206 ABSORPTION OF FOOD-MATERIALS 



of crops from the land end sooner or later in reducing such 

 land to a state in which it refuses to grow a remunerative crop 

 of any kind unless manures are applied to it. 



This more or less barren condition of land from which many 

 crops have been remoVed is explained by the fact that plants 

 lift into their bodies from the soil on which they grow a certain 

 amount of its constituents, and the removal of a crop therefore 

 means the removal of a considerable weight of the most im- 

 portant components of the soil : since the latter does not in 

 any case contain an unlimited supply of these plant food- 

 materials in a soluble and available form, it will be readily 

 understood that the continuous removal of crops from a field 

 must eventually lead to exhaustion, and that plants grown upon 

 it, would starve, unless a new supply of food-material is added 

 to take the place of that previously removed. 



It is true that the soil under such treatment does not become 

 so completely exhausted of its useful constituents that plants 

 altogether refuse to grow upon it, for soluble food-materials 

 are constantly being released or renewed from the store of 

 insoluble material composing the soil by the disintegrating 

 influence of frost and heat, and the chemical action of the 

 air and water upon it. Nevertheless, in this country, for the 

 production of a remunerative crop, the direct application of 

 manure containing food-materials or from which the latter can 

 be readily set free, is necessary in the case of most soils from 

 which two or three successive crops have been taken. 



Plants cannot grow unless they are supplied with all the 

 elements mentioned as essential on pp. 170 to 174 ; should one of 

 these be totally missing from the soil, growth becomes impossible. 

 From this peculiarity the power of the soil to yield a crop is 

 controlled by the essential element which is present in the least 

 amount. 



If a soil contains too small an amount of phosphates for 

 the growth of a crop, the fact that other elements such as 



