The Smallest of Living Things 239 



FIG. 192. The decay of dead vegetable and animal matter is caused by special 

 kinds of bacteria. If decay takes place above ground, much material that would 

 be of value to the soil escapes in the form of gases. If decay takes place in damp 

 earth, or under a layer of leaves in a forest, humus is formed. 



Decay carried to completion does not leave much Prepara- 

 plant food in the soil, but a partial decay in the earth 

 of such material as stubble, roots, leaves, and manure humus 

 produces humus, upon which plants are so dependent. 

 Bacteria, then, aid in the formation of humus. But 

 as the humus itself is not soluble in the moisture of the 

 soil, it cannot be used by the plants as food before it 

 has been changed by other kinds of bacteria, different 

 from those that caused decay, into material that the 

 roots can take up. 



Knowledge of the habits of these soil-enriching Soil 

 ... i i bacteria; 



bacteria is very necessary to every one who cultivates mana re 



a field, a garden, an orchard, or a flower bed. The first 

 thing of importance to remember is that the bacteria 

 must be able to find a supply of food in the soil, and 

 that nothing answers the purpose so well as a plentiful 



