316 THE SOIL. 



If an inch away from them there is a mass of rich food, they 

 cannot make use of it save by sending out new shoots to 

 embrace it but must remain content with the poorer tract 

 in which they lie. Consequently, the uniform distribution. 

 of the plant food, its solubility, and its exposure on the sur- 

 faces of the particles of the soil are quite as important as 

 its quantity. 



Chemical analysis teaches us none of those things at 

 least it does not teach them so definitely as we would need 

 to know them to be able to make any practical use of its 

 assistance. 



In addition to these, fertile soils must also contain carbon, 

 oxygen, nitrogen, and hydrogen, which are called the organic 

 parts of soils, from their great preponderance in vegetables 

 and animals, of which they constitute from 90 to over 99 

 per cent. 



General results of analytical examinations of soils. 



1. A due admixture of organic matter is favorable to the 

 fertility of a soil. 



2. This organic matter is the more valuable in proportion 

 to the quantity of nitrogen it holds in combination. 



3. The mineral part of the soil must contain all those 

 substances which are met with in the ash of the plant, and 

 in such a state of chemical combination that the roots of 

 plants can readily take them up in the requisite propor- 

 tions. 



