29 



is not sufficient to insure its development unless there is a large 

 number of individuals of the desired species ! present. There are 

 many species of bacteria — good, bad, and indifferent — that can 

 develop under the same conditions, just as there are many weeds 

 that grow vigourously under conditions favourable to crops. 

 It may be necessary in some cases to reduce the number of these 

 forms. It is essential that an accurate knowledge be gained of 

 all the organisms occurring in various soils, the changes that they 

 produce, their symbiotic and antagonistic relations and the con- 

 ditions affecting them, and the relation of their activity to crop 

 production. Chester* has suggested a very good method for ac- 

 complishing this in a uniform manner. A few examples of some 

 of his determinations of bacterial or zymotic efficiency of various 

 soils will be instructive. 



A soil from the experiment station garden, Newark, Del., con- 

 sisting of a heavy clay loam which had been brought to a high 

 state of fertility by ploughing under crimson clover for a number 

 of years and kept under active tillage, gave the average results 

 of two analyses as follows : — 



Per gram 

 of dry soil. 



Streptothrix soli . . 1,600,000 



Bacillus tumescens . . 1,200,000 



Bacillus alcaligenes var dclavariensis . 330,000 



Total . . 3,130,000 



As shown in the table on page 28, B. tumescens is the only 

 one of these organisms that can convert nitrogenous matter into 

 peptones with any degree of energy. The relative ammonifying 

 power of these organisms is proportionate to the ammonifying 

 power of the individual organism and to the number of organ- 

 isms. On this basis this soil has a relative ammonifying effi- 

 ciency of 13.73 an d an acidifying efficiency of 2.22. 



Nearly a year later (April 1 1, 1903) an examination of this same 

 soil showed a great reduction in the number and activity of the 

 bacteria present : 



Per gram 

 of dry soil. 



Bui. 65, Del. Col. Agric. Exp. Sta. 



