CHAPTEE V 



BACTBEIA IN THE SOIL 



Methods of Examination — Methods of Anaerobic Culture — Place and Function of 

 Micro-organisms in Soil — Denitrification, Nitrification, Nitrogen-fixation, 

 Bacterial Symbiosis — Saprophytic and Pathogenic Organisms in Soil — 

 Tetanus — Quarter - Evil — Malignant CEdema — The Relation of Soil to 

 Bacterial Diseases, such as Typhoid Fever. 



Surface soils and those rich in organic matter supply a varied field 

 for the bacteriologist. Indeed, it may be said that the introduction 

 of the plate method of culture and the improved facilities for 

 growing anaerobic micro-organisms have opened up possibilities of 

 research into soil micro-biology unknown to previous generations of 

 workers. 



From the nature of bacteria it will be readily understood that 

 their presence is affected by physical conditions of the soil, and in 

 all soils they occur only within a few feet of the surface. As we go 

 down below 2 feet, bacteria become less, and below a depth of 

 5 or 6 feet we only find a few anaerobes. At a depth of 10 feet, 

 and in the "ground water region," bacteria are scarce or absent. 

 This is held to be due to the porosity of the soil acting as a filtering 

 medium. Eegarding the numbers of micro-organisms present in soil, 

 no very accurate standard can be obtained. Ordinary earth may 

 yield anything from 10,000 to 5,000,000 per gram, whilst from 

 polluted soil even 100,000,000 per gram have been estimated. 

 These figures are obviously only approximate, nor is an exact 

 standard of any great value. Nevertheless Frilnkel, Beumer, Miquel, 

 and Maggiora have, as the result of experiments, arrived at a number 

 of conclusions respecting bacteria in soil which are of practical use. 

 From these results it appears that, in addition to the " ground water 

 region " being free, or nearly so, virgin soils contain much fewer than 

 cultivated lands, and these latter, again, fewer than made soils and 



