Scientific Agriculture. 



248 



[September, 1909. 



organisms are rarely met with in 

 the British Isles, but not infrequently 

 soils are found on which such special 

 crops as Lucerne, which requires a race 

 of bacteria considerably differentiated 

 from that which is found in Clover 

 nodules, fail to nodulate and grow pro- 

 pedy. In such cases a preliminary 

 inoculation of the Lucerne seed may 

 prove very effective in establishing the 

 crop, which otherwise fails, although 

 Clover will grow freely on the same 

 land. Examples have been observed of 

 the value ci inoculating Lucerne seed 

 when that crop is being sown in a district 

 in which it has not hitherto been grown. 



But in most of our soils, where Clover, 

 Beans, and Peas have been cultivated 

 in the regular way, the nodule organism 

 is present, and the leguminous crop 

 nodulates and begins to fix nitrogen 

 without any artificial inoculation. In 

 these cases the gain from inoculation 

 is not likely to be large, 10 to 20 per cent, 

 at the outside — a quantity only per- 

 ceptible by careful experiment— and its 

 existence must depend either upon some 

 advantage to be derived from early 

 inoculation or upon the establishment of 

 an improved race of bacteria, more 

 active in fixing nitrogen than those 

 normally in the soil. Neither of these 

 propositions has been established, and, 

 though the work is still being actively 

 pursued, a practical return for inocula- 

 tion on ordinary field or garden soils is 

 not yet to be expected. The nodule 

 bacteria, either pure or mixed with other 

 organisms, have not been induced to 

 enter into partnership with the ordinary 

 non-leguminous plants, which is not to 

 be wondered at, considering the unlimit- 

 ed opportunities the latter have had in 

 ordinary soil of trying the experiment 

 for themselves. An extensive experi- 

 ment tried upon Tomatoes seemed to give 

 an increased yield after inoculation, but 

 this was shown to be due to the nutrient 

 salts introduced by the culture medium, 

 for a similar increase was produced when 

 the same culture medium was given to 

 the plants after it had been first 

 sterilised by boiling. 



Turning now to other soil bacteria 

 which fix nitrogen without the in- 

 tervention of leguminous plants, mention 

 must be made of the organism dis- 

 covered by Beijerinck and called by him 

 Azotobaeter. This organism is widely 

 distributed, having been isolated at 

 Rothamsted from virgin soils obtained 

 from all parts of the world. In order to 

 fix nitrogen it must be supplied with 

 some form of carbohydrate, by the 

 oxidation of which it derives the energy 

 necessary to bring the nitrogen into 

 combination. Carbonate of lime as a 



base in the soil is also necessary for its 

 growth. 



The history of a certain piece of land 

 illustrates the dependence of nitrogen- 

 fixation by Azotobaeter on supplies of 

 carboy hd rates in the soil at Rothamsted. 

 The land in question has been allowed 

 to run wild for the last 25 years, and has 

 been gaining nitrogen during that period 

 at the rate of nearly 50 lbs. per acre per 

 annum, whereas the adjacent arable 

 land has lost rather than gained nitrogen. 

 On the " wild " land the vegetation every 

 year is allowed to die back, thus the 

 soil is continually supplied with com- 

 pounds of carbon by the oxidation of 

 which Azotobaeter is enabled to fix 

 nitrogen ; on the arable land, however, 

 where the crop is almost wholly removed, 

 there is no return of carbon compounds 

 to the soil. 



Certain pot experiments have shown 

 that the application to soil of sugar, a 

 carbon compound containing no nitrogen, 

 is followed by a gain of nitrogen, of 

 great benefit to succeeding crops, but 

 attempts to obtain similar results in the 

 field at Rothamsted have so far yielded 

 negative results. In the Mauritius, how- 

 ever, the treatment of the soil with 

 Molasses has been found beneficial to 

 the following crops, and Azotobaeter 

 has been also shown to be abundant in 

 the soil. 



The piece of "wild" land at Rotham- 

 sted supplies the clue to the accumula- 

 tions of nitrogen in such virgin soils as the 

 black lands of the North-west of America, 

 the Russian Steppes, the Argentine, 

 Pampas, &c, which are naturally 

 occupied by a luxuriant, grassy vegeta- 

 tion. However long such land has been 

 growing grass, the plants themselves 

 could not increase the stock of nitrogen ; 

 they could only take up what was 

 originally in the soil and restore it again. 

 But when the carbonaceous matter they 

 have assimilated from the atmosphere 

 falls back to the soil, material is provided 

 by means of which Azotobaeter, present 

 in all these soils, can proceed to fix 

 nitrogen. The low ratio of carbon to 

 nitrogen in the organic matter of these 

 virgin soils is in itself evidence that very 

 active oxidation of the vegetable debris 

 had been going on ; in this respect the 

 organic matter of the virgin soils re- 

 sembles that which had accumulated on 

 the "wild" plot at Rothamsted, but 

 differs from that which is found in soils 

 devoid of Azotobaeter. The gain in 

 fertility of land laid down to grass, 

 where a mass of stubble and roots 

 accumulate, is also probably in part the 

 work of this nitrogen-collecting micro- 

 organism, 



