4 Fertilizers 



centuries without exhausting them, while actually they 

 are now incapable of producing a single profitable crop 

 of cereals, grasses, fruits or other products of the farm, 

 because certain other conditions which are essential, in 

 order to bring them into activity, are absent. For ex- 

 ample, it may be that water, which is absolutely essen- 

 tial both for the solution of these food elements in the 

 soil, as well as for their distribution in the plant after they 

 have been acquired, cannot be obtained, or that the tem- 

 perature of the soil and of the surrounding air is either 

 too low or too high, thus preventing or interrupting the 

 progress of those changes which must go on, both in the 

 soil and in the plant, in order that normal growth and 

 development may be accomplished. With a full supply of 

 the fertility elements in the soil, the climatic and seasonal 

 conditions exert an important influence upon its produc- 

 tive power. 



It is evident, therefore, that the chemical elements of 

 fertility in themselves are not sufficient to constitute what 

 we understand by the term. Fertility is not measured 

 by them alone ; associated with them there must be other 

 conditions. That is, while crops cannot be grown with- 

 out these elements, it is the conditions which surround 

 them that, in a large degree, determine the power of the 

 crop to secure them. 



The influence of physical character of soil. 



In the third place, the physical character of a soil is 

 also a factor in determining actual fertility. This has 

 reference, first, to the original character of the rocks from 

 which the soil particles were derived, whether hard and 

 dense in their mineral character, thus resisting the pene- 

 tration and the solvent effect of air and water and other 



