88 SOILS 



way : The minute root hairs are always absorbing 

 water, together with the plant food that is dissolved 

 in it; not free water, but the film water clinging to 

 the grains of soil. The soil grains which thus pay 

 tribute to the plants become dry. But they touch 

 grains that are not in direct contact with the plant 

 pump; part of the film moisture clinging to these 

 is passed along to the dry grains, so tnat both be- 

 come equally moist. Now the grains a little further 

 off have more moisture than these which have given 

 a part of theirs to the dry grains in the grasp of the 

 root hairs. These, likewise, give of their abun- 

 dance to the soil grains less favoured. So it comes 

 about that there is always a steady current of film 

 water passing to every root hair of every thirsty, 

 growing plant; not flowing through the soil, but 

 creeping from particle to particle, and space to 

 space. 



In exactly the same way there is always a cur- 

 rent of film water passing upward on every 

 summer day to replace the water that the upper- 

 most soil grains have lost by evaporation. The 

 amount of water lost from common farm soils 

 by evaporation may be as much as five inches a 

 month during the summer. There must be in- 

 equalities in the dryness of the soil that are due 

 to other causes, as difference in texture or com- 

 position; but for the most part we may think of 

 this great volume of film water, equal to a layer 

 of water over fifteen inches deep in the first five 

 feet of some soils, as settling strongly in two currents 

 toward the surface, to replace the loss of water 

 by evaporation, and toward the roots of plants. 

 These invisible currents are not affected by the 

 law of gravitation; they travel up, down, or sidewise 

 in the endeavour to make the soil equally moist 



