AND ITS STATE OF CHEMICAL COMBINATION. 



ie more soluble would act the more immediately, the less 

 soluble, after a longer period, and only by the aid of a greater 

 quantity of moisture. 



Again, suppose chloride of potassium and sulphate of potash 

 to act in certain experiments only by the potash they respec- 

 tively yield then they may act differently, either because the 

 former salt is more soluble than the latter, or because the plant 

 can extract potash more readily from the one than from the other, 

 or because the one more easily than the other undergoes the 

 necessary decomposition in the soil, before it enters into the 

 plant. 



The reader will detect in this statement several interesting 

 points, which can only be cleared up by careful experiments 

 instituted for the purpose. I introduce the matter to his notice 

 among these general considerations, because, in making up 

 recipes containing a mixture of different saline compounds, it 

 has hitherto been usual to substitute one combination of a sub- 

 stance for another often in consequence of its greater cheap- 

 ness a practice which in reality may not be advisable, and 

 may occasion remarkable discordances in the apparent effects 

 of mixtures which on the whole contain nearly the same pro- 

 portions of those kinds of matter on which plants usually live.* 



3. Tendency to decompose in a given soil. This point de- 

 serves further illustration. Suppose a potash or soda salt to 

 act only by its alkali, and that this alkali acts in combining 

 with silica and carrying it into the plant : it is clear that 

 before it can act in this way it must undergo an important 

 decomposition in the soil which decomposition all soils may 

 not be equally fitted to promote. On two soils, differing in 

 this decomposing power, it is clear that the same substances 

 applied in the same way will produce different effects, which a 

 study of the nature of the soils will alone enable us to account 

 for and explain. 



It scarcely requires any knowledge of chemistry to under- 

 stand that a soil rich in vegetable matter peaty, for example 

 will act upon a saline substance mixed with it differently 



* For a discussion of this topic in another point of view, see page 93. 



