THE ART OF THE SECOND GROWTH 



XV. Life in the soil (Compare Swiss L. F. F. 1904, May and 

 June ) . 



The soil lives like a plant or an animal, since it shows con- 

 tinuous changes of form and of composition. Very little, however, 

 is known of the life and the interdependence of millions of live 

 individuals found in the soil. Certain it seems that tree growth i^ 

 bound on the presence of certain fungi and bacteria living on the 

 roots (Mycorrhiza, important, notably, for shade-bearing trees). 



Most important are the bacteria capable of digesting the 

 nitrogen of the subterranean air. Leguminous plants (Clover, Black 

 Locust) are beset with root knobs, containing bacteria busily 

 engaged in the assimilation of nitrogen. The hypliae of a fungus 

 called Frankia play a similar role on the root knobs of Alder and 

 Sweet Fern. After P. C Mueller, Spruce will grow on poor sand 

 lacking nitrogen if Pine is mixed with it, furnishing nitrogen 

 through its mycorrhiza. 



At Biltmore, Cherry plantations have succeeded surprisingly 

 well when there was an admixture to them of Yellow Pines. 



The maximum number of bacteria is said to be found two feet 

 below the surface of the ground, and none exist below six feet. 

 The number of bacteria per pound of soil varies from one hundred 

 million to two hundred and fifty million. 



Important, too, in plant ecology is the life of the larger animals 

 (worms, insects, nematodes, centipeds) changing the vegetable mat- 

 ter of the soil into manure proper, mixing mineral soil and vegeta- 

 ble matter, increasing the porosity, drainage and aeration and 

 neutralizing the acids of the soil. Shade, protection from wind 

 and sufficient moisture are beneficial to animal life in the soil. 



Paragraph III. Influence of the sylva on the 

 ecolo^ic factors. 



The influence exerted by the forest on "the local climate (heat, 

 air, precipitations, etc.) is dwelt upon in the lectiu-es on forest 

 policy. 



Whilst the ecologic factors shown in the previous paragraph 



exhibit the important influence which the soil has on the tree, there 



exists at the same time, although to a lesser degree, an influence 



of the tree on the soil. This influence is invariably such as to 



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