30 



BULLETIN 512, U. S. DEPARTMENT OF AGEICULTTJRE. 



susceptible to overtopping during the next rain. Also the finer, 

 lighter, and more fertile particles of soil remain suspended in the 

 moving water and are carried off the field. In such cases, bj^ the 

 use of excessive grades, the very cream of the soil is lost. Where 

 erosion of a terrace takes place no attempt should be made to culti- 

 vate the terrace. It should be seeded to grass. 



COMPARISON OF TERRACE TYPES. 



In order to show the relative merits of the bench terrace and the 

 various forms of broad-base ridge terraces, the table below was pre- 

 pared. Under the column headed "Least amount of erosion" is 

 found the broad-base level-ridge terrace ranking first and the 

 uniform-graded terrace last. Since the primary object of terracing 

 is to reduce erosion, this advantage should have the greatest weight 

 when considering the merits of the different types. The broad-ridge 

 terrace is much superior to the bench type of terrace when consider- 

 ing the "Least waste land or weeds." The embankment of broad- 

 ridge terraces can be cultivated successfully and hence no land is lost 

 to cultivation or weeds allowed to grow on the terrace. 



SJiotoing how terraces rank with respect to various advantages. 





Least 

 amount 

 of ero- 

 sion. 



Least 

 waste 

 land or 

 weeds. 



Fewest 



ter- 

 races re- 

 quired 

 in field. 



Ease of 

 culti- 

 vating 

 land. 



Bast adapted to- 



Best 



land 



builder. 



Type of terrace. 



Per- 

 vious 

 soils.! 



Imper- 

 vious 

 soils. 



Steep 

 slopes. 



Horizontal and sloping bench 



2 

 1 



4 

 3 



2 

 1 

 1 

 1 



3 

 3 

 2 

 1 



2 



1 

 1 

 1 



2 

 1 

 4 

 3 



3 



t 



1 



1 

 4 

 3 

 2 



2 

 1 



Broad-base uniform-graded ridge — 

 Broad-base variable-graded ridge 



4 

 3 



1 With reference to least amount of erosion. 



Under the column headed " Fewest terraces required in field," the 

 graded terrace ranks ahead of the level type owing to the fact that 

 by giving the terraces considerable fall they may be spaced farther 

 apart than level terraces. However, the greater vertical spacing can 

 be used only at a cost of greater erosion to the field. 



Broad-terraced fields are easier to cultivate than the bench type 

 since implements and large machinery can be moved across the broad 

 terraces and if desirable the rows can be run at any angle. With the 

 bench type each bench must be cultivated separately, and difficulty is 

 encountered in getting implements from one bench to another. 



From the standpoint of erosion the broad-base level-ridge terrace 

 is best adapted for use on pervious soils; on impervious soils the 

 graded terrace can be used to the best advantage since in the former 

 case most of the water is drained off through the soil and in the 

 latter the water is drained off the field over the surface. 



