282 WHY REFINED ANALYSES ARE NECESSARY. 



constitution. This will appear by referring to three important princi- 

 ples established in the preceding ".lectures. 



l"^. That the ash of plants generally contains a certain sensible pro- 

 portion of ten or twelve different inorganic substances (pp. 216 to 221). 



2°. That they can, in general, only derive these substances from the 

 soil, which must, therefore, contain them (p. 181). And — 



3°. That the fertility of a soil depends, among other circumstances, 

 upon its ability to supply readily and in sufficient abundance all the in- 

 organic substances which a given crop requires (p. 228.) 



Now the quantity of some of these substances which is necessary to 

 plants is so very small, that nothing but a refined analysis of a soil is 

 capable, in many cases, of determining whether they are present in it or 

 not — much less of explaining to what its peculiar defects or excellencies 

 may be owing — what ought to be added to it in order to render it more 

 productive — or why certain remarkable effects are produced upon it by 

 the addition of mineral or animal manures. 



Thus, for example, half a grain of gypsum in a pound of soil indicates 

 the presence of nearly two cwt. in an acre, where the soil is a foot deep, — 

 a quantity much greater than need be added to a soil in which gypsum 

 is almost entirely wanting, in order to produce a remarkable luxuriance 

 in the red clover crop. In 100 grains of the soil, this quantity of gyp- 

 sum amounts only to seven-thousandths of a grain-— {j^^^, or 0-007 

 trs.) — a proportion which only a very carefully ..conducted analysis 

 ould be able to detect, and yet the detecting of which may alone be able 

 to explain the unlike effects which are seen to follow the application of 

 gypsum to different soils. 



Again, the phosphoric acid is a no less necessary constituent of the 

 soil than the sulphuric acid contained in gypsum. This acid is gener- 

 ally in combination either with lime, with oxide of iron, or with alu- 

 mina — and, as it is much more difficult even to detect than the sulphuric 

 acid, requires more care and skill to determine its quantity with any 

 degree of accuracy, — and is generally present even in fertile soils in a 

 still smaller proportion — it is obvious that safe and useful conclusions can 

 be drawn only from such analyses as have been made rigorously, accord- 

 ing to the best methods, and with the greatest attention to accuracy. 



There are cases, no doubt, where a rough analysis may be of use, 

 where the cause of peculiarity is at once so obvious that further research 

 is unnecessary — as where mere washing with water dissolves out a 

 noxious substance, such as sulphate of iron (green vitriol). But such 

 cases are comparatively rare, and it more frequently happens, that the 

 cause of the special qualities of a soil only begins to manifest itself when 

 a carefully conducted analysis approaches to its close. I shall, therefore, 

 briefly describe to you the methods to be adopted, in order to- arrive at 

 these more accurate experimental results. [As these methods of analysis 

 involve considerable detail, I have transferred them to the Appendix.— 

 See Appendix, p. 25.] 



