viii CONTENTS 



CHAPTER PAGE 



as a culture medium. Moisture and air. 

 Temperature. Reaction. Number of organ- 

 isms in soil 



VII THE DECOMPOSITION OF ORGANIC MATTER IN THE 



SOIL HUMUS 78 



VIII THE CYCLE OF CARBON 83 



Cellulose decomposition. Retting. Oxida- 

 tion of hydrogen and methane 



IX THE ACTION OF BACTERIA ON THE MINERALS OF 



THE SOIL 87 



Calcium. Phosphorus. Potassium. Sulphur. 

 Iron 



X THE CYCLE OF NITROGEN 94 



The nitrogen of the soil. Ammonification. 

 Nitrification. Conservation of nitrogen. Ni- 

 trate deposits. Denitrification 



XI BARNYARD MANURES AND SEWAGE DISPOSAL . . 107 

 Manures. Sewage Disposal 



XII THE FIXATION OF NITROGEN 117 



The fixation of nitrogen. Leguminous plants. 

 Inoculation of soil 



PART III 

 THE RELATION OF MICROORGANISMS TO FOOD 



XIII THE CONTAMINATION OF FOODS 135 



Factors governing decomposition. Contami- 

 nation of milk: from interior of udder, from 

 air, from the animal. Influence of the milker. 

 Contamination from utensil. Cleaning of 

 milk utensils. Contamination from factory 

 by-products. Straining and clarifying milk. 

 Influence of feed on contamination of milk. 

 Contamination of other foods than milk 



