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important to their growth. This common salt is found in all 

 soils to a certain extent, and generally in quantity sufficient 

 to supply their wants ; in both these soils, for instance, we 

 find much more common salt than the clover crop required, 

 hence this would have no effect in causing its failure in one 

 case and success in the other. In consequence, however, of 

 the soluble nature of common salt, it is apt to be washed away 

 by drainage ; it is therefore advisable to add some in the form 

 of tillage, when such has been the case. Rain water, too, 

 always contains an appreciable amount of common salt, so 

 that we have, to a certain extent, an everlasting supply of it 

 to plants. It is, how^ever, a question whether the quantity 

 supplied by rain is at all equivalent to the amount extracted 

 by some kinds of produce. Indeed, the fact of common salt 

 being found beneficial to the growth of plants, is a sufficient 

 proof that it is not. 



Silica, the substance existing in combination with the 

 potash and soda of soils, and of which I have found a 

 mere trace present in the ashes of clover, need scarcely 

 be mentioned with reference to this case ; it is liberated 

 in a soluble state along with the potash, and is consequently 

 left in the soil, and continues to accumulate during the whole 

 time the clover plant is growing. It is this soluble silica 

 which is so conducive to the growth of the future crop of 

 wheat. But like all other soluble matters, this silica would 

 in a great measure be removed by drainage during the 

 winter, were it not stored up in the roots of the clover, and 

 preserved until required. It is a remarkable fact that these 

 roots contain considerable quantities of silica, while the plant 

 itself contains none. The average of several analyses, con- 

 ducted with great care, gives me five parts of silica in every 

 thousand of dried roots ; hence, in the decay of these roots 

 in summer, the silica will be liberated for the use of the 

 wheat. 



