262 BURNED CLAY MORE SOLUBLE. 



straw, roots, and tops of an entire rotation of four years, on 

 well-farmed land, is only about 1300 pounds.* In other words, 

 a dressing of a hundred cart-loads an acre of burned clay, of 

 good quality, is capable of yielding to water as much mineral 

 matter as is required by twelve successive corn and root crops, 

 supposing all that these crops contain to be permanently removed 

 from the land. 



Burning, therefore, like liming, owes its principal virtue to 

 its power of rendering immediately available to the growth of 

 plants the natural riches often so abundantly contained in the 

 soil. 



I do not insist, because it is unnecessary to the conviction of 

 my readers, on the further fact, that while so much more of the 

 mineral matter of the clay is rendered soluble in water by the 

 burning, a very much greater increase takes place in the mat- 

 ters which are soluble in acid. A clay which, before burning, left 

 81 per cent of insoluble matter, after three hours' digestion with 

 a mixture of strong nitric and muriatic acids, aided by heat, left 

 73 per cent only when burned and afterwards treated with acid 

 in a similar way. The additional proportion thus rendered 

 soluble in acids consists chiefly of alumina, but partly also of 

 lime, magnesia, and alkaline matter. And as the carbonic and 

 other acids produced in the soil do more slowly there for the 

 benefit of plants, what our strong acids do for us more quickly 

 in our laboratories, there is reason to believe that, besides the 

 soluble matter which water can extract from burned clays, they 

 are capable of yielding to plants a much larger quantity still 

 in a more gradual way. The apparent practical benefits of 

 burning clays, as a source of mineral food to plants, when 

 viewed in this aspect, become very much exalted. 



5. Suggestions for comparative experiments with burned clay. 



It will appear to the readers of the preceding section, that 

 the burning of clay is a process of amelioration which is 

 really deserving of extensive trial. But clays vary in com- 



* Johnston's Lectures on Agricultural Chemistry and Geology, 2d edition, 

 p. 409. 





