RETAINING MOISTURE BY TILLAGE 129 



exists, must be prevented. This is accomplished by the fine loose earth 

 which acts as a mulch. When this is attained, only that moisture in the 

 upper surface which comes in immediate contact with the air is evapor- 

 ated, and the balance is retained for the use of the plant. Plants 

 growing, then, in a well-cultivated soil, have the water in the lower 

 soil held for their use, and as fast as they use it the supply is replaced 

 through the firm soil below, which evaporation being stopped, remains 

 moist and permeable by roots which extend freely, seeking the nourish- 

 ment they need. 



Such is a brief outline of the theory which explains the results 

 gained by thorough cultivation of the soil, so far, at least, as retention 

 of moisture is concerned. The practical demonstration is easy. Go 

 into a well-cultivated orchard or vineyard, push aside the soil with the 

 foot, and moisture will be found two or three inches from the surface, 

 or even less in some soils, while on uncultivated land adjacent, digging 

 to the depth of several feet will show nothing but hard earth, baked 

 and arid. In such hard-baked earth, moreover, the sun heat is conveyed 

 or conducted downward very rapidly during a hot day, so that in some 

 cases the roots are seriously injured. When the surface is well tilled, 

 it will act like a blanket, preventing a too rapid conveyance of heat 

 downward, and thus also diminishing the intensity of evaporation. 



Accurate demonstration of these facts has been secured as the 

 result of many moisture determinations in cultivated and uncultivated 

 soil by the University of California Agricultural Experiment Station.* 

 Very striking exhibition of the condition of trees with and without 

 cultivation is found in the engravings which are reproduced herewith. 

 Upon the demonstration, the practice in the uncultivated orchard was 

 radically changed. The exact determination of moisture present at 

 various depths of the soil beneath these contrasted orchards in the 

 month of July is as follows : 



Totals, six feet 6.3 756 4.2 512 



This shows a gain of nearly fifty per cent of soil moisture by cultiva- 

 tion. 



Necessity of Adequate Cultivation. It has been very fully dem- 

 onstrated by California experience that adequate depth of tilth must 

 be attained. The depth of cultivation, or the thickness of the dust- 

 mulch, as some like to call it, must be sufficient to prevent the access 

 of the dry air to the firm soil below. At the East, where they have a 

 moister air a thin mulch may answer, but in California, with a thirsty 

 air for such a protracted period, there must be deeper tilth. Two or 



*Bulletin 121. 



