SOIL ORGANISMS 391 



soil-water. The shape of bacteria is varied in that they may 

 be nearly round, rod-like, or spirals. In the soil the rod- 

 shaped organisms seem to predominate. 



As already stated, the primary method of multiplication 

 of bacteria is by simple division, the process being very rapid 

 under favorable conditions. The phenomena frequently takes 

 place in thirty minutes. This almost unlimited capacity to 

 increase in numbers is extremely important in the soil since 

 it allows certain groups quickly to assume their normal func- 

 tions under favorable conditions, even though their numbers 

 were originally small.^ Bacteria may thus be considered as 

 a force of tremendous magnitude in the soil, held more or 

 less in check by conditions, but ever ready to exert an influ- 

 ence of profound importance on crop gi'owth. 



In the soil bacteria probably exist as mats or clumps, 

 called colonies, on and around the soil particles wherever food 

 conditions are favorable. Natural and artificial forces tend 

 to break up these colonies and, as many groups are flagellated, 

 bacteria becomes well distributed through the soil. In gen- 

 eral the greatest numbers are found in the surface layers of 

 the soil, since conditions of temperature, aeration, and food 

 are here more favorable. Many of the soil bacteria are able 

 to produce spores, thus presenting both a resting and a vege- 

 tative stage. The production of spores is often extremely 

 important as it allows the organisms to survive unfavorable 

 conditions of many kinds. 



The number of bacteria present in soil is quite variable as 

 many conditions markedly affect their growth. The meth- 

 ods ^ of determining the numbers are extremely inaccurate, 



^If a single bacterium and every subsequent organism produced sub- 

 divided every hour, the offspring from the original cell would be about 

 17,000,000 in twenty-four hours. In six days the organisms would greatly 

 surpass the earth in volume. Under actual conditions such multiplication 

 would never occur, due to lack of food and other limitations. 



*The counting of soil bacteria is generally carried out somewhat as 

 follows; A small sample of soil (usually .5 gram) is placed in a sterile 

 Erlenmeyer flask and treated with 100 ce. of sterile water. The sample 



