THE NATURE OF SOIL 31 



adhere to a single grain of soil is, of course, in- 

 finitely small, but the amount of water that can 

 cling to all the soil grains of a field is enormous, 

 especially when we consider the vast surface area 

 of the grains, in the aggregate. A good farm soil 

 often holds more than one-half its weight of film 

 water. 



Film water is far more important in farming 

 operations than free or bottom water, for it is the 

 direct supply of plants. No common farm crops 

 can thrive in free water, but all must have a large 

 area of soil that is moist with film water. Much of 

 this supply of film water, however, is drawn from 

 the natural reservoir of free water below. 



Water absorbed from the air. Under certain 

 conditions the soil absorbs a small amount of water 

 from the air. The air that fills the spaces between 

 the particles of soil usually contains much water 

 vapour; if the soil becomes very dry it may absorb 

 some of this. The surface soil may also absorb 

 water vapour from the air, especially when there 

 are heavy fogs. This " hydroscopic " water, how- 

 ever, is not of much importance as a means of 

 supplying plants with water, except in a time of 

 great drought. 



THE TEMPERATURE OF THE SOIL 



The soil must be warm in order to produce crops. 

 Most farm soils of the United States are not likely 

 to become too warm for ordinary crops; there is 

 far greater likelihood that they may be too cold. 

 This is especially true in the Northern States, where 

 the season is short, and it is very often desirable to 

 make the soil warmer, particularly in early spring. 

 The seeds of most cultivated plants will decay 



