August 13, 1920] 



action of wind or of water and in some local- 

 ities due to the additional influence of im- 

 proper tillage and pasturage bear definite re- 

 lations to the topography of the area affected. 

 Unfortunately the rich, black humas of the 

 top soil, which is the best part of it, is the 

 first to be removed — a fact that makes early 

 prevention imperative. If the losses are per- 

 mitted to continue a great succession of 

 gullies and barren ravines soon develops and 

 a worthless area is formed where valuable land 

 could have been retained. The water table is 

 perceptibly lowered over large areas by in- 

 creased depth of drainage channels or re- 

 moval of protective cover and this is another 

 serious loss. On the other hand proper drain- 

 age may change an alkali soil to a fertile one. 

 The chief processes that cause these losses 

 involve the principle that the transporting 

 power of water various as the sixth power of 

 its velocity. This means that a current whose 

 velocity is three miles per hour can carry more 

 than eleven times as much sediment as one 

 whose velocity is two miles per hour and that 

 a current of three miles per hour loaded to 

 its capacity will, on being reduced to two 

 miles per hour or less, deposit more than 90 

 per cent, of its load. When a flood current 

 subsides or is checked, an area of rich soil 

 may be covered to a depth of several feet with 

 sand or other worthless material. Prevention 

 and partial restoration of losses may be ac- 

 complished as follows: Meandering channels 

 may be replaced by large drainage ditches 

 and with the aid of catchment basins in 

 regions having high rate of rainfall, prevent 

 flooding and erosion of river bottom land. 

 Other losses may be wholly or partly pre- 

 vented by constructing retaining walls, by the 

 use of tiling or of lined open drains, by con- 

 tour tillage, by limited pasturage, or by plant- 

 ing trees, shrubs or grasses. Restoration may 

 be partially made by constructing dams or 

 by other means of ponding to check the cur- 

 rent and arrest the moving sediment thereby 

 changing the area from one of erosion to one 

 of deposition. 



Soil origin finds its explanation chiefly in 

 the field of geology; soil distribution, largely 



SCIENCE \ / ' - - •'^^u J 141 



in that of physiography. Different kinds of 

 spils are produced from different kinds of rock 

 or from the same kind of rock when subjected 

 to different processes during the course of 

 origin. For example, soils originating from 

 a given kind of rock in a warm, wet climate 

 will be very unlike those derived from the 

 same kind of rock in a cool, arid region. A 

 third kind of soil will result if the materials 

 from the same kind of rock are transported 

 and sorted by water before forming the final 

 soil; a fourth kind, if transported by glacia- 

 tion; and a fifth, if deposited by the wind. 

 The various kinds of soil may differ from each 

 other in number of mineral constituents or 

 in the different proportions of each. The 

 development of hills and valleys and other 

 topographic forms by erosion gives rise to a 

 different kind of soil in each topographic 

 location. Kinds of soil arise also in numerous 

 other ways each of which is a response either 

 directly or indirectly to geologic or physio- 

 graphic processes and conditions. 



Classification of soils that they may be sub- 

 jected to treatment conducive to the greatest 

 production depends chiefly on the accurate use 

 of th,e principles of soil origin and distribu- 

 tion. The changes recently made by the 

 United States Bureau of Soils in the revision 

 of classification units that were used in map- 

 ping a number of years ago afford excellent 

 illustrations of this fact and of its recognition 

 by the Soil Survey. The new divisions formed 

 are based almost wholly on genetic and topo- 

 graphic relations — the principles of geology 

 and physiography being applied to a much 

 greater extent and in greater detail than in 

 the earlier work. 



The distribution of vegetation in so far as 

 it is controlled by topography, kind of rock 

 and geologic structure constitutes an impor- 

 tant phase of agricultural geology. The dis- 

 tribution of soils, of rainfall, of temperature 

 and of plant and animal life, the location of 

 water courses, of valleys and uplands, of rail- 

 ways, highways and of markets as well as the 

 adaptability of various areas to their re- 

 spective agricultural uses are, to a remarkable 

 extent, arranged in accordance with the topog- 



