MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 241 



lized by heat or antiseptics the numbers may become enormous, 50,000,000 

 or even 100,000,000 per g. of material. 



Distribution at Different Depths. Most of the soil bacteria are found 

 in the stratum in which the organic residues are concentrated, that is, 

 in the surface soil. Immediately at the surface the rapid evaporation 

 and the germicidal effect of direct sunshine act as disturbing factors, 

 hence the numbers in the uppermost 25-50 mm. (1-2 in.) are smaller 

 than in the layer of soil immediately below. Beyond the depth of 20 

 cm. or 22 cm. (8 or 9 in.) the numbers diminish rapidly. Material 

 from a depth of .6 M. to .9 M. (2 to 3 ft.) is nearly sterile in humid 

 regions. Differences occur, however, in keeping with the mechanical 

 composition of the soil. In light, open soils the bacteria are not only 

 carried down to greater depths by the percolating water, but can also 

 multiply there, thanks to better aeration. On the contrary, fine-grained, 

 compact soils are more effective in holding back suspended material and 

 do not allow the bacteria to pass downward as readily. Moreover, the 

 less thorough aeration of these soils and the accumulation of toxic reduc- 

 tion products in the the subsoil serve as an effective check on the increase 

 of bacteria in the deeper layers. 



In irrigated soils 6f the arid and semi-arid regions bacteria are dis- 

 tributed at much greater depths. Their occurrence 2 M. to 3 M. 

 (8 or 10 ft.) below the surface is made possible not only by the better 

 aeration of these soils, but by the penetration of roots to great depths and 

 the accumulation there of considerable amounts of organic matter. The 

 practical significance of distribution appears, among other things, in the 

 use of soil for inoculation purposes; for instance, it is reported by Sals- 

 trom that in making peat soils arable the addition of small amounts of 

 fertile loam increases to a very marked extent their crop-producing power. 

 The efficiency of the inoculating material decreases as it is taken from the 

 deeper soil layers. Similarly, in the use of alfalfa soil for the inoculation 

 of new fields the most efficient material is secured at a depth between 

 7.62 cm. and 17.78 cm.( 3 and 7 in.). 



Morphological and Physiological Groups. (Morphological Groups). 

 Rod-shaped organisms are numerically the most prominent among soil 

 bacteria. They occur at times to the extent of 80 or 90 per cent of the 

 total number. Spherical organisms usually constitute less than 25 per 

 cent of the bacterial flora. Spirilla and Sarcince are present in slight 

 numbers. Conditions may occur, however, when the proportion of 

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