MANURES. 241 



years without the use of manure. Of this phosphoric acid, how- 

 ever, a very large proportion is contained in the interior of pebbles 

 and coarse particles, or is in such a state of combination as not to 

 be available ; for plants can take up by their roots only such 

 matters as are exposed on the surface of the particles of soil, and 

 of these even, only such as are sufficiently soluble to yield to the 

 absorptive influence of the moisture which is contained in and 

 about the feeding surfaces of the roots, and the same is true of 

 every other element of plant-food in the soil. Therefore, neither 

 the actual amount of material in the soil, as shown by analysis, 

 nor even the amount which could be dissolved by a strong acid 

 from the surfaces of the particles, is the exact measure of the 

 amount which that soil may be able to supply to the crop ; and, 

 in the absence of absolute knowledge on the subject, all that can 

 be considered as strictly demonstrated is : — 



That the amount of mineral plant-food contained in any soil, 

 in such a position, and in such a condition as to solubility, as to be 

 able to supply the demand of roots, is always limited, — limited, 

 indeed, to such a degree that no soil in the world, which does not 

 receive extraneous supplies by means of inundation or irrigation, 

 can, even through the life-time of a single man, be made to pro- 

 duce maximum crops of any given plant, without the return of 

 some form of manure, either by the feeding of the crop to 

 animals pasturing on the ground, by the death and decomposition 

 of the stems and leaves of the plants, or by the return of animal 

 manure or of some form of mineral manure, which will make up 

 the waste. 



Practice has demonstrated, even this early in the history of our 

 country, that in order to cultivate any land, year after year and 

 generation after generation, with success, it is necessary that 

 manures be added to the soil ; and more careful practice and in- 

 vestigation have shown that the most economical return of manure 

 is such as will supply in the cheapest form the leading mineral 

 elements that have been removed by the crops sold ; — or, rather, 

 the leading ones of those which we have stated above to be 

 necessary in artificial application. 



