260 SOIL CONDITIONS AND PLANT GROWTH 



soil. Conn (job) estimated that they were one-quarter as 

 numerous as the bacteria in arable soil and one-half as 

 numerous in grassland. It is very doubtful, however, whether 

 any counting method is reliable. Waksman and Curtis (292^) 

 have made an extensive study of this group 1 and have de- 

 scribed thirty or forty soil species. Apparently the actinomyces 

 decompose cellulose though they do not readily produce 

 ammonia from protein. It is said also that they can reduce 

 nitrates to nitrites. 



4. Bacteria. 



From time to time indications have been obtained that 

 some of the soil bacteria bring about changes harmful to the 

 plant, but the evidence is insufficient to justify any detailed 

 discussion. It has been supposed that plant toxins are 

 produced (p. 247), that soil nitrates are assimilated (p. 210), 

 and that the food, air, and water which should otherwise be 

 available to the plant are taken up by the micro-organisms. 2 



The subject that has been most frequently investigated is 

 the part played by soil bacteria in the decomposition of the 

 organic matter of the soil and the production of nitrates. 

 Both changes are brought about by bacteria and they are of 

 such obvious advantage to the plant as to suggest that re- 

 lationships ought to exist between bacterial activity and the 

 growth of plants. Several methods have been adopted to 

 trace such relationships. 



The direct method consists in picking out definite 

 organisms and studying them in conditions calculated to 

 throw light on their action in the soil. This has proved very 

 difficult, and has been successfully achieved only by a few of 

 the best bacteriologists ; instances are afforded by the work of 

 Winogradsky (pp. 187, 196), Beijerinck (p. 197) and others. 



Three indirect methods have therefore been used : 



1 For a morphological study see Charles Drechsler (Bot. Gaz., 1919, 67, 

 65-83, 147-168). 



2 See, e.g., Dachnowski (Ohio), Expt. Sta. Record, 1910, 23, 122. 



