The light which these discoveries throw upon the extremely complicated question of the fertility of the soil is considerable, as it follows that no soil can be regarded as really a fertile one in which the process of nitrification does not freely take place. They furthermore explain many facts, hitherto observed but not well understood, with regard to the action of different nitrogenous manures. _Ash Constituents of the Plant._ We now come to consider the present state of our knowledge on the essentialness of the ash or mineral portion of the plant. While a portion of the plant's substance which, up to Liebig's time, had obtained little notice, it has, since the publication of his famous "mineral" theory, obtained an ever-increasing amount of investigation. Up till 1800 practically nothing was known of the function of the ash constituents. In 1802 de Saussure wrote that it was unknown whether the constituents of many plants were due to the soils on which they grew, or whether they were the products of vegetable growth. Some two years later, however, he was enabled to carry out a number of experiments which really placed the subject on a firm scientific basis. The essentialness of the ash constituents was only, however, placed beyond all doubt by Wiegmann and Polstorff's researches, carried out in 1840. Reference has already been made to the great stimulus given to research by the promulgation of Liebig's mineral theory. _Methods of Research._ In epitomising the vast amount of work carried on since 1840, with the view of ascertaining the essentialness of the various substances found in the ash of plants, two methods of experimentation have been followed. _Artificial Soils._ The first of these two methods was that adopted in the famous experiments, carried out by Prince Salm-Horstmar, which have done so much to further our knowledge on this question. It consisted in growing plants on an artificial soil--formed out of sugar-charcoal, pulverised quartz or purified sand--to which were added the different food constituents. _Water-culture._ While the results obtained by Prince Salm-Horstmar by this method were of a most valuable nature, subsequent experimenters have abandoned his method for the other method--viz., "water-culture." The medium used in this process is pure water; and it is from experiments carried out in water-culture that much of our present knowledge, in regard to the relation of the ash constituents to the plant, is due. The names of those who have worked in this department are very numerous. Among them may be mentioned Knop, Sachs, Stohmann, Nobbe, Rautenberg, Kuehn, Lucanus, W. Wolff, Hampe, Beyer, E. Wolff, P. Wagner, Bretschneider and Lehmann. The results obtained by these and other experimenters have demonstrated the following facts.