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. 
} 
| 
= | Average slope . Grade of 
Kind of terrace. ann. Type of soil. Pantieol 
Per cent. : 
iHorizontaltandislopingabench =. 2) hae see ce sas see 15 to 20 | Fairly pervi- | Level. 
ous. 
‘Broad-baseleveliridgents te) tee! en he). Dass Sy 0p ayy) eee Gosek eee Do. 
Broad-base graded ridge ..............- (el one ee 3 to 15 | Impervious, Preferably 
Worn out. variable, 0.0 
to 0.5 per 
cent. 1 
Broad-base levelridge with tile drainage...............- 3 to 15 | Any type. ..-- | Level. 
if 1 Grade will depend upon the length of the terrace, but it is advisable not to exceed a grade of 0.5 per cent 
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 
1U. S. Dept. of Agr., Bureau of Soils, Bulletin 71, p. 56. 
