THE SOIL AND ITS EFFECT ON PLANT GROWTH. II 



Soil Nitrates and Bacteria. 



We may now consider some investigations which, when con- 

 ckided, may have far-reaching effects on the important practical 

 question of increasing the fertihty of the soil. It is well known 

 that nitrogen is essential to plant growth, but our knowledge of 

 the process going on in the soil by which nitrogen is supplied to 

 the plant in a form in which it can be absorbed by the roots is 

 still far from complete. We know, however, that an essential 

 and important part in the process is played by bacteria, in con- 

 verting nitrogen compounds into soluble nitrates which can be 

 absorbed by the plant roots. The investigation about to be 

 described was undertaken during last summer in order to throw 

 light on the relation that exists between the number of bacteria 

 and the amount of nitrates present in the soil at any given time. 

 Eight research workers were engaged in the enquiry, which 

 extended over several periods, each lasting about four consecu- 

 tive days and nights. Samples of soil were taken from the field 

 at intervals of two hours, and in the laboratory the number of 

 bacteria present was counted and the quantity of nitrates as- 

 certained. The result showed that a close relation exists between 

 nitrates and bacteria. The amount of nitrates and the number 

 of bacteria both fluctuated considerably and rapidly, and the 

 fluctuations in the two groups seemed to be connected. As the 

 bacteria increased in numbers, so did the nitrates in amount, 

 and vice versa. This gives new light upon the nature of the 

 soil changes which control the supply of nitrogen to the plant. 

 There appear, however, to be two influences at work in reducing 

 the supply of nitrates in the soil ; one limits the numbers of bac- 

 teria responsible for nitrate production, and the other influences 

 directly the amount of nitrates available for the use of the plant. 

 The effect of the inter-relation of these two influences is to keep 

 down the supply of soluble nitrates. The whole question, how- 

 ever, needs much study, and further investigations to this end 

 are in progress. 



On another aspect of plant nutrition we are lacking in informa- 

 tion, namely, the exact form in which nitrogen is absorbed by the 

 plant. It is known that nitrogen compounds present in the soil, 

 and supplied to it in the form of manure, go through a series of 

 changes culminating in the formation of soluble nitrates, and the 

 view generally held is that only in this form — or, in special 

 circumstances, in the form of ammonia — can nitrogen be absorbed 

 by the roots of the plant. This view has never been scientifically 

 proved, and although it may be correct in the case of plants 

 grown in pots under somewhat artificial conditions, it does not 



