112 THE SOIL. 



luxuriantly as if every particle of the turf were tliorouglily 

 saturated with nutritive substances. 



Hence, the higher produce obtained from the compara- 

 tively poorer soil proves that it is only the surface of the 

 soil, containing the nutritive elements, which is eJBTective ; 

 that the fertility of a soil is not proportionate to the 

 quantity of nutritive substances which chemical analysis 

 proves to be present ; and lastly, these facts show that it 

 is not water which, by virtue of its solvent power, has 

 made the nutritive elements available to the roots. 



We know by experiment, that Avhen water has dissolved 

 from a saturated soil a certain quantity of ammonia, pot- 

 ash, &c., the same amount of water will not further 

 dissolve fi'om a half-saturated soil (or a soil ft^om which 

 one-half of the absorbed potash and ammonia has already 

 been extracted) half so much as from the saturated soil ; 

 but that the earth, in proportion as it has thus become 

 poorer in nutritive substances, will aU the more firmly 

 retain the residue of the ingredients absorbed by it. 



In the half-saturated turf the nutritive elements are 

 much more firmly bound than in the fully saturated ; 

 and, again, in the quarter-saturated much more firmly 

 than in the half-saturated. 



Hence, even if the water had been able to dissolve and 

 convey to the roots half as much from the half-saturated 

 as from the fully saturated, and half as much from the 

 quarter-saturated as from the half-saturated, still the 

 produce could not in any case be greater than in propor- 

 tion to the amount of nutritive substances in the soil. 

 But, in fact, tliey Avere far greater, and the roots actually 

 absorbed more nutritive substances than the water could 

 possibly have conveyed to them, even under the most 

 favourable circumstances. 



These experiments have, for the first time, afforded 



