112 RELATIONS OF SOIL TO WATER 



growing crop is generally moist on the surface. This main- 

 tenance of a moist surface is of importance to fertility in 

 many ways ; it is only under this condition that the nitrifying 

 and other organisms can discharge their functions actively. 



Protection from wind by means of hedges diminishes the 

 rate of evaporation. King (Wisconsin nth Rep.> 309) deter- 

 mined the rate of evaporation from a known surface of filter 

 paper, constantly supplied with water (Piche's evapororneter), 

 at various distances from a hedgerow, and other forms of 

 shelter. The evaporometer was placed i foot above the 

 ground. The trials were made on different days, mostly in 

 bright sunshiny weather. The following figures show the 

 number of cubic centimetres of water evaporated in the same 

 time at various distances from shelter. 



Water evaporated at various distances from shelter 



The effect of the Oak Grove was thus felt at a distance of 

 more than 200 ft., but beyond 300 ft. its influence ceased. 

 The influence of the scanty hedge, and of the clover field, was 

 clearly felt at distances exceeding 150 ft. The effects of 

 shelter, even of so low a kind as that afforded by growing 

 clover, are clearly considerable ; they are due partly to the 

 diminished velocity of the wind at the surface of the soil, and 

 partly to the fact that the wind is less dry after passing 

 through the shrubs and plants forming the shelter. On both 

 these points King has made experiments. 



The evaporation from the soil may be considerably dimin- 



