PREVENTION OF EROSION BY TERRACING. 



31 



The bench terrace is best adapted for use on steep slopes where it 

 would be practically impossible to build and cultivate a broad-ridge 

 terrace. 



The broad-base level-ridge terrace contributes to the building up 

 of land possibly more than does any other form. With this terrace 

 practically no fertile parts of the soil are allowed to escape from 

 the field. The bench terrace also is a good land builder. The great- 

 est objection to the use of the graded terrace is that the water drained 

 off the field usually carries in suspension fertile particles of the soil. 



The table below was prepared to assist in the selection of the ter- 

 race best adapted to the needs of a particular field. In this connec- 

 tion it is recommended that the design of the terrace system be made 

 from the curves as given in this paper for each type of terrace. 



Types of terraces most applicable to land of various slopes. 



Kind of terrace. 



Average slope 

 of land. 



Type of soil. 



Grade of 

 terrace. 



Horizontal and sloping bench. 



Broad-base level ridge . . . 

 Broad-base graded ridge . 



Per cent. 

 15 to 20 



3 to 15 

 3 to 15 



Broad-base level ridge with tile drainage. 



Fairly pervi- 

 ous. 



do........ 



Impervious, 

 worn out. 



Any type. 



Level. 



Do. 



Prefe rably 

 variable, 0.0 

 to 0.5 per 

 cent.i 



Level. 



1 Grade wUl depend upon the length of the terrace, but it is advisable not to exceed a grade of 0.5 per cent 

 if possible. 



On the steeper slopes, where the soil erodes easily, clean-cultivated 

 crops, such as cotton and corn, should not be grown. Impervious 

 soils on slopes of 15 per cent or more, and all soils on slopes of more 

 than 20 per cent, are best suited to pasture and timber. 



The result that should be attained by a system of terraces and 

 proper farming methods is well expressed in the following quotation 

 taken from a bulletin ^ on Soil Erosion by W J McGee, of the Bureau 

 of Soils, United States Department of Agriculture : 



The primary object is conservation of both solid and fluid parts of the soil 

 through a balanced distribution of the water supply. The ideal distribution is 

 attained when all the rainfall or melting snow is absorbed by the ground or its 

 cover, leaving none to run off over the surface of the field or pasture ; in which 

 case the water so absorbed is retained in the soil and subsoil until utilized 

 largely or wholly in the making of useful crops while any excess either remains 

 in the deeper subsoil and rocks as ground water or through seepage feeds the 

 permanent streams. 



The above conditions are fulfilled most nearly by the horizontal 

 bench terrace and the broad-base level-ridge terrace, since the move- 

 ment of the water is reduced to a minimum by both. The graded 

 terrace lacks much in meeting the requirements. The broad-base 



1 U. S. Dept. of Agr., Bureau of Soils, Bulletin 71, p. 56. 



