50 THE MECHANICAL ANAL YS1S OF SOILS [chap. 



solution that is formed is akin to the dark liquid draining 

 from a dung heap, which contains humus dissolved 

 by the alkaline carbonates of the fermented urine. 



Occasionally soils are found which naturally pos- 

 sess an acid reaction, and in which the whole or part 

 of the soluble humus is uncombined with calcium. 

 In practice a sharp distinction can be drawn between 

 the acid humus of the upland moors and peat bogs, 

 formed where the water contains no mineral bases, and 

 the neutral humus of the black soils of the Fens, formed 

 from vegetation that has decayed in water containing 

 calcium carbonate. The latter is much more subject to 

 further changes on aeration and in cultivation readily 

 yields to plants the nitrogen it contains. 



Physically the humus of the soil is a colloid and 

 possesses the colloid properties of swelling by imbibi- 

 tion of water and acting as if it possessed an extensive 

 surface. To this extensive surface may be attributed 

 some of the supposed chemical properties of humus, for 

 example, its reddening effect upon blue litmus paper. 

 This does not indicate that the humus is acid, but the 

 base of the neutral blue litmus is more readily absorbed 

 by the surface of the colloid than the acid portion, which 

 is red. 



Although dark brown humic substances can be 

 prepared from carbohydrates, and therefore contain 

 only carbon, hydrogen, and oxygen, yet the soluble 

 humus of the soil, even when dissolved and reprecipi- 

 tated, always contains some nitrogen, nor can it be 

 obtained entirely free from phosphorus and mineral 

 matter. The original vegetable matter is made up not 

 only of carbohydrates, but of other carbon compounds 

 containing nitrogen, and in some cases both nitrogen 

 and phosphorus ; these all break down under bacterial 

 action into dark-coloured substances richer in carbon, 



