CARBON AND NITROGEN CYCLES IN THE SOIL loi 



chief argument in favour of the bacterial hypothesis is that all known 

 soil processes can be reproduced in the laboratory by soil bacteria act- 

 ing under conditions comparable with those known to obtain in nature, 

 whilst they have not been produced by catalysts. The bacterial hy- 

 pothesis, therefore, remains the simplest and most satisfying, but there 

 is room for more evidence before it can be regarded as positively 

 established. 



It is certain that living bacteria occur in the soil in addition to 

 those present as spores. Some idea of the relative proportions of 

 these two forms was obtained by making gelatin plate cultures of soil 

 before and after treatment with toluene, which destroys living forms but 

 not spores, or at any rate not all spores (Table XL). Spores only 

 form about 25 to 30 per cent, of the total numbers, and for some un- 

 known reason do not accumulate. The bacterial numbers are seen to 

 be very high, but even these figures do not represent the true totals, 

 and no medium has yet been devised that allows of the growth of all 

 the organisms known to occur in the soil. 



Table XL. — Numbers of Active Bacteria and Spores Occurrino in Soils and 

 Capable of Growth on Gelatin Plates. (Russell and Hutchinson (240).) 

 Millions per gram of Dry Soil. 



The active forms must be held responsible for some at least of the 

 oxygen absorption, carbon dioxide evolution and decomposition going 

 on. Under comparable soil conditions a distinct relationship exists 

 between the productiveness of the soil and the amount of bacterial 

 activity, although it cannot be expressed in any definite form. Counts 

 of the numbers of bacteria by any particular method fail to give results 

 sharply connected with productiveness (although there is a general re- 

 lationship) because the organisms are of the most varied description 

 (no), and of widely different efficiency as food makers. Nor, on the 

 other hand, have the methods of physiological grouping helped much, 

 since they necessitate growth in culture media wholly different from 

 the soil under temperature and water conditions that never obtain in 

 nature. Not until the fundamental difficulty has been overcome of 

 synthesising a soil identical with natural soil will it be possible fully 

 to interpret the many interesting observations that soil bacteriologists 

 are now accumulating. 



