1892.] on Micro-organisms in their Belation to Chemical Change. 521 



The Bacteria connected with the Soil. 



It is, however, with regard to the bacteria connected with other 

 industries than those of alcoholic fermentation that our knowledge 

 has particularly advanced during the last" few years. Thus some of 

 the most important phenomena in agriculture have recently received 

 a most remarkable elucidation through the study of bacteria. 



Scientific agriculturists are generally agreed that one of the 

 most important plant-foods in the soil is nitric acid, indeed they in- 

 form us that if a soil were' utterly destitute of this material it would 

 be incapa,ble of growing the barest pretence of a crop either of corn, or 

 of roots, or of grass, even if the soil were in other respects of the 

 most superb texture, however favourably it might be situated, how- 

 ever well drained, tilled, and supplied with the purely mineral 

 ingredients of plant-food, such as potash, lime, and phosphoric acid. 



Yet, notwithstanding the commanding importance of this sub- 

 stance nitric acid to vegetation, it is present in ordinary fertile soils 

 in but little more than homoeopathic doses. 



Tbese facts are gathered from those grand experiments which 

 have during the past half century been going on at Rothamsted 

 under the direction of Sir John Lavves and Dr. Gilbert, and which 

 have rendered the Hertfordshire farm a luminous centre of the whole 

 agricultural world. 



From these experiments it appears that sometimes there is in 

 fertile soil under 1 part, and often under 10 parts, of nitrate of lime 

 per million of soil. 



Indeed, in order to detect and estimate these minute quantities, the 

 most refined methods of chemical analysis have to be called into 

 requisition. [Demonstration of the presence of nitric acid in soil by 

 diphenylamine test.] 



Now the cause of such minute quantities only of nitric acid being 

 found in soils is due partly to this material being washed away by 

 the rain and partly to its being so eagerly taken up by plants for the 

 purposes of nutrition ; for it has long been known that by suitable 

 means the quantity can be enormously increased if no vegetation is 

 maintained, and the ground properly protected from rain. The soil 

 in fact, under ordinary circumstances, continuously generates this 

 nitric acid from the various nitrogenous manures which are applied 

 to it, and it is in the form of nitric acid that the nitrogen of manures 

 principally gains access as nutriment to the plant. 



It was in the year 1877 that two French chemists, Schloesing and 

 Miintz, showed that this power of soils to convert the nitrogen of 

 nitrogenous substances into nitric acid was due to low forms of life — 

 to micro-organisms or bacteria. The proof which they furnished of 

 this statement was of a very simple character, and consisted essenti- 

 ally in demonstrating that this production of nitric acid, or process 

 of nitrification, as it is generally called, is promptly inhibited or 

 brought to a standstill by all those materials which have the property 



Vol. XIIL (No. 86.) 2 n 



