SOIL ORGx\NISMS 



19 



The soil flora includes algae, fungi, and bacteria : to these 

 might be added the comparatively rare saprophytic species 

 and developmental stages of higher plants, and of course 

 plant roots. Algae are very common on the surface of damp 

 soils, diatoms and blue-green species often forming a slimy 

 covering. Green algae are also abundant — e.g. Vaucheria 

 and, especially on damp peaty soils, Chlorococcum. The 

 unicellular algse seem to be washed down to considerable 

 depths and to retain their vitality for prolonged periods. 

 Miss Bristol (1920) has described sixty-four species of algae 

 from different soils. Some of them have withstood desic- 

 cation for over sixty years. The presence of algae in the 

 surface regions has doubtless an important influence in 

 reducing the carbon dioxide and in increasing the percentage 

 of oxygen in the soil atmosphere. 



Particularly in soils rich in humus, fungi are abundant : 

 the mushrooms of an old pasture and the toadstools of a 

 wood are the fruiting bodies of subterranean mycelia of 

 great extent. As well as Basidiomycetes there occur many 

 other forms,, such as species of Mucor, and Aspergillus, 

 yeasts, and Actinomycetes. Along with the bacteria these 

 are active in breaking down higher organic compounds, and 

 especially woody tissues. 



Some conception of the numbers of bacteria present 

 may be gained from Table V, compiled from data given 

 by P. E. Brown (1913). 



TABLE V 

 Bacteria in an Iowan Loam Soil 



The relations to certain properties of the soil are also 

 shown. Many factors affect the numbers and nature of the 



