90 METABOLISM 



LECTURE VIII 

 ASH. II 



' THE mineral matters which are present in every plant, so far from being 

 impurities, are quite as important constructive bodies as carbon and nitrogen.' 

 This statement summarizes the chief results obtained in our last lecture. What 

 was demonstrated there by experimental methods may also be established by 

 observation of plants occurring wild and in cultivation. It may be proved 

 without difficulty that the soil whence the materials of the ash are obtained has, 

 quite apart from the water it contains, a most important influence on the 

 development of the plant. Plants do not grow nearly so well in river sand, 

 which is deficient in these mineral constituents, as in garden soil, nor are they 

 so vigorous in their growth when the supply of garden soil is limited in quantity, 

 as it is, for example, in pot cultivation (SACHS, 1892). These examples illustrate 

 sufficiently clearly the vital importance of food-stuffs being supplied in suffi- 

 cient quantity and of the right quality. Further inquiry into the nature of the 

 contents of the soil, regarded as food-stuffs, and the manner of their absorption 

 by the plant will serve only to emphasize this view of the problem. 



Let us consider, first, soil as it occurs in nature formed by disintegration 

 of the rocks, not such a soil as has supported many generations of plants and 

 which has in turn received many of its constituents from them. The characters 

 of the soil have been treated of more fully than we can do here by A. MAYER 

 (1895) and by RAMANN (1893). [A new edition of RAMANN'S work, improved and 

 enlarged, has been recently (1905) published under the title of 'Bodenkunde'.] 

 Since the sedimentary strata have originated from the weathering and aqueous 

 deposition of primitive rocks, all soils must in the long run have been derived 

 from crystalline rock masses. Owing to the composition of these primitive 

 rocks, the soil produced from them must be of varied chemical composition. 

 An examination of granite as a source of soil gives us the following percentage 

 composition (according to GIRARD ; compare MAYER, 1895, II, 12) : 



Silicic acid. Alumina. Ferrous oxide. Lime. Magnesia. Potash. Soda. Water. 

 I. 72-6 15-6 1.5 1-3 0-3 5-0 2-3 o8 



II. 68-6 14.4 5-0 3.9 0-4 2-8 3.4 t-i 



Similar results are obtained when gneiss, mica-slate, and other rocks are 

 analysed ; the differences between them lie merely in the relative amounts of 

 the individual components, the same elements reappearing, but always in 

 varying quantities. When we reduce such a rock to powder we obtain a soil 

 containing the metals potassium,' calcium, magnesium, iron, all important 

 nutritive elements to the plant ; sulphuric and phosphoric acids, however, 

 are not entirely wanting although they may be overlooked, owing to their 

 occurrence in relatively small quantities (the sulphuric acid as gypsum and the 

 phosphoric acid as apatite) (MAYER, 1895, II, i); they are present as a matter of 

 fact in not less quantity than in ordinary soil of cultivation. 



If we now add to such a sample of powdered granite the one element which 

 may be wanting, or present only in very small amount, viz. nitrogen, in the 

 form of nitric acid, attempts to carry out culture experiments in it will lead to 

 very poor results, because the bases are not united with hydrochloric, sulphuric, 

 phosphoric, and nitric acids, as in our water-culture experiments, but chiefly 

 with silicic acid forming for the most part insoluble compounds, more 

 especially since the salts are present usually as double silicates. In consequence of 

 the low temperatures which have prevailed in recent geological time a compe- 

 tition has taken place between the carbonic and silicic acids, which has resulted 

 in the carbonic acid annexing the majority of the bases. These are carried 

 away in the form of soluble compounds, and the rock in consequence is said to 



