WHAT ARE PLANT FOODS? 145 



unnecessary losses of the essential elements from our soils, but there 

 must be some other reason or reasons than that for the good effects 

 obtained by fertilizer applications to fruit crops, especially on new 

 or very young lands. The most readily advanced reason, of course, 

 is that while the soil minerals contain the essential elements in 

 plenty, they are not "available." Let us now study the meaning of 

 this term "available" and see if the consideration of "availability" 

 is adequate to the explanation of the condition in question. 



"PLANT FOODS" AND "AVAILABLE PLANT FOODS" 



The term plant food is a misnomer. It is intended to apply to 

 the essential chemical elements above mentioned, which enter into 

 the composition of plant food, and which is a term that should apply 

 only to the starches, sugars, proteins, and fats which really serve 

 to sustain the plant and which are products of the plant's own 

 activity. The term is, therefore, used incorrectly by most people 

 and should be supplanted by the term "essential elements" to plant 

 growth. 



With this idea clearly in mind, we may next inquire what is 

 meant by the expression "available plant food." Here again the 

 term plant food is used erroneously, as already explained. But the 

 term "available," as commonly used, is intended to mean that the 

 substance to which it is applied is soluble in the water of the soil 

 when introduced there. Recent studies in the chemistry of the soil 

 have revealed the fact, however, that availability of a chemical 

 element or compound from the standpoint of the plant, as well as 

 that of the soil, is not merely the simple question of the solubility 

 of that substance in the soil water, but of something more. A sub- 

 stance, to be available to the plant's roots, must not only be soluble 

 in the soil water, but it must be so balanced with the other con- 

 stituents of the soil water (soil solution) as to be assimilable, which 

 is by no means always true under soil conditions. Moreover, a 

 substance to be "available" in the broadest sense must not only 

 possess the two attributes just discussed, but, in addition, should 

 not be poisonous at the concentration at which it is found in the 

 soil solution. 



It follows from what has just been said that a fertilizer salt or 

 other substance does not necessarily become usable and available 

 to the plant merely because it dissolves in the soil water. It may 

 indeed dissolve in the soil water when first applied to the soil, but 

 soon thereafter become insoluble by reacting with some soil mineral 

 and from such reaction a new soluble substance may be formed and 

 a new insoluble substance, which latter contains in it the essential 

 element which it is desired to make available. This leads us to 

 conclude that the application to a soil of a soluble fertilizer salt like 

 nitrate of soda or sulphate of potash does not by any means insure 

 the increased supply thereof by that amount in the soil solution. 

 On the contrary, it may mean no increase of that substance at all 

 in the soil solution available to the plant's roots. This important 

 fact has been brought out by an investigation recently conducted in 



