THE IRRIGATION AGE. 155 



bulletin, to account for the observed difference in the cultural result. 

 The cause of this difference was that in the uncultivated field there was 

 a compacted surface layer several inches in thickness, which forcibly 

 abstracted the moisture from the substrata and evaporated it from its 

 surfate; while the loose surface soil on the cultivated ground was un- 

 able to take any moisture from the denser subsoil. This is well illus- 

 trated by the familiar fact that while a dry brick will suck a wet 

 sponge dry, a dry sponge (corresponding to the loose suface soil) is 

 unable to take any water from a wet brick. Besides, the tilled sur- 

 face soil forms a non-conducting layer protecting the subsoil from the 

 sun's heat and the dryness of the air. 



In the East, where this principle is well understood, it is considered 

 that a surface layer three inches in thickness is sufficient to afford ef- 

 fective protection. But what Is adequate in the region of summer 

 rains is quite insufficient in California and in the arid region generally. 

 It takes fully twice the thickness mentioned, and preferably more, to 

 afford protection against the drought and heat lasting five or six 

 months at a stretch. Here again we find an important point in which 

 our practice must differ from that of the East and of the Old World. 



The beneficial effects of summer fallow in California are assuredly 

 due quite as much to the conservation of moisture brought about by 

 the tilled surface layer, as by the weathering of the soil to which the 

 efficacy of the fallow is commonly ascribed. Witness the fact that 

 weeds come up freely on summer-fallow as late as August, when un- 

 plowed land is as bare as a barn floor. 



Similarly on our mostly new and unexhausted lands, the bad effects 

 of weed growth are doubtless due fully as much to the waste of mois- 

 ture going on through their leaves as to the competition with the crop 

 plant food. Hence all good orchardists are very careful about keep- 

 ing their ground clean in summer; but it must not be forgotten that 

 by doing so they quickly deplete their lands of vegetable matter, 

 which requires systematic replacement if production is to continue 

 normally. Yet of the two evils, the loss of moisture is more to be 

 dreaded, and very generally in practice the more difficult to remedy. 



