94 BACTERIA AND SOIL FERTILITY 



ber of molds. After leaching from the soil alkali we often find 

 that it takes a number of years before the soil becomes produc- 

 tive. This period of unproductivity can be shortened by adding 

 to the soil other fertile soils, thus carrying to it a bacterial flora. 

 Acid soils contain few bacteria and many molds. A soil rich 

 in the essential elements of plant production, other things being 

 equal, has a numerous bacterial flora. Limestone soils always 

 contain a numerous and varied microflora. 



Antiseptic and weak poisons added to a soil first decrease and 

 then increase the number of bacteria. Possibly the weak poison 

 kills some of the weaker permitting the remainder to multiply 

 unhindered. Some workers claim that the weak poisons destroy 

 the microfauna which normally would feed upon bacteria. 



Nearly all beneficial soil microorganisms are aerobic; hence, 

 we should expect to find all methods of treatment which keep 

 the structure of the soil in good condition to increase soil bacteria. 

 Cultivation, subsoiling, and fall plowing all increase the air en- 

 tering a soil and also the number of bacteria within it. We find 

 a close relationship between the crop grown on a soil and the 

 number of bacteria, as may be seen from figure 24, page 93. 



Probably one of the governing factors is aeration, as we find 

 fallow soil and soils on which hoed or cultivated crops are grown 

 high in microorganisms, whereas the grasses and haylands become 

 tight and impervious to air contain smaller numbers. 





Depth 



Number of Colonies 





2 inches 



981,000 





4 inches 



1,632,000 





6 inches 



1,623,000 





I a inches 



73,000 





1 8 inches 



21,000 





24 inches 



4,000 



Variation with Depth. — The number of organisms in the 

 soil vary greatly with the depth. Due to the lack of moisture and 

 germicidal action of light, the number of bacteria in the upper- 



