predominantly poor agricultural land" we must understand 

 the nature and causes of erosion. The beginning of such 

 understanding is to recall the nature of the hydrologic cycle 

 precipitation, evaporation, absorption, infiltration, run-off, 

 transpiration, and seepage the relations to these of vegeta- 

 tive cover, and the parts played by humus, humified soil, and 

 the root system. 13 



The most immediate and direct cause of destructive erosion is soil dis- 

 turbance, that is, a disturbance, of any kind, of the natural balance 

 between gravitation and the binding power of roots, tending to hold the 

 soil in place, and on the other hand the force of running water tending to 

 move it. 14 



SOIL LOSSES THROUGH CULTIVATION 



iiin 



EAT 



wfp wfp wfp wfp 



WK& wfp w?fp W!P TlPW m& 



FALLOW 



EACH CART REPRESENTS TEN TONS PER ACRC 



Figure 31. 



When forest areas have been cut for their timber and have 

 been left to the almost inevitable following fires, then the 

 litter, humus, and sprouting seedlings are destroyed, acceler- 

 ated run-off begins to carry away the fertile topsoil, and 

 soon the areas are stripped to the clay, gravel, sand, or rock 

 beneath, and productive lands become waste. 



When an excessive number of cattle and sheep are per- 

 mitted to graze on pasture land, or on native grassland, or in 



See p. 6. 



14 C. G. Bates and O. R. Zeasman, Soil Erosion A Local and National Problem, Research 

 Bulletin 99, August 1930, University of Wisconsin Agricultural Experiment Station, p. 94. 



33 



