validity. The fragility of rangeland ecosystems, the irreversibility of erosion damage, and the large margins of error associated with soil loss estimates make it difficult to develop T values for rangelands.
Sustained productivity of an eroding soil cannot be determined unless yield increases from technology advances are separated from soil productivity changes due to erosion. This concern is especially paramount in the Palouse landscape of Whitman County where serious erosion occurs under an intensive dryland wheat production, that has also had significant technology advances. The separation of technology and soil productivity involved the use of long-term wheat yields, measured wheat response to remaining epipedon, historical soil erosion rates, and landscape-distributed soil erosion rates. Current wheat yield in Whitman County increased approximately 36.1 kg/ha (0.54 bu/acre) per year as an average for the whole landscape; meanwhile annual soil erosion losses average 21.1 metric tons/ha (9.4 tons/acre) on a cropland base of 421,200 ha (1,040,000 acres). The soil productivity decrease from an average epipedon loss of 13.4 cm (5.3 in) in a 90-year period was 725 kg wheat/ha (10.8 bu/acre). An average erosion rate, however, does not reveal the true impact on productivity. Isolation of the soil productivity change component by land capability subclass showed that the net increase in yield on lie and IIIe land (67% of the cultivated cropland) has masked a significant decline in productivity of subclasses IVe and VIe land (18% of the cultivated cropland) in the 90-year period of intensive cultivation. The average soil erosion rate in Whitman County over the 1940 to 1978 period has been nearly twice the tolerance value of 11.2 metric tons/ha (5 tons/acre) per year. Average annual soil erosion at the soil-loss tolerance (T value) level is expected to expose the subsoil of IVe land (about 12% of the cultivated cropland) in about 128 years.
A function is developed for defining soil loss tolerance (T value) that provides for permanent preservation of the soil resource, prohibits erosion that contributes excessively to pollution, and is a function of the present soil depth. The relationship is expressed by T(x,y,t) =(T1 + T2)/2 − (T2 − T1)/2 cos [π(z −z1)/(z2 —z1)], where T(x,y,t) is tolerable soil loss rate at point (x,y), and T1 and T2 are lower and upper limits of allowable soil loss rate, T1 corresponds to soil renewal rate, z1 and z2 are minimum allowable and optimum soil depths, and z is the present soil depth. Tolerable soil loss function between the points (T1,Z1) and (T2,z2) is sinusoidal and dependent upon soil depth and (T2 − T1)/2 is the amplitude. The period is represented by the cosine argument and goes from 0 to 180 degrees for values of z between the limits of z1 and z2. Examples of application are given.
Current criteria for determining soil loss tolerance (T value) are imperfect. Much is still unknown about the rates at which a favorable root zone forms and about the effects of erosion on soil productivity. It is known, however, that erosion at the rate of the maximum tolerance of 11.2 metric tons/ha/year exceeds the estimated rate at which most parent rocks weather.
Water quality objectives are too variable to provide a basis for determining T value. Additional work on the sediment delivery ratio is needed to clarify the relationship between soil erosion and water quality. A sediment limit is proposed as an aid in planning conservation systems that can limit soil erosion to meet water quality objectives.
Policies on the amount of erosion that can be tolerated should not be made solely by the scientist. Farmers and other land users, soil and water conservation districts, special interest groups, and political groups must participate in developing such policies. A major responsibility of the scientist is to inform these policy makers of the likely consequences of various options available to them. Political expediency and short-sighted environmental or economic demands cannot be allowed to determine tolerable levels of soil erosion.
A proposal is given for the systematic assignment of soil loss tolerances (T values) based on properties of soil profiles. The T value is conceived as the product of a T value assigned to deep, uniform soils and a T value adjustment factor. The former presumably may change with societal needs. The proposed T-value adjustment factor is calculated from soil properties and would remain constant unless the criteria change. Potential plant rooting depth is pivotal to the T-value adjustment factor and involves a search to the 2-m depth for (1) taxonomic features indicative of root growth limitation; (2) horizons with high strength and weak structure; and (3) horizons high in extractable Al, low in Ca, or both. An additional adjustment is made based on soil property changes within the potential plant rooting depth that are indicative of the effect erosion may have on soil productivity.
The present national objective of erosion control in the USA is to maintain the productivity of the soil resource and still allow for its maximum utilization. for agricultural land use, this objective is applied at the state and regional level as the soil loss tolerance (T) value; it is based on soil interpretation at the series level and ranges from 2 to 11 metric tons/ha/year (1 to 5 tons/acre/year). Used with the Universal Soil Loss Equation (USLE), it serves as a guide to farmers for selecting conservation practices suited to their soil and crop management conditions. The primary criterion currently used for establishing T values is topsoil thickness such that deeper soils can tolerate higher levels of annual erosion than shallower soils. This criterion is examined and several additional factors are considered: soil structure, existing erosion conditions, specific rooting requirements of crops, available erosion control technology, and non-point source pollution control objectives.
Pressure on existing cropland, emanating from a complex set of factors increasing demand for agricultural products and limiting agriculture's production response, is accelerating erosion losses. These losses take place on existing cropland and particularly on cropland converted from noncrop uses. Public concerns regarding effects of increasing soil erosion losses on future productivity of soils and environmental quality of water are being answered with legislation and agency actions to limit soil erosion. These actions require establishment and enforcement of erosion limits of tolerance termed T values.
Establishment and enforcement of T values affect farm income, U.S. capacity to export crops, availability and prices of food for domestic and foreign consumers as well as future soil productivity and quality of water and air. Consequently, all these factors and effects must be considered in establishing particular T values. T values will likely vary due to soil differences and the nature and magnitude of external effects.