2 5 o SCIENCE PROGRESS 



fact that they multiply on stirring up of the soil in response 

 to free aeration and also benefit by a neutral reaction 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 promoted by the aeration 

 of the soil and the increased crop which under normal con- 

 ditions 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 could be explained in 

 similar fashion, until the rapidity or otherwise of the formation 

 of the nitrates came to be regarded 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 temperature between 6o° and 

 ioo° C, is accompanied by a complete destruction of the nitrate- 

 making organisms, yet the productive power 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 investigations go to show that 

 plants can feed as freely upon ammonium compounds 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 that 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 compounds 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 the plants they 

 carry are entirely dependent upon ammonia and other un- 

 oxidised sources of nitrogen. 



