THE SOIL. 29 



time only a relatively small quantity of it can be taken 

 by the plant. Thus in a soil containing QI2% of nitrogen 

 only 1 "50 % of this total may be available. It is the confusion 

 between total and available plant food which renders the 

 chemical analysis of the soil so very undependable. Thus 

 an analysis may show in the soil a great abundance of 

 phosphoric acid, and nevertheless the soil may be infertile 

 through lack of this plant food in an available form. In 

 short there is practically no reliable method of analysing 

 the soil to ascertain the amount of plant food in the soil 

 which can be taken by a crop. Again, the quantity of 

 available plant food in the soil at one time gives no clue 

 as to the amount which will become available, and which 

 the crop can take, during the period of its growth. Even 

 if the soil could be satisfactorily analysed, and the presence 

 of more than sufficient plant food be detected, yet the soil 

 may be infertile ; the soil may be acid, or salt, or the 

 physical properties may be unsuited for plant growth. 

 On the other hand chemical analysis may detect the pre- 

 sence of salts, acids and the absence of any plant food. 



A fertile soil must contain the essential plant foods in 

 an available form, and no excess of one or more foods will 

 compensate for the deficiency of any other of the foods. 

 In the words of the Law of Minimum, the fertility of 

 a soil, i.e., the size of the crop it can yield, depends on that 

 plant food which is present in available form in relatively 

 least quantity. Of the various compounds required by 

 the growing plant those of nitrogen, phosphorus and, less 

 often, of potassium are given special importance by their 

 relative scarcity. The other plant foods calcium, magnes- 



