30 SOILS 



times during the year, according to the dryness of 

 the season. 



We must consider, then, that beneath all farm 

 soils, at some depth, is standing water; that we 

 plow and harrow above subterranean lakes, which 

 are no less lakes because the water is not entirely 

 free but merely fills the spaces between the particles 

 of soil. The importance of this fact lies in its in- 

 fluences upon the production of a crop. If it is 

 only two or three feet from the top of the soil to the 

 surface of the lake, there is not enough dry soil 

 on top for roots to grow in and the plants drown. 

 Such soils are said to be shallow; they are of little 

 value for ordinary farm crops until ditched or 

 under-drained and the level of the underground 

 lake lowered thereby. The draining of land 

 is considered in detail in Chapter IX. 



Film Water. Water is also present in all farm 

 soils as film moisture. Above the water table is 

 the soil in which the roots of farm crops forage. 

 This soil must be moist, else plants would not grow 

 in it ; but water does not fill all the spaces between 

 the soil grains, as it does below the water table. 

 If we look at a handful of this soil we cannot see 

 water standing in it, but it feels moist. The water 

 is sticking to the soil grains, covering them with a 

 very thin film, as when small stones are dipped in 

 water. It is held close to the grains by surface 

 tension, or adhesion. If this soil were put in an 

 oven and heated, the film water would be driven 

 off as water vapour, and the soil would be left per- 

 fectly dry. 



There is always a large amount of film water 

 clinging to the grains of every soil, even in the dryest 

 season. The dryest road dust has some film water 

 clinging to it. The amount of water that can 



