160 THE SOIL: ORGANIC MATTER 



less carbon dioxide and water produced, and more inter- 

 mediate organic compounds, such as acids and alcohols. 

 Sometimes even methane and hydrogen sulphide are pro- 

 duced. Moreover, under these conditions there is produced 

 material which is called " humus." The rate of decomposi- 

 tion of organic matter is dependent on the number of bacteria 

 present and the kind of living matter undergoing decom- 

 position. 



Humus is a black or dark colored, amorphous material, 

 coating soil grains and giving the dark color to the soil. 

 It is one of the final products of decay of organic matter 

 and is a mixture of a large number of different compounds, 

 the several properties of which, however, are sufficiently 

 similar in some respects to permit consideration of humus 

 as one kind of material for certain purposes. Under con- 

 ditions where basic material in the soil is lacking, humus 

 is acid in character and can be dissolved out of the soil bv 

 ammonium hydroxide which forms an ammonium compound 

 with the so-called humic acid. Under conditions where there 

 is sufficient basic material in the soil, humus is neutral 

 in character and cannot be dissolved out of the soil by 

 ammonium hydroxide until after treatment with hydro- 

 chloric acid which decomposes the neutral humate. 



Of the two kinds of organic matter, the active or decom- 

 posing material is important as supplying carbon dioxide 

 to the soil and thus aiding in the solution of mineral particles. 

 It also sets free mineral compounds and thus supplies food 

 to the plants. It is an active chemical agent in the soil. 

 The inactive organic matter, or humus, is a reserve nitrogen 

 supply, but it has more of a physical effect on the soil than 

 a chemical one. 



To render nitrogen available to plants it must be changed 

 from its organic combination to the nitrate form, and this is 

 accomplished by three sets of bacteria in the soil, changing 

 protein material first to ammonia, then to nitrous acid, and 

 then to nitric acid. This process is called nitrification. 

 Another important kind of bacteria is the so-called nitrogen 

 fixers, which have the power of combining the nitrogen of 

 the air with organic matter in the soil and thus putting the 



