4 CHEMISTRY OF PLANT LIFE 



form before they can be utilized by plants. Obviously, therefore, 

 only those compounds of these elements in the soil which are sol- 

 uble in the soil water are available as plant food. The greater 

 proportion of the soil elements are present there in the form of 

 compounds which are so slightly soluble in water as to be unavail- 

 able to plants. The processes by which these practically insoluble 

 compounds become gradually changed into soluble forms are 

 chiefly the " weathering " action of air and water (particularly 

 if the latter contains carbonic acid) and the action of the organic 

 acids resulting from decaying animal or vegetable matter or 

 secreted by living plants. 



THE VALUE OF THE SOIL ELEMENTS AS PLANT FOOD 



Analyses of the tissues of plants show that they contain all of 

 the elements that are to be found in the soil on which they grew. 

 Any of these elements which are present in the soil in soluble form 

 are carried into the plants with the soil water in which they are 

 dissolved, whether they are needed by the plant for its nutrition 

 or not. But in the case of those elements which are not taken 

 out of the sap to be used by the plant cells in their activities, the 

 total amount taken from the soil is much less than is that of the 

 elements which are used in the synthetic processes of the plant. 

 Hence, much larger proportions of some elements than of others 

 are taken from the soil by plants. The proportions of the dif- 

 ferent elements which are used by plants as raw materials for the 

 manufacture of the products needed for their growth varies with 

 the different species; but a certain amount of each of the so-called 

 " essential elements " (see below) is necessary to every plant, 

 because each such element has a definite role which it performs 

 in the plant's growth. A plant cannot grow to maturity unless 

 a sufficient supply of each essential element comes to it from the 

 soil. 



From the standpoint of their relative value as raw materials 

 for plant food, the elements which are present in the soil may be 

 divided into three classes; namely, the non-essential, the essential 

 and abundant, and the critical elements. 



The first class includes silicon, aluminium, sodium, manganese, 

 and certain other rarer elements which sometimes are found in 

 soils of some special type, or unusual origin These elements seem 



