NOVEMBEK 5, 1915] 



SCIENCE 



633 



in the last half century, the relation of bac- 

 teria to soil cultivation has been carefully 

 studied. There are myriads of bacteria in 

 the soil and from the standpoint of the agri- 

 culturalist, they naturally fall into two classes : 

 Those concerned in increasing the fertility of 

 the soil, and those which deplete the soil of the 

 necessary constituents to sustain plant life. 

 Two groups of bacteria (Clostridium and 

 Azotobacter) have the ability of taking nitro- 

 gen directly from the air and incorporating 

 it into their own protoplasm, and it is believed 

 that these organisms are responsible to a large 

 extent for keeping the nitrogen content of a 

 soil nearly constant, as they replace that lost 

 from the soil by the removal of crops, dentrifi- 

 cation (which will be considered later) and 

 seepage. Another method by which the nitro- 

 gen content of a soil is increased is that of a 

 symbiotic relationship between certain bacteria 

 and plants; Ps. radicicola has the ability of 

 I)enetrating the roots of the legumes and form- 

 ing nodules thereon. Under this special con- 

 dition, the bacteria above mentioned are able 

 to " fix " atmospheric nitrogen. This element 

 is rarely, if at all, available to plants in the 

 form of ammonia, but must be built into 

 nitrates first. Here other types of bacteria 

 (nitrifying organisms) take the " fixed " nitro- 

 gen and build it up to nitrites and nitrates. 



It is upon these facts that the excellent re- 

 sults of crop " rotation " depend and this is 

 responsible for the success in intensive farm- 

 ing. In the " rotation " of crops the farmer 

 sows the land between " money crops " with 

 one of the legumes (peas, beans and especially 

 ■ clover are used) and these plants are those 

 above referred to as being capable of a sym- 

 biotic relationship with the nitrogen-fixing 

 bacteria. When these legumes have grown up, 

 they are ploughed under, thus returning much 

 nitrogen, which has been accumulated during 

 their growth to the soil, and replacing that lost 

 by the removal of the " money crops." This 

 process is known as " green manuring " and 

 it is fast becoming the practise to spray the 

 seeds of these legumes with pure cultures of 

 nitrogen-fixing organisms, so that the proper 



bacteria may be present when the seeds are 

 planted. 



The question naturally arises that if nitro- 

 gen must be in the form of nitrates in order 

 to be available to plant life, why is it that we 

 often add materials as fertilizers which con- 

 tain nitrogen in a much more complex form? 

 Bacteriology again answers this query. These 

 complex nitrogenous substances which we add 

 as fertilizers are broken down by the saprogenic 

 and saprophilic bacteria to the ammonia stage 

 from which they are built up, through the 

 agency of the nitrifying bacteria to the nitrite 

 and finally nitrate form, and are now in a con- 

 dition to be assimilated by plant life. Through 

 the agency of other bacteria cellulose, starches 

 and sugars are transformed to organic acids 

 and carbonic acid, which attack otherwise in- 

 soluble minerals and get them into solution 

 ready for absorption by the plant. Bacteria 

 are also involved in the oxidation of hydrogen 

 sulphide and iron compounds. 



So far, we have discussed only those organ- 

 isms which are beneficial to plant life, but there 

 are others which are detrimental and these 

 naturally fall into two classes; first, those bac- 

 teria which are pathogenic to plants, like those 

 causing tobacco and cucumber wilt, potato rot, 

 pear blight, etc., second, those bacteria which 

 are indirectly harmful to plants because they 

 rob the soil of the constitvients necessary to sup- 

 port plant life. These latter fall in the group 

 above referred to as denitrifying organisms 

 and they have the ability to break down ni- 

 trates, either partially or wholly to the am- 

 monia stage, thus robbing the plant of what 

 was originally available nitrogen. This type 

 of organism was much feared by the agricul- 

 turalist for some time after its discovery, but 

 it is now known that it requires an entirely 

 diilerent environment from the nitrifying 

 organisms and that it seldom occurs in a well- 

 ventilated soil. 



The single view of the soil physicist must 

 appear untenable, for while plants need water, 

 air and a proper temperature, these are not 

 the only requirements to be met, for if it is 

 desired to have a fertile soil to insure our crops, 

 " plant food " must be present and also the 



