TRODUCTIVE POWER OF EACH SOIL VARIES. 125 



the same means produce lialf as much again as an average 

 crop. Admitting this might be readily accompHshed, 

 experimentally, on a small area, yet the price of phos- 

 phoric acid, potash, or even of soluble sihca, and the 

 impossibihty of procuring them for a large number of 

 fields, though in a given field only one of these sub- 

 stances had to be increased in the proportion stated, 

 would oppose insuperable obstacles to the conversion or 

 improvement of land. 



The law of the immobility of the mineral elements in 

 the soil explains the agricultural experience of ages, that 

 almost universally, under hke climatic conditions, certain 

 fields are suited for certahi plants only, and that no plant 

 can be profitably cultivated upon a soil, unless the mineral 

 contents of the soil are in proportion to the special re- 

 quirements of that plant. 



In practice, it is quite impossible, by a supply of mineral 

 substances, to improve the land of an entire country, so 

 that it shall yield crops considerably more abundant than 

 the natural store of food elements in the soil enables it to 

 produce. 



Every field has a real and an ideal maximum of pro- 

 ductive power corresponding to the nutritive substances 

 which it contains. Under the most favourable cosmical 

 conditions, the real maximum corresponds to that portion 

 of the total amount of nutritive elements, which is present 

 in the soil in an available form, i. e. in a state of physical 

 combination with the soil ; tlie ideal maximum is what 

 might possibly be obtained if the rest of the nutritive sub- 

 stances, which are in chemical combination, were con- 

 verted into an available form, and distributed through 

 the soil. 



Hence, the art of the agriculturist mainly consists in 

 selecting such plants as will thrive best on his land, in 



