Natural Fertility of the Soil 3 



of these elements are required by all plants. There 

 are many plants which grow to maturity without 

 sodium, silicon and chlorin, but all of the others must 

 be present for normal growth. Carbon, hydrogen and 

 oxygen are also found in plants, but these elements are 

 secured from air and moisture. 



The number of soil constituents liable to rapid exhaus- 

 tion is limited in many cases to three, and at most to four, 

 which are nitrogen, phosphoric acid (phosphorus), 

 potash (potassium), and lime (calcium), the latter only 

 in exceptional cases. These are liable to be exhausted 

 because they exist in larger amounts than the others in 

 the plants that are grown, and in smaller amounts than 

 the others in even the most fertile soils. It has also been 

 proved that it is the one element of these which exists in 

 the smallest amount which measures the crop-producing 

 power, or fertility, in this respect, as one element cannot 

 substitute or exert the full functions of another. That is, 

 there may be a relative abundance in the soil of potash 

 and of phosphoric acid, but practically no nitrogen, in 

 which case good crops of cereals, for instance, could not 

 be grown, because no other element can substitute the 

 nitrogen required by the plant, and it can be obtained by 

 it from no other source than the soil ; and the soil, for all 

 practical purposes, is quite as unproductive, lacking in 

 productive fertility, as it would be if it contained much 

 smaller amounts of the mineral elements mentioned, and 

 thus be poorer in potential fertility. 



Fertility as influenced by water, climate and season. 



In the second place, there are soils that are so rich 

 in all of these elements that if productiveness depended 

 upon them alone, maximum crops might be grown for 



