110 



PLANT PHYSIOLOGY 



Fertilizers. — Of the essential plant elements, only three are 

 likely to be deficient in soils. These are nitrogen, potassium, and 

 phosphorus, although in a few cases, sulphur may also be lacking. 

 (Nitrogen does not occur in the ash but since it is taken into the 

 plant from the soil, it will be treated in the following discussion 

 with the ash constituents.) Commercial fertilizers, consequently, 

 generally contain compounds of nitrogen, potassium, and phos- 

 phorus, which, since they are liable to be lacking, are called the 

 critical elements. Iron, magnesium, calcium, and all the other 

 essential elements are generally present in sufficient quantities for 

 all nutritive purposes and may be neglected, although calcium 

 (Chap. XI) is often added to diminish the acidity of the soil, 

 i. e., for its balancing action. 



The kind of fertilizer used will depend upon the following 

 variable conditions: 



1. The nature of the crop plant. Various plants have different 

 chemical compositions and even the different organs of the same 

 plant have the mineral salts in different proportions. The follow- 

 ing table gives some conception of the varying composition of 

 plant organs (after Duggar and Molliard) : 



