OF AGRICULTURE. 39 



which they arc slowly dissolved by the action of car- 

 bonic acid. 



Phosphoric acid is generally found in combination 

 with lime in the soil, while in the ashes of plants 

 most of the phosphoric acid is in combination with 

 potash and soda. Sulphuric acid is found mostly 

 combined with lime in the soil. Magnesia is absorbed 

 by plants from its solution in carbonic or vegetable 

 acids. 



Silicic acid is probably introduced into plants as a 

 silicate of potash or soda, or dissolved in carbonic acid 

 water. On the outer surface of all grasses or cereals, as 

 well as reeds and rattan, w r e find a layer of "silex" or 

 flint, derived, without doubt, from the solution of sand 

 or silex absorbed from the soil by these plants. These 

 plants owe their support to this flinty substance, as 

 they are wanting in woody fibre and solid heart 

 interior ; and this support is necessary to enable them 

 to bear their burden of grain and foliage, to strengthen 

 their sap vessels and protect them from the ravages 

 of mildew and insects. 



On examining the usual constituents of the soil, w r e 

 find all the mineral ingredients of ashes sufficiently 

 abundant, excepting the alkalies and phosphoric acid. 

 It is safe, therefore, to mix a larger proportion of 

 these ingredients in the manure we wish to introduce 

 into the soil. They are to be found in ashes, guano, 

 and superphosphate of lime, which are ail known to 

 farmers as the most reliable of the saline manures. 



A certain amount of vegetable mold, or humus, is 

 ^necessary to the formation of a good and enduring 

 soil; and, though saline manures will often, by them- 

 selves, produce a good crop on a poor and apparently 



