THE NEW SOIL SCIENCE. 



75 



the rootlets of the Leguminosje. How this group of bacteria fixes free 

 nitrogen cannot at present be definitely stated, but three alternative 

 explanations have been given. 



1. That under the conditions of symbiosis that exist — the living 

 together of two different forms of life — between the plant and the 

 nodules, the plant is able to fix the free nitrogen in the atmosphere 

 by its leaves. 



2. That the nodule micro-organisms get distributed in the soil and 

 • there fix the free nitrogen, the resulting nitrogenous compounds 



becoming available as a source of nitrogen to the roots of plants. 



3. That free nitrogen is fixed in the course of the development of the 

 organisms within the nodules, and that the resulting nitrogenous- 

 compounds are absorbed and utilised by the host. 



Most authorities agree that the absorption of nitrogenous matter must 

 be by the roots, and if so, then the second alternative named will be the 

 correct explanation. There are, however, some who hold that nitrogen - 

 fixing bacteria are present and active both in nodules on rootlets and in 

 the soil. 



As yet we know comparatively little about the germ life of the soil. 

 Some of these germs have been distinguished like the denitrifying, nitri- 

 fying, and nitrogen-fixing organisms, but there are many other organisms 

 whose actions have been studied which have not yet been classified. The 

 first formation of soil, for example, is to some extent dependent upon 

 bacteria. It has been found that some species of bacteria live upon simple 

 foods, such as free nitrogen and the carbonates. These microbes are of 

 very wide occurrence and can develop on bare rock surfaces. Their 

 secretions, being of an acid nature, help to soften the rocks, and thus 

 they perform the first step in weathering. Again, there are certain soil 

 organisms, as noted under nitrification, that are capable of existing on 

 purely mineral food ; for instance, the various sulphates which form 

 important plant foods appear to be formed in part by bacterial agency, and 

 the deposition of iron phosphates and iron silicates seems also to be aided 

 by bacterial action. As data about the germ life in the soil accumulate, 

 it becomes more apparent that the functions of bacterial life in the soil 

 are not limited to the decomposition of organic matter and the preparation 

 of suitable forms of nitrogen for the uses of plants, but extend also to 

 the preparation of mineral food materials for plants. 



The entire fertility of soil is stated to be due to bacteria. The 

 fact is now recognised that a sterile soil which is devoid of germ life is an 

 infertile one. Two soils, for instance, may have the same chemical com- 

 position, yet one may be fertile and the other practically barren ; and the 

 rational explanation seems to be that the one has abundant germ life while 

 the other is deficient in soil bacteria. It has long been known that humus 

 and the nitrogen content of the soil were important factors in soil fertility, 

 and the humus theory was one that was common in the early part of the 

 nineteenth century, till it was replaced by Liebig's mineral theory. The 

 "new soil science," however, seems to favour the former more than the 

 latter. Humus is the workshop in which soil bacteria work, as it contains 



