NORTH-CAROLINA GEOLOGICAL SURVEY. 113 



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of lime, while others have maintained that it is due to the 

 potash. 



85. Now, it is quite possible that all these opinions are 

 right as far as they go. They are erroneous in being re- 

 strictive. If we examine the composition of an ash of any 

 plant, as I have already observed, we shall find all these ele- 

 ments, and we may well suppose, as they are all so generally 

 present, that they are all required ; and hence, we are not to 

 attribute the efficacy of marl to one of its elements exclusive 

 of the others. It may be, that a given soil is notably de- 

 ficient in potash, while the other elements are in sufficient 

 abundance to furnish all that a given plant requires. In such 

 a case it might appear that fertility was restored to the soil 

 by potash alone. Of all fertilizers, wood ashes are the best, 

 and possess a more general application than any other ; being 

 adapted to any crop. They are the best, because they con- 

 tain all the elements the plant needs ; and hence, the nearer 

 a marl is in composition to wood ashes, the better it is. 

 Hence, then, the efficacy of marl is due to its potash, soda, 

 lime, iron, magnesia, phosphoric acid, sulphuric acid and 

 chlorine, and not any one of its elements, exclusive of the 

 others. The only modification which this doctrine requires, 

 is that some of the elements are more important than others, 

 and it may be true, that the controlling influence is to be 

 ascribed to the alkalies, alkaline earths and phosphates ; still, 

 the marl is better with the less essential elements, than it 

 would be without them. The absolute value of a marl is 

 shown : 1., by the amount of soluble matter it contains. 2., 

 by the predominance of the most valuable elements, as pot- 

 ash and phosphoric acid. Marls which contain the most of 

 these bodies are the quickest and the most durable in their 

 effects ; and when the marl is rich in them, a full dressing 

 lasts from fifteen to twenty years. 



86. In forming a theory respecting the active elements 

 in marl, our views should not be limited to the nutrient prop- 

 erties they possess, or simply to the food elements which con- 

 tribute directly something to the weight or growth of the 

 plant. 



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