106 THE SOIL. 



acid, and ammonia or nitric acid, in sufficient quantity to 

 account for the presence of these substances in the 

 cereal plants grown on such a soil ; while, on the other 

 hand, we find that the plant contains a hundred times 

 more sihcic acid than the water could possibly have sup- 

 phed ; the cause of the absorption of silicic acid, which 

 clearly is not in the water, must again here be sought for 

 in the plant itself. Again, if other cases show that an 

 equally abundant crop of corn is obtained on fields, from 

 which water fails to extract phosphoric acid or ammonia, 

 here, too, we are led to the conclusion that the nutritive 

 substances dissolved in the water are of no special import- 

 ance to tlie plants in question ; but that, as an indispen- 

 sable requisite, these elements must possess the form most 

 suitable for the action of the root, be this what it may. 



The beautiful experiments on vegetation made con- 

 jointly by Professor Nageh and Dr. Zoeller, in the Botanic 

 Garden at Munich, most strikingly prove the correctness 

 of the conclusions to which the analysis of drainage and 

 other waters has led. Instead of growing plants in 

 solutions of the mineral elements of thek food, as had 

 been done in all previous experiments, they piu-sued the 

 very opposite com-se ; they placed the seeds of the plants 

 in a soil containing all the elements of their food in an 

 insoluble state. 



Li such experiments, it is not easy to find a mate- 

 rial Avhicli can be used as a substitute for arable soil, 

 and possessing all its properties ; and the difficulty is 

 proved by the fact, that none of the plants grown by 

 Boussingault and others, in an artificial soil, abundantly 

 provided with all the elements of food, could even 

 remotely bear comparison with a plant gro^\m in a fertile 

 arable soil. Pulverised charcoal or pumice-stone have 

 tlie power of extractmg many elements of the food of 



