SOIL BACTERIA, CONDITIONS FOR GROWTH 401 



upper surface. In fact, it is seldom that soil tempera- 

 tures become sufficiently high to curtail bacterial 

 activity. 



272. Organic matter. The presence of a certain 

 quantity of organic matter is essential to the growth 

 of most, but not all, forms of soil bacteria. The or- 

 ganic matter of the soil, consisting as it does of the 

 remains of a large variety of substances, furnishes a 

 suitable food-supply for a very great number of forms 

 of organisms. The action of one set of bacteria upon 

 the cellular matter of plants embodied in the soil 

 produces compounds suited to other forms, and so 

 from one stage of decomposition to another this con- 

 stantly changing material affords sustenance to a 

 bacterial flora the extent and variety of which it is 

 difficult to conceive. Bacteria not only affect the or- 

 ganic matter of the soil, but, in the case of certain 

 forms, their activities produce changes in the inorganic 

 matter that cause it to become more soluble and more 

 easily available to the plant. 



A soil low in organic matter usually has a lower 

 bacterial content than one containing a larger amount, 

 and, under favorable conditions, the beneficial action, 

 to a certain point at least, increases with the content 

 of organic substance; but, as the products of bacterial 

 life are generally injurious to the organisms producing 

 them, such factors as the rate of aeration and the 

 basicity of the soil must determine the effectiveness 

 of the organic matter. 



273. Soil acidity. A soil having an acid reaction 

 makes a poor medium for the growth of bacteria. A 



