Scientific Agriculture. 



138 



[February, 1812, 



With the development of knowledge of 

 the actions of the yeast plant in produc- 

 ing alcohol and the early studies of 

 bacterial change carried out by Pasteur, 

 attention began to be directed to the 

 soil as a possible seat of similar actions. 

 In particular it had always been re- 

 cognised that soil is capable of trans- 

 forming almost any form of organic 

 compound of nitrogen into nitrates. 

 The higher plants, for example, can only 

 take in nitrogen as nitrate and yet is a 

 matter of indifference whether we supply 

 a crop growing in ordinary soil with 

 nitrogen as nitrate, ammonia or the 

 various forms present in such material 

 as farmyaid manure. Clearly some pro- 

 cesses must be at work in the soil trans- 

 forming these compounds into nitrates. 

 This has proved to be the case. In 1877 

 Schloesing and Muntz were able to 

 demonstrate that the change was a vital 

 one as it is suspended when the soil is 

 saturated with chloroform or other 

 antiseptics which will put living or- 

 ganisms out of action by killing them. 



The late Robert Warineton was 

 then working in the Rothamsted 

 Laboratory and immediately attacked 

 the question in the light of Schloesing 

 and Muntz's announcement. After con- 

 firming their result he succeeded in 

 showing that the action must take place 

 in two stages, each broaght about 

 by different organisms, one of which 

 transforms ammonia into nitrite whilst 

 the other completes the oxidation into 

 nitrate. Warington attempted the separ- 

 ation and isolation in the pure stage of 

 two organisms but used the long and 

 tedious method of dilution and before 

 he had succeeded in purifying his cul- 

 tures to the extent that would satisfy 

 himself Winogradsky published his 

 very elegant method of isolation, which 

 depended upon the growth of the orga- 

 nisms upon a non-organic solid medium- 

 silica jelly. Warington's investigations 

 on the processes of nitrification were 

 not confined to the laboratory. He 

 showed that the organisms are almost 

 confined to the surface layers of cultiv- 

 ated soils and his demonstration of the 

 fact that they multiply on stirring up 

 of the soil in response to free aeration 

 and also benefit by a natural re-action 

 and the presence of mineral plant foods 

 went far to explain the efficacy of 

 many of the ordinary operations of 

 cultivation upon a farm. When, for 

 example, land is bare fallowed during a 

 summer the formation of nitrates is pro- 

 moted by the aeration of the soil and 

 the increased crop which under normal 

 conditions follows a bare fallow is largely 

 due to the accumulation of nitrates 



during the previous seasons, provided 

 that they are not washed out of the 

 soil by the winter's rainfall. Many other 

 facts familiar to the practical farmer 

 conld be explained in similar fashion, 

 until the rapidity or otherwise of the 

 formation of the nitrates came to be re- 

 garded as one of the main causes of the 

 fertility of the soil. The point of view has, 

 however, shifted of late in consequence 

 of some other work which is now going 

 on at Rothamsted. It has been shown 

 that whilst a partial sterilisation of the 

 soil, such as follows from heating it 

 during a couple of hours to a temper- 

 ature between 60° and 1C0°C, is accom- 

 panied by a complete destruction of the 

 nitrate-making organisms, yet the pro- 

 puctive pjwer of the soil is enormously 

 increased. It may even be doubled. 

 This increase in fertility depends upon 

 a great speeding up of the rate at which 

 the nitrogenous residues in the soil are . 

 broken down by bacteria to a state of 

 ammonia, though the ammonia is not 

 oxidised to nitrates, Other investiga- 

 tions go to show that plants can 

 feed as freely upon ammonium com- 

 pounds as upon nitrates, indeed many 

 of the soluble organic nitrogen com- 

 pounds like the amino-acids appear to 

 be capable of furnishing the plant 

 with the nitrogen it requires. It had 

 been known before the plants under 

 laboratory conditions can draw their 

 nitrogen from ammonium compounds, 

 but this was not supposed to take place 

 to any general extent in the field. We 

 have now come to regard nitrification 

 as only the end process, the rapidity of 

 which is determined entirely by the 

 rate at which the other preliminary 

 breaking down of organic nitrogen com- 

 pounds to ammonia is taking place, this 

 latter being the significant change which 

 limits the supply of nitrogen to the crop. 

 Moreover, soils are to be found in nature 

 in which, owing to their acid condition, 

 nitrification is at a standstill, so that 

 plants they carry are entirely dependent 

 upon ammonia and other unoxidised 

 sources of nitrogen. 



Warington also accumulated several 

 important facts upon another bacterial 

 process going on in the soil which works 

 in an opposite direction and transforms 

 nitrates into nitrogen gas. Somewhat 

 similar organisms again set free nitrogen 

 gas from ammonia and the organic 

 compounds of nitrogen ; the whole group 

 of these changes, to which the name 

 of denitrification is sometimes given in- 

 volves the impoverishment of the soil 

 by loss of combined nitrogen. Waring- 

 ton investigated the conditions under 

 which these losses are likely to be of 



