SOILS. 19 



suggesting the fact now well understood that it is not 

 so much the quantity of a needed element in the soil 

 which is required as its availability to plants. The 

 conditions induced by proper tilth moisture, and the use 

 of such fertilizers as will by chemical combinations re- 

 lease and put in proper condition for plant assimilation 

 the otherwise inert elements of the soil, are matters 

 determined by experiment. 



Drainage Properties of Soils and Subsoils, 



Soil is the surface land that is cultivated and which 

 produces vegetable growth. In general terms, it is the 

 surface stratum of earth. 



Subsoil is the stratum of earth upon which the soil 

 rests. 



The dividing line between the two is not clearly 

 marked as a rule, so that the terms are usually under- 

 stood to apply respectively to that depth of surface land 

 which may be cultivated, and the layer immediately be- 

 neath. Soils and subsoils are of almost every con- 

 ceivable color, composition, and physical structure. 

 Their treatment for the growth of plants and for the 

 support of important engineering and architectural 

 structures, and their investigation respecting their re- 

 lation to human diseases, engaged the attention of men 

 of thought for ages. Men and animals move upon 

 them, plants derive their nutriment from them, and 

 water is stored among their particles. The soil is a 

 great laboratory wherein is developed and from which 

 is dispensed the Creator's supplies for man's temporal 

 wants. 



