FIGURE 219.—A steel terrace grader in operation
Necessity for Drainage Outlets
TERRACING to Control Erosion If Well Done Is Paying Investment
Most of the worn-out farms scattered throughout the United States were not worn out by producing crops but because the owners did not take steps to prevent the washing away of the fertile topsoil. Many productive farms will join the worn-out class in a few years unless soil erosion is prevented. Some of the Federal farm loan banks realize this danger and now require that all rolling land upon which they make loans be terraced. They realize that unless erosion is controlled there is great danger that the security for their loan will be washed away.
Terraces, such as are meant in this connection, are not distinct steps, as is often believed, but are broad ridges of earth thrown up across the slopes. In constructing these ridges, broad shallow channels are formed along their upper sides; in these channels the water flows off at low velocity, so that its power to erode and carry off soil is destroyed.
The narrow-base terrace—that is, a terrace from 5 to 8 feet wide at the base and from 6 inches to 1 foot high—has been used extensively in the Piedmont section of the South. It is cheap to construct and easy to maintain. However, attempts to cultivate this type of terrace have not been successful generally; considerable land is lost to cultivation and the growth of weeds and grasses on the terrace is objectionable.
The broad-base terrace has been developed from attempts to render cultivable the narrow-base form. It has all the advantages of the latter form, with the added advantage that no land is lost to cultivation. Such a terrace has a base width of from 15 to 25 feet and is from 1½ to 2 feet high. When properly constructed it can not only be cultivated but it can be crossed at any angle with farm machinery. It is the type of terrace best adapted to general farming conditions and is especially suited for areas where modern farm machinery is employed.
The success or failure of a terrace system is largely a matter of properly locating the terrace lines. A very common cause of failure is the fact that the upper terrace in the field is made to drain too large an area. As a result the upper terrace breaks and a large volume of water rushes down the slope, breaking all terraces below. Frequently a farmer desires to terrace his farm, but his neighbor’s farm lies at a higher elevation and the upper terrace would be required to handle run-off water from the higher land. In such cases an attempt should be made to induce the neighbor to terrace his farm also. If this cannot be done the water from above must be intercepted by a hillside ditch to carry it to the nearest drainage channel below.
Whenever possible terraces should end at natural drainage channels The absence of a suitable drainage outlet within the limits of the field often necessitates ending terraces at fence lines, depressions, or draws. The volume of water which is discharged from the ends of a system of terraces often erodes unsightly and objectionable ditches along the ends of the terrace to the foot of the slope. Erosion in such channels can be reduced greatly by seeding them with grass or placing soil-saving dams at frequent intervals along the ditch line.
In laying off terrace lines various kinds of homemade devices are employed, but unless the operator exercises special care in the use of them the results usually are poor. Many landowners realize the inefficiency of these devices and use as a substitute a farm level on a tripod that can be purchased for about $20. Such a level gives very satisfactory results in the hands of a careful operator. However, no attempt should be made to lay off a terrace system unless the operator has a thorough understanding of the terrace practices and the operation of the farm level. Both are simple and easily understood.
In constructing terraces the work should begin invariably with the highest terrace in the field and each terrace should be completed before work is started on the one next below. If time cannot be spared to terrace the entire field it is better to construct well the first few terraces near the upper side of the field than to terrace the whole field poorly, for a break in a terrace near the upper side of a field is followed by breaks in all below.
Terraces are sometimes built with a plow alone. Several plowings are required to throw the terrace up to the desired height. A large 16-inch plow with an extra large wing attached to the moldboard is used very successfully. The disk plow and ordinary road grader are also effective. However the most commonly used and the cheapest implement for throwing up a terrace is a wooden V-shaped drag that can be made in the field with very little difficulty. Steel terrace graders costing from $60 to $125 are now on the market and give satisfactory results. (Fig. 219.) After the first three or four rounds have been plowed on each side of the center line of the terrace, the drag is used to push the loose earth toward the center and thus build the terrace higher. The plowing is resumed and the drag used again, and this is continued until the terrace has attained the desired width. If the terrace is not built sufficiently high the first time, the work is started again at the center and the plowing and dragging are repeated.
In order to finish up terraces properly some work with the slip scraper is generally required. The top of the terrace should be tested with the level and rod to see that it conforms to the proper grade. Any low places detected should be filled with a shovel or scraper. All large embankments across draws and gullies should be built with slip scrapers, and it is necessary to build such embankments considerably higher than the rest of the terrace to allow for settling of the loose earth. Most breaks in terrace systems occur at crossings of gullies or draws, and it is therefore very important that high, broad, substantial banks be built across these places.
The terracing of farm lands requires considerable work, but the results obtained in increased productivity more than warrant the efforts expended.