6 Professor William Caldwell on 



In olden times natural manures wQve sufficient to feed the 

 lands with the requisite nitrogen to be converted into nitrates, 

 previous to absorption and assimilation by the plants and 

 vegetables. Now we must procure artifieal fertilizers. One of 

 these artificial fertilizers is procured from the gas works. The 

 coal is destructively distilled and the ammonia evolved is absorbed 

 and converted into ammonium sulphate. But the production of 

 ammonium sulphate in the whole world is relatively a small 

 amount. It was only one million tons in 1910 — an amount 

 quite insufficient to meet the demands of agriculture. We must 

 have recourse to Chili saltpetre, a naturally occurring sodium 

 nitrate found on the plains of Chili. For ages the nitrogen of 

 the atmosphere has been absorbed by the soil, and the nitrifying 

 organisms convert it into nitrates. 



In 1830 only 935 tons of saltpetre were used from this 

 source, whilst in 1912 2,500,000 tons were used, and 75% of 

 these imported nitrates were used as artificial fertilizers. In a 

 short space of time, however, these sources of supply will have 

 vanished, and therefore we must apply our scientific knowledge 

 to make good the deficiency. 



Let us see what methods we may adopt. Plants, belonging to 

 the Leguminosae order, have their roots covered with nodules, 

 which are Aast collections of nitrifying organisms, which possess 

 the power of using the free nitrogen of the air and assimilating 

 it in their tissues. At the ordinary temperature without any 

 extraordinary phenomenon these organisms are secreting and 

 storing vast supplies of nitrogen for their own use. Yet these 

 organisms which flourish on the roots of leguminous plants are 

 not specific. They may be cultivated and used by other plants. 

 Land barren and poor in quality may be inoculated with these 

 organisms and made rich and fertile for the production of 

 foodstuffs. 



But theie is another and more prolific method of fixing the 

 nitrogen of the air. The method was suggested by Sir William 

 Crookes at a conversazione of the Royal Society in 1898, 



