492 



SCIENCE 



[N. S. Vol. XLV. No. 1169 



lets, as dynamite it explodes our mines, and 

 as cyanide it extracts our gold. Under the 

 general name of protein it is an indispen- 

 sable element of animal food, and as animal 

 waste, it is returned to the soil as food for 

 plants. Thus, nitrogen not only gives life, 

 health and prosperity, but it is the most 

 terrible element of death and destruction 

 known to modern warfare. 



In considering nitrogen as an element of 

 food, we may note in passing that the nor- 

 mal adult human body requires a certain 

 amount of the element every 24 hours, and 

 that this must be supplied in the form of 

 meat, eggs and various vegetables, all of 

 which are either directly or indirectly prod- 

 ucts of the soil. Now, every one knows that 

 continual cropping causes a soil to become 

 poor, and chemical analysis shows that the 

 change is due, in part, to the loss of nitro- 

 gen. Fertilizers are applied to the land to 

 increase the yield. 



The problem will become clearer if we 

 take another view. There was a time when 

 fewer people lived in cities, and when this 

 depletion of the soil was largely restored by 

 the refuse from farm animals and even 

 from man himself, from which it follows 

 that the greater portion of the nitrogen re- 

 moved as food for animals was returned to 

 the soil in some other form available for 

 plant growth. 



"We may next consider the complications 

 that arise in dealing with such aggregates 

 of population as our large cities. Suste- 

 nance must still be drawn from the soil, and 

 since nitrogenous waste is no longer re- 

 turned to the land it is clear that the latter 

 is continually being depleted. A single ex- 

 ample will illustrate the point. The sewage 

 of the city of London has an estimated an- 

 nual value of $100,000,000 as fertilizer, and 

 that of any other city would represent an 

 amount corresponding to its population. 

 These drains, of course, must in some way 

 be accounted for if the soil is to- continue to 



produce. What, now, has science done to 

 relieve the situation 1 



When chemical analyses of soils and 

 crops had clearly defined the problem and 

 had told the story of nitrogen, a search was 

 made for some material that could supply 

 the deficit. First, natural guano, as it is 

 called, deposits consisting largely of the 

 dried excrements of birds, and found in 

 such arid districts as Peru, was obtained in 

 as large quantities as possible. But mate- 

 rial of this kind was limited in amount and 

 is now practically exhausted. 



Next, the chemist saw that the waste 

 liquors of the gas works contained combined 

 nitrogen that could serve as plant food, and 

 we are now using in the United States $20,- 

 000,000 worth of ammonium sulphate as 

 fertilizer per year. This, again, is limited 

 because in this country about 80 per cent, 

 of the coke is made in the wasteful "bee- 

 hive" oven in which the vapors are simply 

 burned on the spot. The ammonium sul- 

 phate derivable from those vapors in 1911 

 could have been sold for $24,000,000, to say 

 nothing of the other compounds present. 

 But gas plants can not be operated econom- 

 ically for the ammonium sulphate alone, 

 and our coal beds are not inexhaustible, 

 which shows that some other nitrogen sup- 

 ply must be found. 



The third source of plant nitrogen is the 

 Chile saltpeter bed, from which millions of 

 tons of sodium nitrate have been taken as 

 fertilizer to different parts of the world. 

 This supply, also, is limited. Indeed, con- 

 servative estimates indicate that the depos- 

 its will be exhausted within a relatively 

 short time, which means that man must 

 solve the nitrogen problem in some other 

 way, or he must eventually starve. The 

 necessity for research in this field has been 

 urged for many years, and it is now in 

 order to ask what chemistry has done to re- 

 lieve the situation. 



It has already been stated that nitrogen 



