Methods ot Studying the Soil Popuhitioii 49 



Tablk 6. Infiaknch ok Moisti-hk Content ok Sfyi, on thk Ximukks or |{\(Tkhi\ 



(from Eiigl)or<liiig) 



Niiinbors of hactcria in IIkiiisiiikIs ])c'f Kram dry soil. 



IVrfonlage of 



Moisture Moisture-Holding Total Relative 



Content Capacity Bacteria Numbers 



per cent per cent 



6.5 30 !),!)80 33 



10. !> 50 11,890 40 



U.l 65 16,410 5.5 



17.4 80 29,960 100 



'21.7 100 25,280 84 



The numbers of microorganisms in the soil vary with the season 

 of year, being highest in spring and fall and lowest in summer and 

 winter. The abundance of the individual constituent groups of 

 bacteria may also \ary with the season of year. Hiltner and Stormer 

 reported that actinomycetes make up 20 per cent of the microbial 

 population de\eloping on the plate in spring, 30 per cent in autumn, 

 and 13 per cent in winter. Conn found larger numbers of bacteria 

 in winter, e\en in frozen soil, than in summer; he explained this 

 by the existence of two types of bacteria, "winter" and "summer." 

 Further studies suggested a simpler explanation for this observation: 

 the freezing of the soil and subsequent thawing result in the break- 

 ing up of the clumps of bacteria usually present; this gives an appar- 

 ent increase in numbers, as determined by the plate method. 



In addition to seasonal variations, there are also short-term varia- 

 tions among the microorganisms in the soil. These are believed to 

 arise from different competitive factors among the microorganisms. 

 The long-term fluctuations reflect the seasonal changes in climatic 

 conditions, as affecting the supply of energy for microorganisms pro- 

 vided by plant materials. These variations offer a more logical ex- 

 planation than the "inherent urge" concept suggested by some soil 

 investigators. The abundance and distribution of microorganisms 

 in soil, as well as the composition of the population of different soil 

 types, are influenced primarily by additions of organic matter. 



The bacteria of the sofl are capable of adapting themselves readily 

 to changes in temperature. A definite correlation was found be- 

 tween the a\erage yearly temperature of the air and soil and the 

 optimum temperature for bacterial development; this optimum tem- 

 perature is considerably higher than the soil temperature even dur- 

 ing the warm periods of the year. 



