ORGANISMS IW THE SOIL 429 



It is not usuall^^ the entire absence of bacteria from the 

 soil that is to be avoided in practice^ for all arable soils con- 

 tain bacteria, although sometimes not all of the desirable 

 forms ; but, as great bacterial activity is required for the 

 large production of crops, the practical problem is to main- 

 tain a condition of soil most favorable to such activity. 



351. Distribution of bacteria. — Bacteria are found 

 almost universally in soils, although they are much more 

 numerous in some soils than in others. A number of in- 

 vestigators have stated that in soils from different locali- 

 ties and of .different types that they have examined, the 

 numbers of bacteria were proportional to the productive- 

 ness of the soils. The number of bacteria present has in 

 some cases been shown to be proportional to the amount 

 of humus contained in the soil. It is natural to expect 

 that within certain limits both these findings will hold. The 

 conditions obtaining in a productive soil are those favorable 

 to the development of certain forms of bacteria, and these 

 kinds constitute a very large proportion of those generally 

 found in soils- However, there is evidence that compara- 

 tively unproductive soils may contain a large number of 

 bacteria that are presumably not favorable to plant growth. 



Samples of soil taken from certain productive and rela- 

 tively unproductive parts of a field on the Cornell Uni- 

 versity farm contained a larger number of bacteria in the 

 poor soil, although the two soils were equally well drained 

 and the good soil had slightly more organic matter. 

 They had also received practically the same treatment 

 during the preceding few years : — 



Character .of Number of bacteria 



soil per gram of dry soil 



Good 1,200,000 



Poor 1,600,000 



