PRODUCTIVE POWER OF EACH SOIL VARIES. 129 



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 like climatic conditions, 

 certain fields are suited for certain 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 requirements 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 

 productive power corresponding to the nutritive sub- 

 stances 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 ; the 

 ideal maximum is what might possibly be obtained if 

 the rest of the nutritive substances, which are in chem- 

 ical combination, were converted 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 adopting a proper system of rotation, and in using 

 all the means at his command to make the nutritive 

 elements in chemical combination available for plants. 



The achievements of practical agriculture in these 

 respects are wonderful, and they demonstrate that the 

 triumphs of art far exceed those of science, and that 

 the farmer, by aiding the agencies which improve the 

 chemical and physical condition of his land, can obtain 

 much more abundant crops than by supplying nutritive 

 matters. Because, what he can supply in the shape of 

 manure, with due regard to a proper return, is so small 

 in comparison with the store of nutritive matter con- 



