SOIL TEXTURE AND COMPOSITION 



93 



j^oeo 



aoo 



"fSO 



ffCO 



i ASP c£f/r a^ srom^ 





% p£p cavr OP jSi^cnsp//} 

 § PEPC£y/roPS4cr£m^, tfi^uowj 

 — pepce/vrop^^iAfMiy^/^ 



g ASPce/vroPAf/T^c/wrncKSSAf 



cent, while ten tons of manure increased the numbers 75 per cent. 

 In soil deficient in organic matter the increase due to manure 

 may be many times greater than 

 this. The addition of sugar and 

 various other easily decomposed 

 carbohydrates may increase the 

 numbers even more than do ma- 

 nures. One of the great benefits 

 coming from the use of manures 

 is the great increase in the bac- 

 terial flora. 



Bacteria like higher plants are 

 dependent upon a suitable tem- 

 perature. They cannot multiply 

 in a frozen soil. Although we 

 often find that the number in a 

 soil is increased after freezing 

 this is due to many factors. The 

 loosening up of the soil, suppress- 

 ing of microorganisms which 

 feed upon bacteria, and the ac- 

 tual stimulation which comes 

 from the cold — all of these may 

 be responsible. 



We have seen that bacteria are 

 directly dependent upon mois- 

 ture for their life activities; 

 hence, we find the number vary- 

 ing with the moisture content. 

 They increase with the moisture, 



probably until about 70 per cent of the water-holding capacity of 

 the soil is reached. Then as the water increases the number of 

 bacteria decreases. 



Alkali soils support a sparse, special vegetation. The same is 

 true of the microflora of an alkali soil. A soil containing large 

 quantities of alkali contains few bacteria but a considerable num- 



-a? 



/H3» Mtf/i/pfr 



S TZiMS 

 OPAUWi/P£ 



OP/'ii'Wijipe- 



Fig. 24. — Relationship between 

 crop and bacteria in a soil. 



