THE ELEMENTS OF PLANT FOOD 33 



partly decayed, or oxidized, is called humus. Organic 

 matter in the soil is not properly called humus till it 

 has passed the most active stage of decomposition and 

 has lost the physical structure of the substances from 

 which it is made. It has a dark color and partakes of 

 many of the properties of charcoal. New soils gener- 

 ally have an abundance of carbonaceous matter, but 

 by constant cropping and failure to add manure or to 

 turn under green crops this carbonaceous matter be- 

 comes exhausted. Although it is true that plants can- 

 not take it through their roots, yet the carbonaceous 

 matter mellows the soil, and absorbs and holds am- 

 monia gas so that it may be acted on chemically, and 

 nitrates may be formed that may be taken in by the 

 rootlets as plant food. The presence of carbonaceous 

 matter is of the utmost importance in providing a means 

 of making other plant food available for the use of the 

 plant. It also encourages bacterial growth of certain 

 kinds that are essential in the preparation of plant foods. 



Note. Carbon monoxide (CO) is very poisonous. The pres- 

 ence of CO is shown by the blue flame over a fire of coke or hard 

 coal. It is formed by the red-hot coals taking from CO 2 one of its 

 atoms of O. This gas has a direct poisonous effect when it escapes 

 into a room, often causing death. It is also present in the illuminating 

 gas furnished by cities. 



Some Hydrocarbons and Derivatives. --The com- 

 pounds of hydrogen and carbon without O are called 

 hydrocarbons. The number of such carbohydrates is 

 very great. They exist in nature as the constituents 

 of petroleum, natural gas, and asphalts. They may be 

 formed by the heating of carbon compounds without 

 the free mixture of O, as in the manufacture of charcoal 

 and illuminating gas. 



M. & H. AG. 3 



