156 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



composition of the soil. The quantity and character of the sub- 

 stances brought into solution by means of various solvents, may 

 be compared with the effect of the same solvents under the same 

 conditions upon minerals in the proportions in which they may 

 occur in soils. For example, 1 known phosphates of lime are com- 

 pletely dissolved by fifth normal nitric acid under certain condi- 

 tions ; some phosphates of iron and aluminium are also completely 

 dissolved, while basic phosphates of iron and aluminium are dis- 

 solved by the solvent only to a slight extent. Hence treatment of 

 the soil with this solvent gives an idea as to the condition of the 

 inorganic soil phosphates. This matter will be discussed later. 

 Hilgard 2 determined the alumina dissolved and the silica which 

 was liberated by strong acids, and by comparing the relative 

 quantities, came to the conclusion that the quantity of silica was 

 not sufficient to combine with the alumina to form known silicates, 

 and that a portion of the alumina in certain soils is present as 

 hydrated alumina, probably Gibbsite A1(OH) 3 . 



Organic Matter. The organic matter and nitrogen of the soil 

 are closely related, for nitrogen is chiefly in organic combination. 

 The organic matter of the soil consists of the residues of plants 

 and animals, animal excremenls, and the products of their decay. 

 All the compounds which are found in plants or animals, enter the 

 soil, through the presence of some of them is very transitory. 3 

 Sugars, urea, and similar substances, are rapidly changed into 

 other bodies. Cellulose and lignin, which make up the woody 

 matter of plants, decay much more slowly, and may remain in the 

 soil for some time. Lactic, acetic, and butyric acids are pro- 

 duced in the fermentation of sugars and starches. Vegetable 

 acids, such as oxalic, citric, tartaric, and malic acid are introduced 

 into the soil in plant residues, but are quickly destroyed by 

 bacteria. Proteids and fats persist for a longer or shorter time, 

 according to their nature. It is thus possible for all the organic 

 compounds found in plant or animal residues to be present, in 



1 Texas Station Bulletin 126. 



2 The Soil, p. 389. 



8 Wollny, Die Zersetzung d Org. Stoffe. 



