386 NATUEE AND PKOPERTIES OF SOILS 



decomposition o£ woody matter, which is disintegrated by the 

 penetrating mycelia of the fungi. These break down the 

 structure, and thus greatly facilitate the work of the decay 

 bacteria. Action of this kind is largely confined to the forest 

 and is not of great importance in cultivated soil. Another 

 function of the large fungi is exercised in the intimate, and 

 possibly symbiotic, relation of the fungal hyph^e to the roots of 

 many forest trees, in soil where nitrification proceeds very 

 slowly, if at all, for nitrates are apparently not abundant in 

 forests. 



Algse, except in special cases, do not exist in the soil to 

 any large extent. Certain Colorado soils,^ however, seem to 

 contain appreciable numbers of this form. While the pres- 

 ence of both the larger fungi and the alg^ is interesting, their 

 importance in soil fertility is probably rather slight. 



The roots of plants are important in the soil both by con- 

 tributing organic matter and by leaving, on their decay, open- 

 ings which render the soil more pex^meable to air and water. 

 The dense mass of rootlets, with their minute hairs, that is left 

 in the soil after every harvest, furnishes a well-distributed 

 supply of organic matter, which is not confined to the furrow 

 slice, as is artificially incorporated manure. The action of 

 roots on the soil is not by any means entirely physical. Dur- 

 ing the life of the plant the elimination of tissue and the 

 presence of exudates make the rootlets rather important chem- 

 ical agents.^ The chemical and biological importance of de- 

 caying organic matter has already been adequately empha- 

 sizeoL. 



211 . Micro-organisms — ^protozoa. — The micro-organisms 

 of the soil belong to the following groups: (1) protozoa, (2) 

 fungi and algas, (3) actinomyces, and (4) bacteria. 



^ Bobbins, W. W., Algm m Some Colorado Soils; Colo. Agr. Exp. Sta., 

 Bui. 184, 1912. 

 *See paragraphs 156 and 157. 

 *See paragi'aphs 64 and 132. 



