LECTURE XIII. 



Exact chemical constitution of soils — their oi^anic constituents — Analysis of soils — Compo* 

 sitiou of certain characteristic soils— Physical characters of soils. 



In the two preceding lectures we have considered the general consti- 

 tuiiuv. and origin of soils, and their relation to the geological structure of 

 tiis country in which they are found, and to the chemical composition of 

 the rocks on which they rest. . We have also discussed some of the 

 causes of those remarkable differences which soils are known to present 

 in their relations to practical agriculture. But a more intimate and pre- 

 cise acquaintance with the chemical constitution of soils is not unfre- 

 quently necessary to a complete understanding of the causes of these dif- 

 ferences — of the exact effect which its chemical constitution has upon the 

 fertility of a soil — and of the remedy which in any given circumstances 

 ought to be applied. 



Some persons have been led to expect too much from the chemical 

 analysis of a soil, as if this alone were necessary at once to explain all its 

 qualities, and to indicate a ready method of imparting to it every desir- 

 able quality, — while others have as far depreciated their worth, and have 

 pronounced them in all cases to be more curious than useful. — [Boussin- 

 gault, ' Annal. de Chim. et de Phys.' Ixvii., p. 9.] The truth here, as 

 on most other subjects, lies mthe middle between these extreme opinions. 



If you have followed me in the views I have endeavoured to press upon 

 you in regard to the necessity of inorganic food to plants — which food 

 can only be derived from the soil, and which must vary in kind and 

 quantity with the species of crop to be raised, — you will at once perceive 

 that the rigorous analysis of a soil may impart most valuable knowledge 

 to the practical man in the form of useful suggestions for its improvement. 

 It may indeed show that to apply the only available substances to the 

 soil which are capable of remedying its defects, would involve an expense 

 for Avhich, in existing circumstances, the land could never give an equiva- 

 lent return. Yet even in this latter case the results of analysis will not be 

 withoift their value to the prudent man, since they will deter him from 

 addiiig to his soil what he knows it already to contain, and will set him 

 upon the search after some more economical source of those ingredients 

 which are likely to benefit it most. 



It will be proper, therefore, to turn our attention briefly to the conside- 

 ration of the exact chemical constitution of soils. 



§ 1. Of the exact nature of the organic constituents of soils, and of the 

 mode of separating them. 

 We have already seen in Lecture XL, p. 229, that all soils contain a 

 greater or less admixture of organic — chiefly vegetable — matter, the 

 total amount of which may be very nearly determined by burning the 

 dried soil at a red heat till all blackness disappears (p. 233). But this 

 vegetable matter consists of several different chemical compounds, the 

 nature and rela<ive weights of which it is occasionally of consequence to 

 be able to determine. 



