122 



DISCOVERY 



organic matter, not to the ash constituents as Liebig 

 had suggested, but to the ash constituents + nitrogen 

 compounds. 



The First Artificial Manures 



With characteristic energy Lawes and Gilbert de- 

 veloped this discover}-. There was then (as nearly 

 always) a shortage of farmyard manure on farms, and 

 agriculturists had for generations sought for substi- 

 tutes, but with little success. Lawes and Gilbert saw 

 that the mixture of ash constituents and nitrogen 

 compounds would form an effective and more con- 

 centrated substitute, obtainable in very large quanti- 

 ties, and of course independently of farmyard manure. 

 Vast deposits of calcium phosphate were kno«-n, and 



Fig. I.— effect of FERTILISERS OX THE GROWTH OF WHE.\T. 

 The left-hand sheaf is the produce of the uamanured plot, the others show 

 the elfects of successive increases in the fertiliser given. 



large quantities of ammonium sulphate were made in 

 the manufacture of coal gas, while potassium compounds 

 could be obtained without difficulty from wood ashes. 

 Lawes and Gilbert therefore proceeded to extend their 

 experiments with these substances, whUe Lawes set 

 up the first factory for producing superphosphate. 

 Farmers gradually came to recognise the value of these 

 artificial manures, and before long they were using 

 many thousands of tons a year. 



When the Lawes and Gilbert experiments began the 

 ordinary yield of wheat was about 20 bushels per acre : 

 before twenty years had elapsed it had become 30 

 bushels per acre, and other crops had advanced in like 

 manner. It cannot, of course, be claimed that the 

 whole cause of the improvement lay in the artificial 

 fertilisers, but this was one of the most important 

 factors. 



The story of the nitrogen compounds in their role 

 of fertUisers reads almost like a romance. Lawes and 

 Gilbert used ammonium salts obtained as a by-product 



in the manufacture of coal gas. Large quantities 

 are now made every year ; just before the war the 

 world's production from all sources (e.g. coke ovens, 

 etc.) was approximately 1,400,000 tons per annum. 

 At Rothamsted and at many other centres i ton of 

 ammonium sulphate brings about an increase of some 

 2h tons of wheat, so that the world's production, if ^ 

 devoted entirely to this purpose, would at this rate 

 be equivalent to 3,500,000 tons of wheat. The quantity 

 sounds immense, but when the large and growing 

 population of the world is considered, it is not really 

 very great. But the ammonium sulphate cannot 

 be given wholly to wheat ; other crops — potatoes, 

 sugar-cane, animal foods, etc. — all need supplies of 

 nitrogen. 



A second source has long been known — nitrate of 

 soda. This comes from ChUe, and just before the war 

 the world's consumption was 2,600,000 tons per annum. 

 In some cases this material is better than, and in 

 some inferior to, ammonium sulphate : on an average 

 it is at least as good. The deposits are large, and the 

 world could look forward with confidence to the future 

 if they were inexhaustible ; but they are not, and 

 although the life of the beds has been put at 200 to 

 300 years, the period is not long in the history of the 

 world. And as our own coal deposits must one day 

 be exhausted, and with them the supply of ammonium 

 sulphate, there is clearly the possibility that our 

 present civilisation might perish solely for want of 

 nitrogen compounds for the food of crops. 



It is a familiar story how Sir \Mlliam Crookes im- 

 pressed these facts on the community in his address 

 to the British Association at Cardiff in 1S90, and how 

 in consequence of his own investigations and those of 

 the technical chemists and engineers who followed him, 

 methods were elaborated for preparing nitrogen com- 

 pounds from the air. Although this discovery also 

 made possible the prolongation of the Great War, and 

 was indeed used for that purpose, its beneficent effects 

 are now showing themselves in the enormous develop- 

 ments taking place in the supply of artificial manures 

 for human food crops. 



Atmospheric Nitrogen 



The best known of the new compounds is calcium 

 nitrate, made in Norway by passing air through an 

 electric arc and oxidising the resulting oxides of nitrogen 

 and absorbing them in water, with formation of nitrous 

 and nitric acids. The mi.xture is fully oxidised and 

 neutralised with lime to produce calcium nitrate. 

 The pure substance readily absorbs moisture from the 

 air — a character which would be objectionable — an 

 impure product is therefore sent out free from this 

 undesirable feature. This product, known as nitrate 

 of lime, has proved very valuable as a fertiliser, and 



