THE NEW SOIL SCIENCE. 



77 



It must be borne in mind, even at the expense of reiteration, that we 

 have yet much to learn regarding the germ life that exists in the soil, 

 and many questions await a definite answer. Are soil bacteria the great 

 preparers of food material for plants ? With manures do we return plant- 

 food material to the soil or simply food for soil bacteria ? Is cultivation 

 of the soil in the first instance the growing, sowing, and feeding of soil 

 germs ? Can manuring with chemical agents be replaced by inoculating 

 the soil with specific germs ? 



The value of the bacterial point of view will have to be judged by the 

 explanation it affords of the rationale of practical details, for instance, 

 why is it that gardeners do not care to use fresh manure ? The old 

 reply used to be because the nitrogenous constituents in such a manure 

 are not available for the use of plants, or that the chemical reactions 

 which took place on decomposition were hurtful to the roots of plants. 

 Bacteriological science has shown that fresh farmyard manure abounds 

 in denitrifying bacteria, and their introduction into soil rich in organic 

 matter will therefore do more harm than good unless the conditions are 

 such that the products evolved can immediately be utilised. Again, the 

 action of the nitrifying organisms has made clearer the theoretical explana- 

 tion of many questions connected with the washing out of nitrates from 

 fallows, the various processes occurring in the upper soil as contrasted 

 with subsoil, and the advantages of autumn and winter sowing. 



Before the germ life of the soil attracted attention it was demon- 

 strated that the majority of agricultural and garden crops appropriated 

 their nitrogen in the form of nitrates derived either from manures 

 applied to the land or from nitrogenous substances in the soil which 

 were converted into nitrates by the action of nitrifying bacteria. No 

 hard and fast rules, however, can be laid down, for a few plants are able 

 to utilise to a limited extent nitrogen in the form of ammonia or humafce 

 of ammonia, and some others are able to secure their nitrogen supply by the 

 action of micro-organisms which develop tubercules upon their roots, and 

 thus enable them to appropriate supplies of nitrogen which may be secured 

 from the air in the soil or from nitrogenous compounds decomposing in it. 

 These bacterioidal nodules are common to one family in the order 

 Leguminosce — i.e. the Papihonacece — but the faculty is also possessed by 

 the other two families which form with it the order. Apart from the 

 Leguminosce, the organs in question are found in Alnus, ElceagriUs 

 angtistifolia, Hippophac, and Podocarptis, all of which are able thereby 

 to thrive in soils destitute of nitrogen. Bacterial life in the soil seems 

 also to have at least two different ways of reclaiming dissipated free 

 nitrogen : one is by the nodule bacteria, and the second is by bacteria in 

 the soil, for it has been found that soil entirely free from all common 

 plants but containing certain kinds of bacteria if allowed to stand in 

 contact with the air will slowly gain in its nitrogen content. The 

 nitrogen compounds in such a soil are manufactured by the bacteria in 

 the soil, for if they are not present they do not accumulate. It further 

 appears that as a rule this fixation of nitrogen is not performed by any 

 one species of bacteria, but by two or three of them acting together. 

 The fact that fixation of free nitrogen occurs in the soil affords a new 

 explanation of the activity of the nodule bacteria, viz. that the actual 



