56 THE SOIL. III. 



Alumina ... ... 3'5 



Ferric oxide ... 3*1 



Phosphorus pentoxide ... 12-4 

 Sulphur trioxide ... 1*0 



Carbon dioxide ... 1*6 



Insoluble ... ... 62-0 



Analyses of four samples of humus obtained from soils gave 

 Carbon ... ... 4450% 



Hydrogen ... ... 3 6% 



Nitrogen ... ...6-510% 



Oxygen ... ...2835% 



Ash ... ... 412% 



Humic acid and the soluble humates are colloidal bodies. 

 According to Grandeau, humic acid and the humates combine 

 with phosphoric acid, lime, potash, oxide of iron, or silicic 

 acid to form double compounds, which, though soluble in am- 

 monia, do not give the reactions characteristic of their con- 

 stituents. When solutions of these double compounds are 

 submitted to dialysis decomposition occurs and the ash ingre- 

 dients pass through the membrane entirely free from the 

 organic matter. This has, to some extent, been confirmed by 

 Simon. * These absorptive properties of humus for substances 

 in solution, as well as those of clay, are of great importance in 

 agriculture and will be considered hereafter. Its alleged power 

 of absorbing nitrogen from the air and converting it into am- 

 monia, described by Simon and others, is denied by Prevost.f 

 According to LogesJ dilute hydrochloric acid extracts from a 

 soil along with the humic acid a nitrogenous compound, whose 

 composition could not be exactly ascertained ; two samples from 

 different soils contained 6*5 and 6'8% nitrogen and 37'3 and 

 45*4% carbon. 



Classification of Soils. Soils are usually divided by prac- 

 tical agriculturists into 



Sandy, containing less than 10 % clay and less than 3 % 



calcium carbonate. 



Loam ,, from 40% to 70% clay and less than 



3% calcium carbonate. 



* Lamlw. Versuchs. Stat. 18, 452; J.C.S. 1876, 731. t J.C.S. 1881, 371. 



I Land. Versnch. Stat. 1885, 201. 



