6o BACTERIA AND SOIL FERTILITY 



ash-free protein, carbohydrates, and fats die sooner than if given 

 only water. Likewise, bacteria require ash. They use it for 

 various purposes: (l) For maintaining the osmotic pressure of 

 the cell which gives to it its plump contour and assists in the tak- 

 ing up of foods. (2) For the neutralizing of acids, the nitrify- 

 ing organism and many others produce acids which if left to ac- 

 cumulate would soon cause the death of the cell. (3) Some 

 of the ash is built into the body of the microbe which gives to it 

 its peculiar chemical and physical properties. Some bacteria use 

 sulfur for energy. These comparatively large organisms when 

 taken directly from sulfur springs have within their bodies yellow 

 granules of sulfur, but on fasting these soon disappear, and sulfur 

 dioxid, hydrogen sulfide, or sulfuric acid appear in their place 

 depending on the specific microorganism which is being 

 studied. 



Mutual Relationship between Plants. — There are some 

 species of bacteria which live in close association with other plants 

 and in this manner get their food. The legume bacteria live in 

 the small nodules of the alfalfa plant drawing from it the food 

 needed and give the alfalfa in return nitrogen compounds. Such 

 a friendly association of one organism with the other is called 

 symbiosis* This mutual association plays a prominent part in soil 

 bacteriology, for probably there are many bacteria which thus 

 associate the one with the other. 



Sometimes one microbe changes the food so that another can 

 readily use it. There are bacteria in the soil which break down 

 plant tissues into acids which are used by the nitrogen-fixing bac- 

 teria. The yeast acts on sugar and changes it to alcohol. This 

 can be used by bacteria in producing vinegar. This association is 

 known as metabiosis. 



There are other species of bacteria which are mortal enemies 

 and cannot live together, one species killing out the other almost 

 as fast as they come in contact with each other. The details of 

 this miniature warfare are not well understood, nor do we know 

 the weapon in all cases by which it is carried on. It is probable 

 that some poison is secreted which is without effect upon its pro- 



