In conclusion, the nature and characteristics of nitrate of soda as a manure may be briefly summed up as follows:-- 1. It is a whitish, crystalline salt, extremely soluble, and is quickly diffused in the soil. It should contain 95 per cent of pure nitrate of soda--_i.e._, 15-1/2 per cent of nitrogen, equal to about 19 per cent of ammonia. 2. Next to sulphate of ammonia, it is the most concentrated nitrogenous manure; the relative quantities of nitrogen these two manures contain being as three is to four. 3. It contains its nitrogen in the most valuable and readily assimilable form--_i.e._, as _nitric acid_, the form into which all other forms of nitrogen have first to be converted before they become available for the plant's uses. 4. That, at present market prices, nitrate of soda may be safely affirmed to be the cheapest form of nitrogenous manure. 5. That nitrate of soda, in addition to its direct value as a manure, probably exercises a slight influence on the mechanical properties of the soil, by increasing its compactness and water-absorbing capacities; that it further tends to promote deep roots, and thus to increase the soil area whence the plant may derive its nourishment, at the same time rendering the plant more able to withstand the injurious influence of drought. 6. That a plentiful supply of the other manurial constituents should be present in the soil, if nitrate of soda is to exercise its full value. 7. That it may be profitably applied in the case of nearly all kinds of crops, but that great care should be taken as to the mode of its application. That this should be almost invariably as a top-dressing, and that it should be applied in several doses if possible. 8. That its effects can be regarded as lasting only during the first year after application. FOOTNOTES: [202] This substance is also largely known under the name Chili saltpetre, to distinguish it from potassium nitrate or common saltpetre. [203] See Appendix, p. 351. [204] We may remind our readers that these nitrate deposits were largely the cause of the late war between Chili and Peru, which resulted in the cession to Chili by Peru of the province of Tarapaca, where the most important deposits are situated. [205] The other nitrate deposits are found in the provinces of Antofagasta and Atacama, and a certain amount of the refined article is exported from these places. The amount, however, is inconsiderable as compared with that which comes from the province of Tarapaca. [206] See his elaborate article on the Geology of Bolivia and Peru, published in the 'Quarterly Journal of the Geological Society' for November 1860. [207] The source of the boracic acid is probably volcanic.