4 PLANT PRODUCTS 



There will also be the need to consider a classification 

 which is mainly physical as follows : — 



1. Cementive or binding. 



2. Opening or aerating. 



And lastly we may have to consider fertilizers from a dy- 

 namic, rather than a static point of view, as in the following : — 



1. Lasting. 



2. Readily available to the plant. 



3. Soluble in water and easily diffusible. 



4. Stimulating and only suitable for top dressings. 



5. Reactive, i.e. those that induce chemical or biological 

 activity in the soil. 



The purely chemical classification, depending as it does 

 upon the most important chemical element present, is com- 

 paratively simple and devoid of ambiguity. In practice it is 

 not quite so simple as it looks. Later we shall have to discuss 

 cases where the use of a manure dependent for its value on 

 one element produces ultimate effects which are best 

 measured in terms of another element. Also in many cases 

 the fertilizers are compound and contain more than one 

 element of value. 



The physical classification demands a knowledge of the 

 soil to which the fertilizer is applied. But the ultimate 

 physical effects resulting from the applications of the 

 fertilizers are of a very varied kind, some even tending to 

 destroy completely the proper physical condition of the soil 

 unless some remedial measures are employed. 



The power of a fertilizer to act quickly or slowly is a very 

 important property. In some cases a rapid effect is desirable. 

 For example, when a fertilizer is used as a top-dressing it 

 must always be soluble, otherwise the action would be too 

 slow. The case of applying such a fertilizer as dung to the 

 surface of a permanent pasture might be considered a case 

 of top-dressing, but this term is usually applied to the use of 

 a soluble manure on a hay or corn crop when in fairly full 

 growth, under which circumstance quick action is necessary. 

 When a fertilizer is applied in the winter or period of little 

 growth, a much less degree of solubility will suffice and it 



