Evening Republican, Volume 21, Number 126, Rensselaer, Jasper County, 13 June 1917 — PREVENTING EROSION OF FARM LANDS. [ARTICLE+ILLUSTRATION]

PREVENTING EROSION OF FARM LANDS.

(Prepared by the United States Depart- • ment of Agriculture.) ‘ The many disastrous attempts to cultivate the narrow-base, level-ridge terrace on all types of soil have led to the development of a terrace*with a broader base, known as the broad-base level-ridge terrace. The broad-base embankment of earth provides the strength necessary to withstand the weight of the Impounded water above, and the terrace is built sufficiently high to hold all run-off water from the drainage area above the terrace. Figure 1 represents a cross section of two adjoining broad-base, levelridge terraces, with the various dimensions designated by letter. The vertical height of the terrace above the point c is represented by h; w is the width of the base of the terrace, d the horizontal distance, and v the vertical distance between terraces. These dimensions were obtained from surveys of eight fields representing the best practice in the use of this form of terrace.

Height of Terrace. From observation of field conditions, and a study of the data secured, It is believed that a broad-base, level-ridge terrace should be not less than 1% feet high and at least 10 feet broad at the base. Methods of plowing and cultivation should be adopted which will tend to Increase the base width from year to year and thus virtually transform the whole field into a series of terraces. Since the stability of a broad-base level-ridge terrace with closed ends depends upon its ability to retain the surface run-off water due to rainfall over the area between It and the next terrace above, it Is apparent that the reservoir capacity above the terrace must be sufficient to store this water. Upon' this principle is based the design of a system of broad-base, level-ridge terraces. From a general study of the rainfall records for the United States it is found that rainfalls exceeding 8 inches per 48 hours do not occur frequently in a given locality, and it is believed that provision for 8 inches of rainfall in the design of a system of terraces would give satisfactory results. To determine the proper vertical spacing for a system of terraces for any particular field it is necessary to know the average slope of the land surface and the approximate percentage of the rainfall that will percolate into the soil. The former can be measured readily by some forni of leveling instrument and the latter can be ascertained by a knowledge of the physical character, the humus content, and the tillage condition of the soil. The susceptibility of the subsoil to the percolation of water also is an Important factor to be considered in estimating the run-off. Rates of Percolation. It is by no means an easy matter to estimate the percentage of rainfall

that will run off for the’ various types and conditions of soils. For instance, the difference in the rates of percolation for day and sandy soils is very marked, the latter permitting a much higher rate than the former. Thlsis due to the fineness of the particles and the compact structure of the clay soils as compared with the open, porous structure and coarse particles of the sandy soils. The open structure of a soil facilitates the entrance and rapid circulation of both air and water, Since resistance to flow varies inversely as the size of the individual pore spaces. After a long dry period the pores in the upper layers of a soil become filled with air which, until it is expelled, tends to retard the entrance of soil water. A deeply plowed soil will absorb a greater percentage of rainfall than one where shallow ploa’ing is practiced, and the greater We amount of humus in a soil the greater will be Its capacity to absorb water. The rate of absorption after the top soil is saturated with water depends upon the permeability of the subsoil. A close. Impervious subsoil checks the rate of percolation and thereby increases the run-off at the surface. The water capacity of the top foot of farm land in good tilth has been stated to be 4or 5 Inches; thus a soil 12 inches deep could absorb this amount of-rainfall provided the rain is supplied to the surface at the same rate at which the soil is capable of receiving it. If the former rate is greater than the latter, the excess water runs off over the land surface with a velocity depending upon the slope. The steeper the slope the more rapid the run-off, and correspondingly less would be the time allowed for the absorption of water by the soil. Hence, the steeper the slope the greater will be the percentage of tbrj^ainfall flowing off. Reduce Height of Terrace. Were it not for the fact that the terraces would need to be placed very close together on steep slopes, thus necessitating a greater nufllber of terraces, it would be well to reduce the height of the terrace as the slope of the land increases. This would obviate the difficulty encountered in the construction of large terrace embankments on steep slopes. In the field investigations many terraces with closed ends were foundSome followed contours completely around ar-knoll or hilltop, forming a closed circuit with no outlet. But most of the level terraces examined had outlets at either one or both ends. In the foregoing discussion the terrace was taken as 1% feet high; with closed ends it would overflow for a rainfall in excess of 8 inches in 48 hours. How’ever, if one or both ends of a terrace be left open a liberal factor of safety against overflowing is provided. To provide a factor of safety for terraces with closed ends it is recommended that they be made about 1% feet high.

STEEP SLOPE CLEARED, NOW ERODING BADLY.

CROSS SECTION OF LEVEL-RIDGE TERRACES.