_Moisture-requirements of Crops in the Arid Region._—Plants (particularly broad-leaved ones) which have made a brash growth during a period of abundant moisture, will wilt quickly when sunshine returns, and take some time to adapt themselves to the drier conditions. On the other hand, plants accustomed to dry air and scanty soil-moisture, will not wilt or suffer under what would elsewhere be considered very rigorous conditions. Loughridge[80] has made numerous determinations of moisture in soils in which crops were beginning to suffer, and others on similar soils that were growing normally, and found that in general, not only were the differences in moisture content considerably less than in the case above quoted from King’s observations, but that the amounts of free moisture required by various crops in the arid climate of California were surprisingly small. [80] Rept. Cal. Expt. Sta. 1897-08, pp. 65-96. The tables below show the results of observations made by Loughridge during several drought years in California; so arranged as to show the differences of moisture content for the same crop in different soils. It will be observed that in all cases where a crop growing on a clay soil could be compared with the same on a lighter soil, the moisture required to keep the crop in good condition was very much greater in the clay than in the loam or sandy soils. In the case of apples, _e. g._, 8.3% of water was abundant to keep the trees in excellent condition on a loam soil, while on a clay soil holding 12.3% the condition was very poor. That this difference is due in the main to the difference in the hygroscopic-moisture coefficient of the respective soils, is plainly apparent in several cases. It is therefore not the _total_ moisture content, but the free moisture present in excess of what is held by hygroscopic absorption, that determines the welfare of the plant. By determining, first, the total moisture in the soils, as taken in the field, then, after allowing them to become air-dry, determining the maximum of hygroscopic moisture they would absorb (see p. 198), Loughridge found by difference the amount of free moisture, or liquid water which must be present in the soil to prevent the crops from suffering. An exceptionally good opportunity for these observations was offered by the dry season of 1898, during which crops suffering and not suffering, on identical lands, could easily be found. The determinations were always made for each foot of the upper four feet of the land in the immediate neighborhood of the trees or among the field crops. The first table exemplifies the method of procedure; the second gives the summary of results for the several crops and trees, as calculated from observations made during the season.