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SCIENCE 



[N. S. Vol. XXXVII. No. 953 



nures; e. g., in dunged soils, greenhouse soils, 

 sewage farm soils, etc. Thus the limiting fac- 

 tor can be reintroduced from untreated soils. 



6. Evidence of the action of the limiting 

 factor in untreated soils is obtained by study- 

 ing the effect of temperature on bacterial 

 numbers. Untreated soils were maintained at 

 10°, 20°, 30° C, etc., in a well moistened 

 aerated condition, and periodical counts were 

 made of the numbers of bacteria per gram. 

 Eise in temperature rarely caused any in- 

 crease in bacterial numbers, sometimes it had 

 no action, often it caused a fall. But after 

 the soil was partially sterilized the bacterial 

 numbers showed the normal increase with in- 

 creasing temperatures. Similar results were 

 obtained by varying the amount of moisture 

 but keeping the temperature constant (20° 

 C). The bacterial numbers in untreated soil 

 behaved erratically and tended rather to fall 

 than to rise when the conditions were made 

 more favorable to trophic life; on the other 

 hand, in partially sterilized soil, the bacterial 

 numbers steadily increased with increasing 

 moisture content. Again, when untreated 

 soils are stored in the laboratory or glass- 

 house under varying conditions of tempera- 

 ture and of moisture content the bacterial 

 numbers fluctuate erratically; when partially 

 sterilized soils are thus stored the fluctuations 

 are regular. 



7. When the curves obtained in (6) are ex- 

 amined it becomes evident that the limiting 

 factor in the untreated soils is not the lack of 

 anything" but the presence of something ac- 

 tive. 



8. This factor, as already shown, is put out 

 of action by antiseptics and by heating the 

 soil to 60° C, and once out of action it does 

 not reappear. Less drastic methods of treat- 

 ing the soil put it out for a time, but not per- 

 manently: e. g., heating to 50°, rapid drying 

 at 35°, treatment with organic vapors less 

 toxic than toluene (e. g., hexane), incomplete 

 treatment with toluene. In all these cases the 

 rise induced in the bacterial numbers per 



' The soils we used included fertile loams well 

 supplied with organic matter, calcium carbonate, 

 phosphates, etc. 



gram is less in amount than after toluene 

 treatment and is not permanent; the fac- 

 tor sets up again. As a general rule, if the 

 nitrifying organisms are killed, the limiting 

 factor is also extinguished; if they are only 

 temporarily suppressed the factor also is only 

 put out for a time. 



9. The properties of the limiting factor are : 

 (a) It is active and not a lack of something 

 (see (7)); (&) it is not bacterial (see (3) and 

 (4)); (c) it is extinguished by heat or poi- 

 sons and does not reappear if the treatment 

 has sufficed to kill sensitive and non-spore- 

 forming organisms; it may reappear, however, 

 if the treatment has not been suiBcient to do 

 this; {d) it can be reintroduced into soils 

 from which it has been permanently extin- 

 guished by the addition of a little untreated 

 soil; (e) it develops more slowly than bacteria 

 and for some time may show little or no ef- 

 fect, then it causes a marked reduction in the 

 numbers of bacteria, and its final eilect is out 

 of all proportion to the amount introduced; 

 (/) it is favored by conditions favorable to 

 trophic life in the soil.' 



10. We see no escape from the conclusion 

 that the limiting factor is a living organism. 

 We were, therefore, led to search for organ- 

 isms not bacteria, slower growing, less resist- 

 ant and larger. Protozoa naturally sug- 

 gested themselves. We soon found numbers of 

 ciliates, amoebs and flagellates and induced 

 Mr. Goodey to study them in detail. This 

 work is still continuing, and promises highly 

 interesting results : some remarkable forms 

 have been picked out, and it is already evident 

 that the zoological survey of the soil will be 

 a prolonged business but will be eminently 

 worth while. The ciliates and amoebse are 

 killed by partial sterilization. Whenever they 

 are killed the detrimental factor is found to 

 he put out of action, the bacterial numbers 

 rise and maintain a high level. Whenever the 

 detrimental factor is not put out of action the 

 protozoa are not killed. To these rules we 

 have found no exception. Some exceptions 

 have been found to the converse proposition, 



'This is dealt with fully in the Jmmial of 

 Agricultural Science, 1912, 5: 27, 86. 



