PROPERTIES OF HUMUS 147 



distributed among the soil grains with considerable uni- 

 formity within certain limits. That is, it does not ordinarily 

 extend to any great depth, nor does it extend laterally 

 over a field with uniformity, owing to changing conditions 

 in soil masses, but the grains themselves are fairly well 

 covered with the humus. It must therefore have been more 

 or less soluble in water, or in liquid form at one time or 

 another at least, to surround the particles. Its distribution, 

 moreover, is aided by the growth and final decay of the hyphse 

 or root-like hairs of certain fungi which feed on a decaying 

 bit of organic matter. It is no uncommon occurrence to 

 note the wide ring of darker soil surrounding a decaying 

 root. Earthworms also distribute humus throughout the 

 soil to a very great extent by passing soil through their bodies 

 and drawing after themselves into their burrows particles 

 of leaves and blades of grass. In some localities where 

 earthworms are fairlv numerous, Darwin has estimated 

 that thev work over in a vear from 0.1 to 0.2 of an inch 

 of surface soil. Ants, burrowing insects, and animals, all 

 help to distribute organic matter and, subsequently, humus 

 throughout the soil. 



123. Properties of Humus. — If a dark soil containing 

 considerable humus be first treated with a dilute mineral 

 acid like hydrochloric, and then with ammonia, a black 

 liquid will be obtained. The soil residue after washing is 

 very much lighter in color, almost white in some cases. 

 Bv this means the humus has been dissolved from the soil 

 grains, although in many cases it may not be of exactly the 

 same composition as it was in the soil. If the water is 

 evaporated from this solution there remains a shiny black 

 substance, rather hard and scaly, and absorptive of water. 

 If it is burned there remains behind an ash of inorganic 

 material. 



Chemically, humus is by no means a single compound. 

 It is a mixture, probably a mechanical mixture, of many 

 substances. Research bv the Bureau of Soils has shown that 

 it is composed of acids, carbohydrates, fats, waxes, hydro- 

 carbons, resins, nitrogenous compounds of various kinds, 

 black compounds or pigments, and undoubtedly many com- 



