2 BULLETIN 512, U, S. DEPARTMENT OF AGKICULTTJEE. 



matter per acre of land drained, containing plant food sufficient to 

 produce a crop. Unless this loss be replaced by natural agencies or 

 by the application of fertilizer, it is obvious that the land soon will 

 deteriorate greatl}^ in productiveness and eventually be abandoned. 



In addition to the loss of the soluble elements of the soil, a notice- 

 able impairment occurs in the physical condition of the soil. A^^ien 

 the moving Avater washes the soil particles from the surface of the 

 hillside and deposits them on the land below, the heavier particles, or 

 the sandy constituents of the soil, are deposited first, and the finer, or 

 clay, parts last. Since neither pure sand nor pure clay possesses the 

 productive characteristics observed in a soil composed of the proper 

 intermixture of sand and clay particles, it is apparent that the 

 effect of this sorting process is to diminish greatly the fertility or 

 productive power of the soil. Hence, not only the eroded land suffers 

 but also the land at a lower level upon which the eroded material is 

 deposited. Portions of the flood plains of small streams often are 

 covered with a layer of sand, the fertility of the land so covered be- 

 ing practically destroyed, since it is a most difficult task again to 

 build up a productive soil over such areas. Drainage channels, also, 

 co'nstructed at considerable cost, often become filled with soil 

 washed from the hill lands. (See PI. I, fig. 1.) As a result the ad- 

 joining bottom land reverts to swamp and becomes unprofitable for 

 cultivation. 



FORMS OF EROSION.i 



Erosion due to moving water occurs in two forms — sheet washing 

 and gullying. Small areas are practically ruined by gullying (PL I, 

 fig. 2), while sheet washing (PL II, fig. 1) diminishes the produc- 

 tive power of large areas. 



Gullying generally is the most dreaded of the two types on account 

 of its more apparent destructive effects. Where the ravages of ero- 

 sion proceed unchecked, deep gullies invariably develop in the field. 

 Their appearance causes not only a;bsoiute loss of land and incon- 

 venience in cultivating, but a marked lowering in the water table, 

 with a possible accompanying inability of the soil to retain the 

 proper moisture content for the production of crops and to withstand 

 periods of drought. 



The injury due to sheet washing, which occurs throughout the 

 United States, generally is underestimated and is regarded by many 

 farmers as of no particular consequence. It is this type of erosion 

 that slowly carries away the very fertility of the soil without appris- 

 ing the farmer — except through slightly diminished crop yields each 

 year — that the application of remedial measures is imperative in 

 order to save his farm. To the very slowness of its action can be 



^ For a more extended discussion of the translocation of soils, see U. S. Dept. Agr. Bui. 

 180, by R. O. E. Davis. 



