316 ANNUAL REPORT OF THU Off. Doc. 



and (3) the relatively low number in soils covered with sod. The 

 reason for these differences is apparent. Woodland soils, although 

 rich in humus, are usually too acid for the best development of bac- 

 teria therein. Pasture lands, or lands for a long time in sod, are too 

 compacted or imperfectly aerated. Most soil bacteria develop best 

 in the liberal presence of atmospheric air, hence the opening up of 

 such soils by tillage to the action of the atmosphere is essential be- 

 fore the best development of bacteria can take place. Pasture lauds 

 also have a tendency to become acid, a condition unfavorable to bac- 

 terial development. 



In the studies at the Delaware Station, the highest numbers of bac- 

 teria were always found in soil which had been under active culti- 

 vation, especially when liberally supplied with humus, either by plow- 

 ing under of green crops or by the use of stable manure. Thus in soil 

 No. 6, of the preceding list, where the number of bacteria was 2,200,- 

 000 per gram, the latter had been enriched by repeated crops of crim- 

 son clover plowed under, accompanied by active tillage. Soil No. 11, 

 a vegetable garden, had annual di'essings of stable manure for a 

 series of years, and had also been under constant tillage. The 

 value of stable manure in increasing the number of bacteria in the 

 soil has been shown by Miquel,^ who found that after the applica- 

 tion of this fertilizer the number of bacteria in the soil was increased 

 from 700,000 lo 900,000 per gram. 



It may therefore be stated as a general principle that the com- 

 hined efect of hvjh manuring and (ultivat'Oii is to decidedly increase 

 the number of bacteria in the soil, thus in tm^n setting free an in- 

 creased quantity of OA^ailahle ])lant food. 



Soil Bacteria in Their Kelation to Atmospheric 



Oxygen. 



It has been stated that the great majority of soil bacteria develop 

 best in the presence of atmospheric oxygen. Bacteria differ as re- 

 gards their relation to this important element, and thus it has been 

 the custom to divide them into three classes, (1) obligate aerobes, or 

 those which do not grow except in the presence of oxygen; (2) anae- 

 robes or those which grow only with the complete exclusion of oxy- 

 gen, and (3) facultative anaerobes or those which are indifferent to 

 the presence or absence of this gas. In recent times it has been rec- 

 ognized that no such sharp lines as these can be drawn; on the 

 other hand these different classes merge into one another by indis- 

 tinct stages of gradation. 



A bacterium may, in a measure, show the ability to grow with the 

 partial or complete exclusion of atmospheric oxygen, but it grows 



