THE DUST MULCH IN MINNESOTA. 465 



from the leaves on which it collects without touching the ground at 

 all and from them is returned to the air again. 



A matter of the greatest importance to the farmer and horti- 

 culturist is the fact that for each pound of dry matter contained in 

 the vegetation grown upon a field or garden four hundred pounds 

 of water, on an average, have been evaporated from the foliage of 

 the plants or weeds, which have pumped it up from the moist earth 

 only to breathe it out again from the leaves. 



To the foregoing must be added the direct evaporation from 

 the surface. This varies greatly with varying conditions. In a fine- 

 ly comminuted soil underlaid with clay, where the pores have not 

 been disturbed and capillarity is well established, it may be nearly 

 equal to the entire rainfall, and the land in a dry year be worthless 

 on that account. In a claye}^ loam, containing one-fourth or less 

 of the amount of moisture required to bring it to the point of satura- 

 tion and receiving tlie most favorable tillage', direct evaporation may 

 be less than one-tenth of the precipitation. 



There is another way by which a part of the rainfall disappears 

 from the soil and fails to be of benefit to the crops. I refer to the 

 downward and lateral movement by percolation. This, however, 

 amounts to but little except where the soil is almost pure sand or 

 gravel with no impermeable understructum within a long distance 

 of the surface. In loams or clays the movement is exceedingly 

 slow and, following, as it does, the general slope of the land, it takes 

 a long time to get beyond the roots of the plants or out of the reach 

 of capillary attraction. 



But this attraction has a tendency to bring the moisture to the 

 surface to be carried away by the air. It must be remembered that 

 by the action of this force water may move downward or laterally 

 as well as upward. It tends from the wettest parts towards the 

 driest ones. Whenever the surface by reason of evaporation be- 

 comes drier than the earth several feet below, the subterraneous 

 water begins to creep upwards through the minute pores of the 

 ground till it reaches the top, whence it is spirited away by the ab- 

 sorption of the atmosphere. This is not only a dead loss of so much 

 life giving water, but it also to a considerable degree cools the soil 

 and retards the growth of the crops, which is equivalent to shorten- 

 ing the season. 



Now we are brought face to face with the great problem which 

 every tiller of the soil, in order to achieve the highest success, must 

 solve. How shall the vast amount of water now carried ofif by 

 evaporation, and worse than wasted, be retained in the soil and made 

 to subserve some useful purpose? As it is now, for the most, the 



