430 



SCIENCE. 



[N. S. Vol. XX. No. 509. 



methods of soil management have much to 

 do in favoring or hindering the activities 

 of these helpful little creatures. Some day 

 farmers will come to understand that spe- 

 cialists working in laboratories and who for 

 long hours gaze through the microscope are 

 gradually gaining knowledge of funda- 

 mental importance to them. 



EOOT-TUBERCLE BACTERIA. 



As has been said, there are astonishing 

 numbers of bacteria in all fertile soils, often 

 attaining many millions in a little cube one 

 centimeter (two fifth inch) in diameter, 

 and numerous different kinds have been 

 identified. Each of these kinds has certain 

 special activities as well as peculiar struc- 

 tural characteristics. Most of them are 

 concerned in the decomposition of organic 

 matter and in this sense are destroyers, 

 though we must not by any means call them 

 enemies. If they do cause the fermenta- 

 tion or putrefaction of organic substances, 

 these very operations are widely advan- 

 tageous, since by this means useless ma- 

 terials are put out of the way or, perhaps, 

 are converted into forms of utility. In 

 this way the humus ingredient of fertile 

 soils is mainly supplied. 



But most of these innumerable organisms 

 do not add anything to the soils, they 

 simply change the state or condition of 

 matters previously present. This is true 

 of the nitrifying kinds mentioned above 

 except that they do absorb the carbon di- 

 oxide of the air and contribute carbon to 

 the soil in the solid form— a slim advan- 

 tage, however, in the case of soils already 

 foi'med, since green-leaved plants accom- 

 plish this on a miich larger scale. The 

 nitrifying bacteria are decidedly useful 

 little creatures becaiise of the desirable 

 changes they bring to pass in organic ma- 

 terials present in the soil, not because of 

 any contributions from an outside source 

 through their agency. 



There are, however, other peculiar bac- 

 teria not everywhere present among the 

 hosts of living organisms commonly at 

 home in fertile soils which do aid in such 

 additions. If they are peculiar in dis- 

 tribution they are apparently more peculiar 

 in their mode of action and in the results 

 produced. After much careful investiga- 

 tion and no little difference of opinion it 

 has been conclusively settled that green 

 vegetation can not make direct use of the 

 free nitrogen of the air, though combined 

 nitrogen, mostly in the form of nitrates, 

 is absolutely essential to their life and 

 growth. These nitrates are freely soluble 

 in water and tend to leach away. If, there- 

 fore, we should return to the soil every- 

 thing that grows upon it, we should still 

 find fertility decreasing, unless in some 

 way nitrogen could be added from an ex- 

 traneous source. The store of this sub- 

 stance is not inexhaustible in any soil. In 

 spite of any contributions which could be 

 made from the products of the soil itself, 

 whether these were first fed to animals or 

 not, this essential element of fertility would 

 considerably and constantly diminish. A 

 small amount of ammonia, the product of 

 fermenting matters or of electric combina- 

 tions, is washed from the air by rain, but 

 too little seriously to affect the problem of 

 the maintenance of soil fertility. 



Now we know that soils in nature have 

 not deteriorated. "Without help from man 

 they have, through thousands of years, 

 gained instead of lost their nitrogenous 

 supplies ; though it must be true that great 

 losses have in the meantime occurred by 

 drainage. There must, therefore, be some 

 means of supply, some accession from with- 

 out which more than balances the losses. 

 If green vegetation can not absorb and 

 combine the free nitrogen of the air, of 

 which the quantity is practically unlimited, 

 something else must do so. 



