168 AGRICULTURAL ECONOMICS 



into prominence only quite recently, but now they are recognized to 

 be of as much, if not more, importance than the other two groups. 



All bacteria may be included in one of two large classes, depending 

 on their functions or the character of their activities. These are the 

 parasites and the saphrophytes. The saphrophytes are the decay- 

 producers. These saphrophytic, or decay, bacteria are invaluable. 

 They have been called the "link between the world of the living and 

 the dead." They transform dead materials back into living matter' 

 and thus complete the cycles through which, in nature, all substances 

 must go. 



Enormous numbers of bacteria inhabit the soil, some of them 

 harmful, but the vast majority beneficial. Actual counts have shown 

 that the numbers present in soils may vary from a few thousand per 

 gram (about one-thirtieth of an ounce) to over fifty million per gram. 

 The growth of bacteria in the soil is greatly influenced by certain 

 chemical and physical conditions. Thus, a proper amount of water 

 in the soil is as necessary for the growth of bacteria as for crops. 

 Either excessive moisture or severe drought interferes with bacterial 

 growth very considerably. Temperature conditions are also impor- 

 tant. Every organism grows best at a certain temperature, which is 

 called its optimum temperature. The optimum for most soil organ- 

 isms ranges from 65 to 90 F., although of course there are exceptions 

 to this statement. 



In general it may be said that the beneficial bacteria in the soil 

 need air. Hence in heavy clay soils, where there is not enough air, 

 methods which increase the circulation of oxygen in the soil increase 

 bacterial activity; these increase the solution of plant food, and this 

 ultimately increases crop production. On the other hand, if there is 

 too much air present, as in light sandy soils, the bacterial activities 

 will be too great and the humus will be burned up too rapidly, plant 

 food will be produced in too large quantities to be utilized by the 

 crops, and more or less extensive losses of valuable soil elements will 

 occur. 



The reaction of a soil (i.e., its relative acidity or alkalinity) means 

 much from a bacterial standpoint. Soils which have become acid or 

 sour are notably unproductive and this is largely due to the fact that 

 the growth of beneficial bacteria in such soils is checked. 



Lastly, bacteria require food for growth just as truly as do crops, 

 and it is because of this need that they influence fertility. In the 

 process of taking up food from the chemical compounds in the soil, 



