3 i6 GENERAL SCIENCE 



Soil full of earthworms is usually fertile, as they feed on 

 organic matter. The most important of all living organ- 

 isms in the soil are the bacteria and yeast plants. 



Bacteria are so small that they have to be magnified 

 500 or more times before they can be seen. It takes 

 from 25,000 to 150,000 of them laid side by side to make 

 an inch in length. But what they lack in size they make 

 up in numbers and rapidity of reproduction. If they have 

 all the food they need, and if the other conditions are 

 right, there will be a new generation every fifteen to thirty 

 minutes. They increase in numbers simply by one bac- 

 terium (singular of bacteria) dividing into two equal parts, 

 each of which is a bacterium. If they divide every 

 fifteen minutes there will be four generations per hour, 

 and at the end of the hour there will be sixteen new 

 bacteria from each bacterium. If enough food could be 

 secured, the offspring of one bacterium during a period of 

 four or five days would be sufficient to fill all the oceans 

 of the earth. But a limited supply of food and unfavor- 

 able conditions for growth prevent this rapid increase of 

 bacteria from continuing. 



Bacteria of various kinds are found in all soils. They 

 range from less than 30,000,000 per ounce up to billions 

 per ounce of soil. The most fertile soils, like those in 

 gardens, contain the most bacteria. Some experiments 

 have shown that the soils which produce the greatest 

 crops contain the most bacteria. Some soils will produce 

 a large crop of one kind but will not produce a crop of a 

 different kind. This is sometimes due to the absence 

 of the bacteria that a certain plant needs. Some plants 

 have specialized bacteria and will not grow without them. 

 Alfalfa is such a plant, and if it does not grow the soil 

 must be inoculated with alfalfa bacteria taken from the 



