Power of Soils to ahsorh 3Ianure. 



137 



decomposition, they would also indirectly increase the supply, 

 not only of ammonia, but of all the other alkaline sul)- 

 stances which are bound up in the form of silicates of small 

 solubility. 



Again, the influence of common salt, when used as manure, 

 may depend on a like cause — an explanation which is the more 

 probable, since we have reason, as before said, to doubt the value 

 of soda as an element of the food of plants. And this leads me 

 to mention, very shortly, a conjecture which I have been led to 

 make with regard to the deposition of silica in the straw of 

 wheat and other crops of the same kind. 



It has always been a matter of question with chemists, in what 

 way the beautiful coating of silica could be laid on wheat straw ; 

 and as the soluble compounds of silica hitherto known have 

 been those of potash and soda only, it has been necessary to sup- 

 pose that solutions of these salts were decomposed by carbonic 

 acid, and that the silica, in solution in water, was subsequently 

 carried to the straw and there deposited. The discovery of the 

 silicate of ammonia, however, affords a much more satisfactory 

 explanation of this phenomenon. When the double silicate of 

 ammonia and alumina is treated with water, silicate of ammonia 

 dissolves, and this solution, when carefully evaporated, leaves 

 on the dish in which the operation is performed a transparent 

 varnish of silica, hard and brittle, and splitting into thin plates 

 like mica. In the act of evaporation the water carries with it 

 the ammonia, leaving only silica behind. What more natural 

 than to suppose that the silica of cereal crops is thus left 

 by the constant transpiration of water from the surface of their 

 leaves and stalks ? A circumstance which in an interesting 

 way favours the view now suggested, is, the observation made by 

 Mr. Lawes that, in the growth of wheat, much more ammonia is 

 removed from the soil than is found in the crop in the shape of 

 albuminous matters ; that, indeed, to produce 1 bushel of wheat, 

 containing in round numbers 1 lb. of nitrogen, between 4 and 

 5 lbs. of nitrogen as ammonia are required in the soil. This 

 singular observation, which has been hitherto without explana- 

 tion, becomes intelligible enough if it be conceded that the 

 ammonia is engaged in carrying the silica to the straw, and is, if 

 we may so say, wasted in the act. It is also remarkable that 

 this loss of ammonia is apparently confined to the cereal crops and 

 grasses, and is not found to occur in plants that have not silicious 

 stems. I do not wish to push this conjecture beyond its proper 

 limits, and therefore merely mention it as worthy of being borne 

 in mind. If it be in any degree correct, then the action of 

 common salt in strenpftheninor and briefhtenina: the straw of wheat 

 and barley, which is the best ascertained of its effects as manure, 



