The Romance of Modern Chemistry

CHEMISTRY AND AGRICULTURE which must be added to the soil have been definitely ascertained and their effects on various crops have been studied. The waste products of the animal body contain much of the material which is required for the enrichment of the soil, and hence farmyard dung is an excellent general manure. Guano, the dried excrement of sea-birds, also contains nitrogen, phosphate, and potash, and so has been largely employed for the same purpose. Occasionally, for special crops and in special circumstances, it becomes necessary to supply to the soil a particular plant food— nitrogen, for instance. In this case one may use as manure either sulphate of ammonia from the gasworks, or nitrate of soda from Chili. The nitrogen from ammonium sulphate is not so rapidly available for the use of the plant as the nitrogen from the Chili saltpetre, inasmuch as the ammonia in the former has first to be interviewed by the nitrifying bacteria and converted into nitrate. The approaching exhaustion of the Chili saltpetre beds has stimulated chemists to discover ways and means of utilising the nitrogen in the atmosphere for plant-feeding purposes, and the reader may remember the reference made in a previous chapter to the work already done in this direction. At the high temperature of the electric arc the nitrogen and oxygen of the atmosphere combine to a small extent, and the compound so formed is easily converted into nitric acid. As already indicated, the small amount of nitric acid occurring in the atmosphere is to be traced to the influence of electric discharges, so that the method now in vogue for the manufacture of nitric acid from the atmosphere depends really on the production of artificial lightning. 224