12 THE FERTILITY OF THE SOIL [CH. 



We shall find the study of the soil very unsatisfying 

 and uninspiring if we become too much absorbed in 

 its utilitarian aspects and forget to stop and reflect 

 on the infinite wonder of its honeycombed structure 

 and its dark recesses, inhabited by a teeming popu- 

 lation so near to us and yet so hopelessly beyond our 

 ken that we can only form the dimmest picture of 

 what the inhabitants are like and how they live. 



CHAPTER II 



HOW PLANT FOOD IS MADE IN THE SOIL 



BY far the greater part of the food of the plant 

 comes from the atmosphere : oxygen, carbon dioxide 

 and water between them furnish most of the material 

 out of which the plant is built But it was discovered 

 long ago that something is taken from the soil, and 

 that this part, although small, is absolutely indis- 

 pensable to the growth of the plant. The food thus 

 furnished by the soil is really composed of a number 

 of substances, the most important of which are nitrates, 

 phosphates and other salts of potassium, calcium, 

 magnesium, sodium, etc. It is convenient to divide 

 these into two groups, the nitrogenous group, such 



