EVAPORATION FROM BARE SOIL 117 



to its less permeability to air. For a time then the rolled soil 

 is moister at the surface than one left rough, and this fact is 

 made use of by the farmer when sowing seeds in spring or 

 summer. 



King, when sowing oats and barley broadcast, and then 

 harrowing, found that on the portions of the land rolled a 

 greater number of seeds germinated, and that germination 

 was quicker than on the unrolled land. When however the 

 same seeds were drilled, and thus buried deeper in the ground, 

 a subsequent rolling was without advantage. Rolling is 

 undoubtedly often of groat use in aiding the germination of 

 turnip seed in a dry summer. A consolidated condition of the 

 surface cannot however be usefully maintained. We have 

 already seen that the amount of evaporation is in the long 

 run greater in summer time from a consolidated soil, than 

 from one covered by a thin layer of loose earth. This is due 

 in part to the greater velocity of the wind over a smooth 

 surface, and in part to the more continuous supply of water 

 at the surface in a consolidated soil. There are also other 

 distinct advantages from a loose surface. The farmer is thus 

 adopting the best plan when he hoes the land as soon as the 

 turnip plants are sufficiently grown. The hoeing is not 

 effective merely because the plants are thinned and the weeds 

 destroyed, its benefit is partly due to the renewal of the loose 

 condition of the surface soil. 



The effect of saline matter in the soil, or of additions of 

 saline manures, on the rate of evaporation, must in some 

 cases be considerable. In the experiments of Johnson and 

 Armsby, and of King, already quoted (pp. 97, 101), the for- 

 mation of a slight crust of salt on the surface greatly lessened 

 the evaporation, presumably by choking the interstices of the 



