146 THE SOIL: ORGANIC MATTER 



(4) Reaction of the Soil. — Most beneficial bacteria require 

 for their optimum growth a neutral or alkaline medium in 

 which to work. The presence of an acid inhibits their 

 growth to a certain extent. Some bacteria produce mineral 

 or organic acids as a result of their own activities, and these 

 acids after reaching certain concentrations not only check 

 the growth of the bacteria which produce them, but that of 

 other bacteria as well. If there are sufficient acid-neutral- 

 izing compounds in the soil, such as calcium carbonates, 

 these acids are neutralized as fast as formed and bacterial 

 life is not suspended. 



(6) Kinds of Living Matter. — Although all portions of 

 vegetable and animal remains are attacked by one kind of 

 bacteria or another, and eventuallv can be entirelv decom- 

 posed under ordinary soil conditions, some parts of these 

 remains resist decay more strongly than others. This is due 

 ordinarily to the few species of bacteria which can attack 

 these parts of the plant or animal body. 



Cellulose and lignin — that is, the hard, woody portion of 

 plants — resist decay more strongly than the softer, more 

 succulent portions. Large roots, bits of bark, particularly 

 bark containing tannins, and wood in general, are more 

 resistant than leaves, soft stems, and fine roots like those of 

 grass. As a rule fats and waxes are less easily decomposed 

 than sugars and most of the proteins. In forests, however, 

 logs and stumps gradually decay and disappear entirely, 

 but here the conditions for one thing are most favorable 

 for rapid oxidation, and those bacteria and fungi which are 

 the most active oxidizers thrive. 



Most animal remains decay rapidly, but such things as the 

 organic matter in bones, hair, or hide decompose very 

 slowly, 



122. Humus. — The accompaniment of almost all kinds 

 of organic decomposition in soils is the production of a black 

 or dark amorphous material which is called humus. It is 

 one of the final products of decay, formed for the most part 

 where there is insufficient oxygen to allow complete destruc- 

 tion to carbon dioxide and water. From the original source, 

 whether it be root or leaf or stalk or animal, the humus is 



