CHAPTER VIII 

 SOIL ORGANISMS 



The number of lower plants and animals which are temporarily or 

 permanently housed in the soil and draw nourishment from it is quite 

 astonishing. Nine-tenths of all insects spend some time in the soil. 

 Their harmful activities through devouring of roots (grubs), damage to 

 seedlings, unfavorable physicochemical influences on the soil, etc., are 

 exceeded by their profoundly beneficial activities. 



Darwin was the first to call attention to the importance of earth- 

 worms in working over the soil. By their burrows, which penetrate 

 to a maximum depth of 7 m., they open up the lower layers of the soil. 

 They grind up large amounts of earth with their principal food, which 

 is decomposing plant materials. The excreta of worms are deposited 

 occasionally within their burrows but usually on the surface of the 

 ground in little spiral heaps. This material, as D'Auchald has ascer- 

 tained, is richer in nitric acid and calcium carbonate than the original 

 earth. For this reason the soil reaction of the worm excreta shows a 

 somewhat lower H ion concentration. Salisbury, too, found a higher 

 carbonate content in worm excreta, and thus, as compared to the acid 

 soil, a lower H ion concentration. 



Earthworms in this manner not only further the loosening and 

 crumbing of the soil, but they render the nutritive substances of the 

 soil more easily available for plants and thereby increase the store of 

 nutrients. They are most abundant in nearly neutral soil and are 

 rarely found in soils with an acidity higher than pH 6. However, 

 Wherry (1924, p. 309) has shown the presence of Helodrilus lonnhergi 

 in peat soils of 4.7 pH. 



Soil Fauna. — Compared with the activity of earthworms, the 

 importance of insect larvae, ants, centipedes, mites, etc., in the soil is 

 probably very small, but very little is definitely known on this subject. 

 "Springtails" (Collembola), according to Handschin (1925), are per- 

 manent inhabitants of the surface soil, but they withdraw, under 

 certain circumstances, to greater depths, in pursuit of more favorable 

 life conditions. In cemeteries great aggregations of springtails are 

 found upon corpses, at depths of 120 to 180 cm. 



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