350 THE EEGOLITH 



the needed constituents are supplied. Investigations have, 

 however, shown that, though varying in different species, the 

 proportional amount of food demanded by plants which can be 

 supplied by the atmosphere and meteoric waters is very large. 



It seems to be now pretty well conceded that of all the con- 

 stituents found in soil aside from moisture, only potash, lime, 

 magnesia, phosphoric and sulphuric acids, can be considered 

 absolutely essential as plant food. The ash of all plants, to be 

 sure, contains silica, soda, — and it may be iron and other min- 

 eral ingredients, — but such are to be regarded as accidental 

 rather than otherwise. Of the constituents enumerated as 

 essential, magnesia and sulphuric acid are almost invariably 

 present in sufficient quantities, while potash, lime, and phos- 

 phoric acid, even though sufficiently abundant in a virgin soil, 

 are liable to exhaustion under the ordinary methods of culti- 

 vation. The source of these materials has been shown in the 

 previous pages and need here be only touched upon. The 

 potash and the lime must have come originally from the de- 

 composition of potash-lime-bearing silicates, as the feldspars and 

 micas, amphiboles and pyroxenes. The original source of the 

 phosphoric acid was undoubtedly the apatite of the eruptive 

 rocks, though now to be found in bones and skeletons of ani- 

 mals, whose remains become entombed in sedimentary rocks 

 of all ages. How small and proportionally insignificant are 

 the percentages of these constituents in any soil, fertile or 

 barren, is shown in the table on p. 351,^ in which are given the 

 general average composition of a large number of soils, seden- 

 tary and transported. The sulphuric acid, which is not men- 

 tioned in this table, rarely amounts to more than from 0.05% 

 to 0.5% when calculated as sulphuric anhydride (SOg). 



So small, comparatively, are these percentages, that it is rare, 

 indeed, to find a soil which on complete analysis will not be 

 shown to contain them in sufficient quantity. The varying 

 degrees of fertility in such cases are due then, not to differ- 

 ences in ultimate composition, but to difference in combination 

 of these elements whereby they are or are not available for 

 plant food, and to physical and climatic differences as well. 

 Naturally a growing plant can take up only that which is 

 soluble by the means at its command. A high percentage of 



^From Part A, Vol. II, Part II, Chemical Analyses, Geological Survey 

 of Kentucky, p. 113. 



