The failure to appreciate this necessary difference often leads to heavy losses on the part of newcomers to the arid region, who in this as in other respects are apt to follow blindly the precepts familiar to them in the East, until taught better by sore experience. In the East and Middle West a depth of three inches is considered the proper one for the protective surface-layer; and in the case of maize even this is considered excessive in many cases. In the arid region this depth should be at least doubled where irrigation is not practiced at least every four to six weeks; and in some sandy soils even seven and eight inches is not too much for effective protection. _Illustrations of Effects of Surface Tillage._—The efficacy of loose surface tilth in preventing evaporation, as compared with mere superficial scratching or with the total omission of cultivation, is well exemplified in a series of investigations conducted on this subject during the extremely dry season of 1898, by the California Experiment Station; the seasonal rainfall having during that year been on an average from one-third to one-half only of the usual amount, so as to test to the utmost the endurance of all growing plants. Some of the details of this investigation have been given above (p. 214) in connection with the question of moisture requirements of crops. Loughridge[97] also investigated the moisture conditions in adjacent orchards differently treated in cultivation. In one of these cases two orchards of apricots were separated only by a lane, and the soil identical; but one owner had omitted cultivation, while the other had cultivated to an extra depth in view of the dry season apparently impending. The results are best shown by the plates below, showing representative trees and the annual growth made by each. The table annexed shows the differences in the moisture-content of the two fields to the depth of six feet, in July: MOISTURE IN CULTIVATED AND UNCULTIVATED LAND. ===================+=====================+================== | Cultivated. | Uncultivated. Depth in soil. +------------+--------+---------+-------- | Per cent. |Tons per|Per cent.|Tons per | | acre. | | acre. -------------------+------------+--------+---------+-------- First foot | 6.4 | 128 | 4.3 | 86 Second foot | 5.8 | 116 | 4.4 | 88 Third foot | 6.4 | 128 | 3.9 | 78 Fourth foot | 6.5 | 130 | 5.1 | 100 Fifth foot | 6.7 | 134 | 3.4 | 68 Sixth foot | 6.0 | 120 | 4.5 | 90 | --- | --- | --- | --- Total for six foot | 6.3 | 756 | 4.2 | 512 -------------------+------------+--------+---------+-------- [97] Rep. Calif. Expt. Sta. for 1897-98, p. 65.