Soil Loss Tolerance - USDA-ARS Wind Erosion Research Unit

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92 SKIDMORE 1.4 1.2 % \ E E 1.0 - W 0 z .8 2 w I I- n W n cn cn .4 3 I I I I I I I I I 0 0 0 2 4 6 8 10 12 14 16 18 20 ELAPSED TIME. centuries Fig. 2. Change in soil depth and T value as soil loss proceeds at the tolerable rate. 2.0 1.8 1.6 - 1.0 I F n .8 W n .6 i 0 .4 UJ .2 2.0 1.8 1.6 1.4 1.2 1 .o .8 .6 .4 .2 L \ E E W 0 1 I I l l l l l l l l l l l l l l l l I l O 0 2 4 6 8 10 12 14 16 18 20 ELAPSED TIME, centuries Fig. 3. Change in soil depth and T value as soil loss proceeds at the tolerable rate. The optimum soil depth could also be defined in terms of the present soil depth or according to the judgment of soil scientists who know local conditions and limiting resources. In addition to meeting T values, we should increase the quantity and quality of the soil resource wherever the
SOIL LOSS TOLERANCE 93 cost of doing so is lower than the value of the increased production capacity. So far we have discussed the soil property depth as if it were measurable, which it is. However, we must establish some guiding criteria for that measurement. Do we measure A + B horizons, rooting depth of commonly grown crops, depth to impervious layer, or something else, and do we establish a weighting factor for quality of soil material giving more credit to the more desirable topsoil These questions and those pertaining to defining limits can be answered best by consensus of concerned and knowledgeable scientists representing various groups like the Soil Conservation Service, Agricultural Research, and Cooperative State Research Service. With appropriate limits, Eq. [2] would satisfy criteria for defining erosion tolerance mentioned earlier and be very useful for determining T value. Then, using the WEQ (Woodruff and Siddoway, 1965; Skidmore and Woodruff, 1968) and the USLE (Wischmeier and Smith, 1978), we can implement erosion control practices to maintain a soil resource that can produce food and fiber to meet our present and future needs. LITERATURE CITED Clean Air Amendments of 1970. 1970. Public Law 91-604. U.S. Code Congressional and Administration News. 1954-2001. Federal Water Pollution Control Act Amendments of 1972. 1972 Public Law 92-500. U.S. Statutes at Large 86:816-903. Food and Agriculture Organization of the United Nations. 1974. A world assessment of soil degradation-an international programme of soil conservation. Report of an expert consultation on soil degradation, Rome. 40 p. _--- . 1977. Assessing soil degradation-report of an FAOIUNEP expert consultation, Rome, FA0 Soils Bull. 34. 83 p. Lowdermilk, W. C. 1951. Conquest of the land through seven thousand years. USDA, SCS, MP-32.38 p. Revelle, Roger. 1976. The resources available for agriculture. Sci. Am. 235:164-178. Skidmore, E. L., and N. P. Woodruff. 1968. Wind erosion forces in the United States and their use in predictingsoil loss. USDA, ARS, Agric. Handb. No. 346.42 p. Stamey, W. L., and R. M. Smith. 1964. A conservation definition of erosion tolerance. Soil Sci. 97:183-186. Wischmeier, W. H., and D. D. Smith. 1978. Predicting rainfall erosion losses-a guide to conservation planning. USDA, Agric. Handh. No. 537. 58 p. Woodruff, N. P., and F. H. Siddoway. 1965. A wind erosion equation. Soil Sci. SOC. Am. Proc .29:602-608.
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Soil Loss Tolerance - USDA-ARS Wind Erosion Research Unit

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