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During the past twenty years or more, considerable attention has been given to the relation of bacteria to nitrogen-transformations in soil and in manure. This particular phase of soil microbiology has been so extensively studied because of the manifest importance of nitrogen in the nutrition of plants. There still remain, however, many other problems connected with the micro-organisms in the soil that are of fundamental importance and which must be studied before a true appreciation of the significance of soil bacteria to agriculture is possible.

There are, for example, many unanswered questions relating to the distribution, function, and associative action of the microorganisms known to be abundant in soil. These questions require a method of attack almost the opposite from that used in studying the nitrogen-transformations. The latter type of investigation starts with a known function and is directed toward the unknown organisms (or at least unknown at the outset) to which this function belongs; the other type starts with known organisms and is directed toward their unknown functions. The study of nitrogen-transformations has been centered around a few special physiological activities; while the other type of study is concerned with the general characteristics of the entire soil flora. Therefore, investigations of the latter sort may be distinguished from those of the former by calling them soil flora studies.

In making a study of the soil flora, the first step necessary is classification. Investigations relating to the distribution or function of the micro-organisms cannot proceed far without a thoro knowledge of the kinds of organisms in question. It is not yet possible to make a complete classification of the microscopic flora of soil; but it is possible to make a preliminary grouping of the organisms. Nothing more than this is attempted in the present work; but it is hoped that this preliminary grouping may be used by later investigators and developed by them into a more extensive classification.

The results now ready for publication are to be written up in a series of five papers. The present bulletin, containing the first two of these papers, is really an introduction to the three that are to follow. The three later papers are to be published in separate bulletins, and each one is to take up one of the three most important groups of soil bacteria: Actinomycetes, spore-forming bacteria, and non-spore-forming bacteria. An outline of these papers follows the " Acknowledgments."


This work was begun at the suggestion of Dr. H. A. Harding (formerly Bacteriologist at this Station), whose criticisms and advice proved most valuable during the two years while the work was proceeding under his direction. During the last three years the helpful suggestions of Dr. R. S. Breed have also proved of great value. Equal acknowledgments are due to the writer's father, Dr. H. W. Conn, for the inspiration of conversation and correspondence with him in regard to the work. Lastly, credit is due to Dr. W. W. Ford, who has given much valuable assistance in determining the species of spore-forming bacteria encountered in this work.

Technical Bulletin No. 57.

I. The general characteristics of the microscopic flora of soil.
Previous investigations.
Soils investigated.
Kinds of micro-organisms in soil.

Limitations of the study.

Peculiarities of the soil flora.

Groups of micro-organisms found.

Importance of these different groups.
II. Methods best adapted to the study of the soil flora.
Sampling soil.
Plating soil for quantitative purposes.

General technic.

Comparison between soil-extract gelatin and tap-water gelatin.

Comparison between different brands of gelatin.

Comparison between plain asparaginate agar and asparaginate agar with

Comparison between gelatin and agar.
Plating soil for qualitative purposes.

Advantages of gelatin.

Advantages of agar.

Technic at present recommended.
The study of pure cultures.

The classification card of the Society of American Bacteriologists.

The group number.

Value of the card in studying soil bacteria.

Diagnostic value of the tests given on the card.

Reasons for the inconsistent results.

Technical Bulletin No. 58.

III. Spore-forming bacteria in soil.

The three most common species.
B. megatherium DeBary.
B. mycoides Fltigge.
B. cereus Frankland.
Less common species.

Gelatin colonies of the spore-forming bacteria.
Significance of the spore-forming bacteria in soil.

Technical Bulletin No. 59.

IW Non-spore-forming bacteria in soil.
The two principal groups.
Rapid liquefiers.

Ps. fluorescens (Fltigge) Migula.
Orange liquefying type.
Other types.

General characteristics.
Rod-shaped forms.
Recognition of colonies on plates.

Abundance in soil.
Fluctuations in numbers.
Significance of the non-spore-forming bacteria in soil.

Technical Bulletin No. 60.

V. Actinomycetes in soil.

General description of Actinomycetes.

The question of nomenclature.
Early attempts at classification.
Krainsky's work.
Waksman's work.

Methods used in the present work.
Morphological criteria.
Growth on glycerin media.
Growth on other media.
Weakness of present methods.
The most abundant types in soil.
The pink-spored type.
The dark-brown type.
The potato scab type.

V. Actinomycetes in soil (continued). Classification (continued). The numerous less common types. Recognition of colonies on plates. Distribution. Occurrence in nature generally. Occurence in soil. Significance of the Actinomycetes in soil. Discussion of the whole series of soil flora studies

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