170 THE LIVING ORGANISMS OF THE SOIL [chap. 



improved types to replace already existing kinds of 

 less effective character. 



The Changes of Organic Matter in the Soil. 



The surface layer of soil is constantly receiving 

 additions of organic matter, either leaves and other 

 debris of vegetation covering the ground, together with 

 the droppings of animals consuming that vegetation, 

 or dung and other animal and vegetable residues which 

 are supplied as manures to cultivated land. These 

 materials rapidly change in ordinary soil, losing almost 

 immediately any structure they possess, becoming 

 dark-coloured humic bodies, or even burning away as 

 thoroughly as if placed in a furnace. That these 

 changes are due to micro-organisms is seen by their 

 immediate cessation if the soil be treated with anti- 

 septics like chloroform or mercuric chloride : or if the 

 mixture of soil and organic matter be sterilised by heat- 

 ing. Attempts have been made to estimate the number 

 of bacteria contained in the soil : the prodigious numbers 

 obtained, 2 up to 50 millions or more per cubic centi- 

 metre of the upper soil, show little beyond the fact that 

 the soil is tenanted much as any other decaying organic 

 material would be. The soil bacteria are always associ- 

 ated with a certain number of fungi and yeasts, 

 especially when the reaction of the medium is at all 

 acid: the organisms are most numerous in the surface 

 layer, though they are still to be found in the deepest 

 subsoils. Below a certain depth they must disappear, 

 because deep well water often comes to the surface in 

 an absolutely sterile condition. The changes which 

 organic materials undergo in the soil may be roughly 

 grouped into two classes ; according as there is free 

 access of oxygen or not, either decay (eremacausis) 



