28 THE PRINCIPLES OF 



the roots of higher plants penetrate and erode the surface, 

 and the result is the breaking down of the continuity of 

 the substance, facilitating the action of further destructive 

 agencies. 



Neither is this all. The presence of these forms of vege- 

 tation fosters a condition of moisture favourable to decay. 

 And further, as time goes on, and generations of plants 

 succeed each other, we have the death and decay of plants 

 with the evolution of carbon dioxide, or carbonic acid gas. 

 This gas enters into solution with rain-water, and helps still 

 further to dissolve out the lime and the alkalies, and thus we 

 have in vegetation itself a very potent force in bringing about 

 the gradual formation of that loose material which we call soil. 



This concludes what I have to say upon those forces which 

 have during long ages operated in the production of soils. 



I propose, in the next place, to deal with what we may 

 call the proximate constituents of soils, the word proximate 

 being used in contradistinction to ultimate. I am not going 

 to speak of the ultimate chemical or elementary constituents 

 of soils, but of those substances or groups of substances which 

 together make up the entire bulk of every soil. 



In the first place, let us turn our attention very briefly to 

 the soluble portion that is, the portion soluble in water 

 and clearly understand that this is the true available mineral 

 plant food, the presence of which is of vast importance as 

 a cardinal element of fertility. There are some nine sub- 

 stances which together form what are called the mineral or 

 ashy constituents of plants. They are readily enumerated, 

 but space will not allow me to do more than name them. 

 They are worthy of our closest attention, and should form 

 an important item in any instruction given upon agricultural 

 chemistry in lectures or in text-books. They constitute 

 the available mineral plant food of crops, and are constantly 

 being drawn from the soil, especially by crops which are 



