III. THE SOIL 43* 



the type of humic acid and also carbon dioxide, which is always found 

 im large proportion in the air of a soil. These acids have powerful 

 solvent properties. 



5. Bacteria. As will be discussed hereafter, ordinary vegetation re- 

 quires the presence of nitrogenous organic matter, "humus," in the 

 soil, and since the main source of this organic matter is the remains of 

 previous plants, the question naturally suggests itself What is the first 

 source of organic matter in the mineral debris resulting from the dis- 

 integration of rocks ? According to the observations of Miintz l the bare 

 surfaces of the rocks, even near the summits of mountains, yield large 

 numbers of nitrifying and other organisms which are able to withstand 

 the lowest temperatures. Direct experiments showed that these or- 

 ganisms, with no other food than the mineral matter of the rocks and 

 small quantities of ammonia and alcohol vapour in the moist atmosphere 

 around, were able to live and produce nitrates, also to accumulate car- 

 bon. According to Winogradsky - these organisms, by the energy 

 liberated by the oxidation of ammonia to nitric acid, are able to obtain 

 carbon from mineral carbonates. It is evident, if these conclusions are 

 correct, that bacteria must play an immensely important part in in- 

 itiating the formation of the indispensable humus in soils. They are 

 equally important in carrying on the functions of a fertile soil (vide 

 Chap. IV). 



It is also to be noted that certain low forms of vegetation, e.g., lichens 

 and algae, are apparently able to grow -on a purely inorganic soil ; they 

 must therefore obtain the nitrogen they require from the air, possibly 

 by the aid of micro-organisms associated with them. Such vegetable 

 growth furnishes humus to a soil and renders it suitable for maintain- 

 ing the life of higher plants. 



PROXIMATE CONSTITUENTS OP SOILS. 



The constituents of a soil are popularly divided into four groups, an 

 arrangement which, though perhaps not scientific, is often convenient. 



These proximate constituents are : 



(i) Sand, consisting mainly of silica, but containing small frag- 

 ments of felspar, mica, or even of limestone. 



(ii) Clay, mainly kaolin, but also containing finely divided felspar. 



(iii) Finely divided limestone. 



(iv) Humus, the somewhat indefinite product resulting from the 

 decay of organic matter in the soil. 



Sand, from a chemical standpoint, is one of the least important 

 constituents of a soil, furnishing as it does only very little of the food 

 of plants. In fact, the essential constituents, the grains of quartz, are 

 probably of no value whatever as plant food. The little plant food 

 "sand " does provide is furnished by the fragments of felspar, mica, or 

 calcium carbonate which may be present in it. From a practical 

 point of view, however, sand is of the greatest value, because of the 



1 Ann. Chim. Phys. [6] 11, 136 ; Jotir. Chem. Soc., 1887, Abstracts, 1135. 



2 Ann. Agron. 16, 273; Jour. Chem. Soc., 1890, Abstracts, 1180. 



