PEEVEJSTTIOlSr OF EEOSION BY TEEEACING. 



15 



by a knowledge of the physical character, the humus content, and 

 the tillage condition of the soil. The susceptibility of the subsoil 

 to the percolation of water also is an important factor to be con- 

 sidered in estimating the run-off. 



12 3 4 5 



Verf/ca/ Disfcrnce Befyveen Terraces jn Feef 



Fig. 7. — Curves showing vertical distances between broad-base level-rldge terraces, for 

 diflEerent rates of run-off and land slopes. 



It is by no means an easy matter to estimate the percentage of 

 rainfall that will run off for the various types and conditions of 

 soils. For instance, the difference in the rates of percolation for 

 clay and sandy soils is very marked, the latter permitting a much 



