CULTURE OF RIAIZE 241 



usual period. Aside from its influence in destroying weeds, 

 which appears to be the main purpose of all intercultural tillage, 

 a moderate amount of stirring to a depth which will not seriously 

 injure the roots appears, therefore, to be somewhat beneficial. 



If two inches of cut straw are spread upon the surface of the 

 soil, the evaporation of water from the soil will be checked. If 

 the surface of the soil is sheltered from rain, but exposed to the 

 sun, and at the same time stirred to the depth, say, of two inches, 

 the stirred portion rapidly becomes dry, and when in this condi- 

 tion acts as a mulch to the soil below, although not as effectively 

 as does the cut straw. Under the above conditions the author 

 has checked evaporation of water from the soil equal to one- 

 fourth of an inch of rainfall per week. 



When, however, the soil is exposed to the usual atmospheric 

 conditions of the humid regions, where it rains one day in three, 

 it becomes a question whether the evaporation from the lower 

 •soil is checked more than that from the stirred portion is increased. 

 The author has concluded from pot experiments conducted 

 during three seasons that for humid regions during ordinary 

 weather these two factors were nearly equal, although during 

 rainy weather the evaporation may be increased and during 

 periods of severe drouth it may be decreased by constant surface 

 stirring of the soil, both of which may be desirable. The average 

 evaporation in inches of rainfall from two pots treated as indi. 

 cated below was determined for eighty-six days during the months 

 of June, July, August and September at the Pennsylvania Station : 



Evaporation in Inches of Rainfall During Eighty-Six Days. 



Water . . . . . , . 19.8 



Bare soil not cultivated . • • • 16.0 



Bare soil cultivated 16.7 



One maize plant, soil not cultivated . . 19.3 



One maize plant, soil cultivated . . . 18.8 



One maize plant, soil mulched with cut straw 10.6 



Oats followed by oat stubble , . . 37.7 



