rOMPosiTinX OP ROIL8 79 



matter of plants may bf BSed OW and over again foi 

 plant food. 



'I'lu- decay or rotting of organic matter in soils ifl due 



to the action of bacteria which inhabit tha Boil. These 

 bacteria feed on the organic matter ami canae decay. 

 They are sensitive to heal and cold, and require b con- 

 stant supply of fresh air ami water. That t! 

 mentf are true is shown by the following well-known 

 facts: Dead organic matter does not decay when frozen; 

 thm frozen meat or vegetables may he kept for yean 

 and -how no sign <>f decay. I Nad organic matter may 



he dried and kept for a long time, hut when moistened 

 it decays. Dried or smoked meat is a good example of 

 this. If Sealed up from the air. organic matter keeps 

 indefinitely. Canned L r <><>d< arc an example of this fact. 

 The conditions under which the bacteria of decay act 

 beai arc the same a- tl Baary for the Bpronting 



of Med. They require heat, moisture, and air. and. U 

 in the case of the -n^t], it i< the oxygen of the air that is 

 used. 



As we have already learned, the decay of the dead 

 organic matter of the soil results in the formation of a 

 n<e called humus. Humus is a dark, almost black, 

 product, which gives to garden soil its rich, dark color. 

 It is a mixture of many different compounds, all of 

 which are made up of the four elements — carUm. hydro- 

 gen, oxygen, and ni tro ge n, Humus is of great import- 

 in the soil, and we shall have more to say of it 

 later on. 



80. Inorganic Matter. — The mineral matter which 

 mikes lip more than '»<) per cent of mosi soils U a mix- 



