660 PLANT GROWTH AND PLANT COMMUNITIES 



would seem to rest primarily on their relative abilities to deal with the 

 prevailing substrate. 



The adoption of management practices involving incorporation of 

 crop residues has been reasonably successful in the control or suppres- 

 sion of certain troublesome root pathogens, which presumably have 

 not been favored as saprophytes by the organic matter supplied. How- 

 ever, in any discussion of antagonism or competition between organ- 

 isms the question of the production of microbial products that may 

 have antibiotic eflFects must arise. If an organism indeed produces an 

 antibiotic which can inhibit the growth of other soil forms, it may have 

 a substantial competitive advantage and may become dominant over 

 others equally well or better able to utilize the available substrate. The 

 property of antibiotic production may therefore be a vital aid to sur- 

 vival in the intensely competitive microbiological world of the soil 

 (Brian, 1957b). 



This is not the place to review the controversy as to whether anti- 

 biotics are or are not produced in soil. Direct experimental proof of 

 antibiotic production in normal soil is difficult but has been accom- 

 plished ( Brian, 1957b ) . Some of the approaches made in this connec- 

 tion have been unrealistic, because they have failed to take into ac- 

 count the generally colonial habit and discontinuous distribution of soil 

 organisms. It is hardly to be expected that the whole soil mass would 

 be permeated by antibiotics that could be extracted and identified. 

 Moreover, the antibiotic efi^ects are revealed only by organisms in- 

 hibited by the particular product in question. To unresponsive organ- 

 isms the antibiotic is merely a potential energy source which can be 

 metabolized. Some have maintained that if antibiotics are produced in 

 soil, they are of no consequence, because they will be immediately 

 utilized by species to which they are not inhibitory. But this presup- 

 poses that at every point in the soil there is a wide array of viable 

 forms capable of developing rapidly if an appropriate substrate ap- 

 pears. This is contrary to the concept of discontinuity, of colonial de- 

 velopment in micro-habitats, and of varied persistence when the sub- 

 strate is depleted. 



It is well established that a substantial percentage of the micro- 

 organisms isolated from soil— bacteria, fungi, and actinomycetes— are 

 antibiotic producers when cultured on laboratory media (Brian, 1951; 

 Benedict, 1953 ) . In a few cases the experiments have been extended to 

 show production when inoculated into sterile soil supplemented with 

 an energy source (see Brian, 1957b). Antibiotics have been detected 

 and identified in fragments of plant material recovered from unsteri- 

 lized soil (Wright, 1956a) and, perhaps significantly, from the decom- 

 posing seed coat after germination in soil ( Wright, 1956b ) . The weight 

 of the evidence, admittedly mostly indirect, is that microbial antago- 



