which has got deteriorated by heavy cropping is recuperated 

 by growing on it a crop of clover, arahar, or some other 

 leguminous crop. The power of fungi to make use of the 

 free nitrogen of the air has been demonstrated by various 

 observers. Probably Ammonium nitrite is formed first (N2 + 

 2HO = NH 4 NOz) and afterwards the more complex organic 

 compounds, which ultimately go to form the protoplasmic 

 matter of which the fungi are composed. The multiplication 

 of the fungi (bacteria) at the root-nodules of leguminous 

 plants has been chiefly studied. Hellriegel was the first to 

 point out that the root-nodules in different plants are caused 

 by different microbes. It is now also known that the root- 

 nodules are not confined to the leguminosae, and probably 

 bacteria have a wide influence in feeding plants with nitrates. 

 But it is only the bacteria that cause root-nodules that have 

 a marked effect in enriching the soil, though some nitrogen 

 is accumulated in the soil by all fungi whether they live on 

 roots or not. Berthelot's experiments went to show that 75 

 to 100 Ibs. and in some cases over 900 Ibs. of nitrogen per 

 acre was accumulated by Bacteria and that humus rather 

 hindered than helped accumulation of nitrogen from this 

 source. Fixation of nitrogen in the soil goes on by day and 

 by night, but more actively in day time, and in high tempera- 

 tures (50 to 104 F), though all nitrification due to Bacteria 

 ceases at 230 F. Free access of air, and moisture from 12 

 to 15 per cent., are also most helpful. Though nitrifying 

 Bacteria accumulate largely in the roots of leguminous plants, 

 soils in which leguminous crops have recently grown are not 

 helpful to nitrification, thus there is a limit to the accumula- 

 tion of nitrogen by the growth of leguminous crops, and it is 

 not possible to go on increasing the fertility of soils by taking 

 one leguminous crop after another. Rotation therefore is 

 necessary. It is not possible to go on increasing the quantity 

 of microbes in a given quantity of any culture-medium and 

 soil is no exception to the rule. Even manures by keeping 



