RELATION OF CROP TO FOOD IN SOIL. 



According to weight, the ^-saturated turf contained, in 

 every cubic centimetre, only Jth of the nutritive sub- 

 stances found in the completely saturated turf ; but, by 

 mixing 1 volume of saturated with 3 volumes of unsat- 

 urated turf, the former had become far more distrib- 

 uted, and its volume or efficient surface had been made 

 larger. Supposing it were possible to coat 3 volumes 

 of ordinary turf-powder with 1 volume of saturated, so 

 as completely to surround every fragment of the former 

 with saturated turf particles, the bean-plants would, in 

 a soil so prepared, grow as luxuriantly as if every par- 

 ticle of the turf were thoroughly saturated with nutri- 

 tive substances. 



Hence, the higher produce obtained from the com- 

 paratively poorer soil proves that it is only the surface 

 of the soil, containing the nutritive elements, which is 

 effective ; 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 when water has dis- 

 solved from a saturated soil a certain quantity of am- 

 monia, potash, &c., the same amount of water will not 

 further dissolve from a half-saturated soil (or a soil from 

 which one-half of the absorbed potash and ammonia has 

 already been extracted) half so much as from the sat- 

 urated soil ; but that the earth, in proportion as it has 

 thus become poorer in nutritive substances, will all ,the 

 more firmly retain the residue of the ingredients ab- 

 sorbed 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, 



