A NATIONAL PLAN FOE AMERICAN FORESTRY 327 



the original soil, is stiffer, more difficult to plow, less penetrable to plant roots, 

 less absorptive of rainfall, and less retentive of that which is absorbed, and appar- 

 ently its plant-food elements frequently have not been converted into available 

 plant nutrients to anything like the degree that obtains in the displaced surface 

 soil. . . . Such raw material must be given more intensive tillage in order to 

 unlock its contained plant food^and on much of it lime and organic manures will 

 be needed in order to reduce its stiffness sufficiently to make it amenable to 

 efficient cultivation, to the establishment of a desirable seed-bed tilth. It bakes 

 easier and, as a consequence, crops growing on it are less resistant to dry seasons, 

 because of rapid evaporation from the hardened surface, and the many cracks 

 that form deep into the subsoil to enlarge the area exposed to direct evaporation. 

 Crops also suffer more in wet seasons because the material becomes more soggy or 

 water-logged than did the original soil. On much of it both fertilizer and lime 

 will be required for satisfactory yields. 



While these difficulties of tillage and the lowered productivity are being attended 

 to by the farmer in those fields not yet abandoned, the unprotected fields con- 

 tinue to wash. Unfortunately the farmers in many localities are doing little or 

 nothing to stop the wastage and much to accentuate it. 



Even on moderate slopes the soil losses from the cultivated fields on 

 certain s-oil types, under unfavorable climatic conditions, are enor- 

 mous. Forest Service studies at Holly Springs, Miss., in the loessial- 

 soil belt, show that a single torrential rain falling on a cornfield 

 having a 10 per cent slope washed soil from a study plot at the rate 

 of 23 tons per acre. Preliminary results show that under such con- 

 ditions only 2 to 3 years are required to wash away 1 inch of topsoil. 

 These data, substantiated by observations, indicate that the culti- 

 vable life of these upland soils ranges from 5 to 20 years. Yet the 

 serious danger of erosion from the cultivation of fields of slight slope 

 in this region which have readily erosible soils is not so generally 

 recognized and many such fields are being cleared and plowed to 

 take the place of other fields which have lost their productivity. 

 Bennett points out (op. cit.) that "some soils can not be cultivated 

 without steady decline due to erosion, even where the slope does not 

 exceed 1 or 2 percent. The Knox silt loam, for example, is such a 

 soil. On this soil erosion goes on in all tilled fields where there is any 

 slope whatever. " 



The high run-off from slight slopes is further shown by Duley and 

 Hays 31 in their studies in Kansas. They found run-off increased 

 rapidly as the slope increased from to 3 percent grade. Over 63 

 percent ran off with a 2 percent grade in their experimental tank. 

 The increase in run-off was then very slight for each 1 percent of 

 increase in slope, reaching about 86 percent with a slope of 20 percent 

 grade. Erosion, on the other hand, increased gradually until the 

 slope was about 4 percent; then the increase was found to be more 

 rapid up to about 7 or 8 percent, after which there was a still greater 

 increase in the rate at which the soil was removed from the plots. 



If level agricultural land were scarce in the United States, and there 

 were a great need to increase crop production, very intensive farm 

 management could unquestionably be applied to rather steep slopes 

 to meet the situation. Under existing circumstances, however, it 

 seems likely that clearing new ground on slopes of over 10 to 15 per- 

 cent is destined eventually to swell the area of abandoned land and 

 add to the problem of reclaiming gullied land by reforestation. ^ 



The process of planting trees on actively eroded land is not simple. 

 Preliminary measures, in the form of temporary terracing, "plowing 



si Duley, F. L., and Hays, O. E. "The Effect of the Degree of Slope on Run-off and Soil Erosion.?' 

 Jour. Agr. Research, vol. 45, no. 6: 349-360. 1932. 



168342 33 vol. 1 22 



