170 AIR AND LIFE. 



the form of nitrogenous manures. But notice must be taken of the fol- 

 lowing facts. I u the first place, forests — whose age is often very great — 

 goon growing, although for centuries no manure has been added to the 

 soil on which they grow, and the same is true of pasture land. Again, 

 it is a well-known fact that if soil is manured with any nitrogenous 

 manure, it yields more nitrogen in the crop than was given to it in the 

 fertilizer. These facts, ascertained by Boussingault many years ago, 

 suggested the idea that atmospheric nitrogen might play some part in 

 the nutrition of plants, and that in some way or other they might borrow 

 nitrogen from the atmosphere which contains such an amount of this 

 substance. 



To be sure, the atmosphere contains some ammonia (nitrogen and 

 hydrogen combined), but the amount is very small. Mayer, of Heidel- 

 berg, while cultivating in the open air plants whose roots were immersed 

 in nutrient solutions from which nitrogenous compounds were excluded, 

 and protecting them against rain so as to exclude the influence of such 

 nitrogenous comj>ounds as exist in rainwater, obtained a crop contain- 

 ing exactly the same amount of nitrogen as the seeds from which the 

 plants grew — not a milligram more. This shows that the amount of 

 ammonia, or other nitrogenous compounds, which may be borrowed 

 from the atmosphere by plants in a direct manner is quite insignificant. 

 But while plants may obtain very little or nothing from the atmosphere 

 by direct process, the case is entirely altered when indirect processes 

 are allowed to operate. Under such circumstances atmospheric ammonia 

 when combined with the elements of the soil, plays an important part, 

 as shown by Berthelot. Instead of remaining useless, as when con- 

 tained in the atmosphere, it then becomes useful, and is utilized by 

 plants. This process by which atmospheric ammonia combines with 

 soil elements is not a spontaneous one such as that by which hydrogen 

 burning in oxygen forms water — there is no unavoidable chemical reac- 

 tion — it is effected by the agency of definite micro-organisms. While a 

 specimen of soil left to itself under normal circumstances acquires more 

 nitrogen, the same specimen remains unaltered (neither loses nor 

 acquires nitrogen) when it has been previously sterilized by subjecting 

 it to a heat above 105° or 110 c C, by which all micro-organisms are 

 killed. Again, M. Schloesing and Muntz have shown that it is by dif- 

 ferent micro-organisms that the nitrogen contained in nitrogenous 

 organic; matters of arable land is made to combine with other matters, 

 and to form nitrates. One generates ammonia; another transforms 

 ammonia into nitrous acid, which forms nitrates by combining with 

 basic elements, and lastly a third micro-organism transforms the nitrites 

 into nitrates; and this triple process is what is called nitrification — an 

 operation fully investigated by Munro, Winogradsky, and Frankland. 



Thus, by one means or another, atmospheric ammonia may be put 

 within reach of plants and be used by them. But ammonia is however 

 a very small proportion of the nitrogenous contents of the atmosphere. 



