40 BULLETIN 512, U. S. DEPARTMENT OF AGRICULTURE. 
The graded terrace is adapted particularly for use on impervious 
and worn-out soils, and on shallow open soils with an impervious 
_ foundation—in short, soils that will not absorb much water and that 
necessitate the removal of most of it over the surface. 
By the selection and proper construction of suitable types of ter- 
races erosion can be controlled on slopes up to 20 per cent, or even 
more. Instances were found where erosion was controlled by the use 
of terraces on land which had a slope of 30 per cent. However, slopes 
steeper than 20 per cent usually can be devoted more profitably to 
grasses or timber than to cultivated crops. Of all types of terraces, 
the use of the broad-base level-ridge terrace is recommended wherever 
conditions will permit. This type, supplemented with efficient tile 
drains, offers the most ideal method of preventing soil erosion on 
any type of soil. 
The success of a terrace system depends largely upon its proper 
laying off. A good leveling instrument in the hands of a competent 
and experienced levelman is the best insurance against failure. 
Construction always should begin with the highest terrace in the 
field, and each terrace should be completed before starting the next 
lower one. The late fall and early winter is the best time to build 
terraces. 
A terrace is susceptible to failure until it has become thoroughly 
settled. To facilitate settling it is best not to cultivate a terrace the 
first year, but to sow it to a cover crop. The best results are obtained 
where crop rows are run parallel with the terraces. 
The instructions given herein for the selection and design of ter- 
race systems are based upon the results of surveys, observation, and a 
study of terraced fields in the best-terraced sections in this country - 
and it is believed that if they are followed carefully a great increase 
in the efficiency of terrace systems will result and that much better 
opportunity will be afforded to observe the results with a view to fur- 
ther improving the practice of terracing. At the same time a close 
study of local conditions—particularly of soil—should be made 
which no doubt will afford more definite information for improving 
further the design of a terrace system adapted to a particular 
locality. 
Since the primary purpose of terracing is to hold the soil of the 
farm in place and thereby both maintain its fertility and render pos- 
sible an increase of fertility by proper farming methods, all of the 
benefits, such as greater yields and land values, which result from the 
preservation and increased fertility of the soil may be attributed di- 
rectly to the practice of terracing. In short, the terracing of farm 
lands saves the soils the most substantial and valuable asset of the 
country. 
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