110 ACTION OF SALTS. 



manence of this action must be sought. That is due to the 

 acid constituent of the salt. That, when the plant decom- 

 posed the salts, was let loose, and now acts on the silicates 

 of the soil. It decomposes these, uniting first with the alka- 

 lies, and thus reproducing itself. It is again decomposed by 

 the growing plant. The same round of action continues. 

 Suppose all this had been witnessed on a worn-out, almost 

 barren field. It is concluded at once, that there is some 

 peculiar virtue in the salt applied, that it is of itself food, or 

 manure; whereas the whole action is in obedience to a gene- 

 ral law applicable to all salts. 



151. Suppose plaster or gypsum has been applied; the 

 effects of a bushel of plaster per acre, or even the one four- 

 hundredth part of one per cent, of the soil, produces eflTects 

 on alluvial land, which shows its good results, as far as eye 

 can reach. It seems almost incredible that so minute a 

 portion of a mineral can act at all, yet how beautifully is 

 this result explained, by the principle that plants decompose, 

 first, this salt; the lime, for plaster is a sulphate of lime, 

 then acts on geine, which is thus rendered soluble ; while the 

 acid, the oil of vitriol or sulphuric acid, immediately acts on 

 silicates. If silicates of alkali exist in the soil, we have now 

 changed sulphate of lime for an alkaline sulphate, and if 

 silicate of lime is also present, the potash or alkali, having 

 been exhausted, plaster of Paris is formed anew. So long 

 as there is in the soil organic matter, this action continues, 

 and will continue till the plant has gradually withdrawn for 

 its own. use, the acid of the salt which was introduced. 



152. Fertility depends wholly on salts and geine. With- 

 out the last there is no fruit formed ; without the salts the 

 geine is locked up, is insoluble. Consider now the applica- 

 tion of this principle, that the base of the salts acts always 

 in one uniform way, its action is wholly upon geine ; that 



