30 PHYSICAL AND CHEMICAL CONSTITUENTS OF SOILS. 



constituents of soils, are manures. The same substance (whether 

 silicious sand, clay, chalk, or humus) which, when in quantity, and 

 for the much larger proportion of such quantity, is a mere earth, 

 or mere physical material, also, for a very small proportion, in the 

 same or other soil, acts chemically and as a manure. And these 

 different operations of the same substance may even oppose each 

 other ; and then it will depend on other circumstances whether the 

 manuring action of a minute proportion of the substance will do 

 more good than is produced of injury by the excess of the same 

 substance as an agricultural earth and physical material of the 

 soil. 



If I have succeeded in clearly showing the distinction of me- 

 chanical and chemical action in soils of even the same substances, 

 it will serve to remove much of the obscurity and mystery which 

 have attended the general subject. When the application of cal- 

 careous matter as manure is new, or but beginning in any country 

 (as in Virginia thirty years ago), it has been deemed (by many par- 

 tially informed persons) a sufficient objection to the promised 

 benefit of a small application, that much larger natural proportions 

 elsewhere did not always make rich lands. It seemed incredible 

 that a proportion of calcareous earth less than 1 per cent, of the 

 soil could much promote its fertilization and productiveness, when 

 other soils had 5, 10, or 50 per cent, of that material, and were 

 not always rich, and in some cases were extremely barren. But, 

 in such cases, 1 per cent, (or less), perhaps, was as large a pro- 

 portion of carbonate of lime as could act chemically and as a ma- 

 nure. All beyond that proportion would be mere physical material ; 

 and if in excess even for it mechanical operation, would be injuri- 

 ous in proportion to its excess. Thus (as will be shown hereafter) 

 a very small proportion of this earth serves to lessen the evil effects 

 to soils of both too much wetness and too much dryness, and the 

 opposite evils of too much heat and also of low temperature. But 

 in a chalky soil, where this ingredient is in great quantity, the 

 mechanical action predominates and overpowers the chemical ; and 

 such constitution of soil serves to aggravate all the opposite evils 

 of dryness and moisture, heat and cold, which the chemical action, 

 if alone, would greatly mitigate. 



The perplexity and erroneous deductions which have prevailed 

 have been much increased by some writers of scientific celebrity. 

 From analyzing specimens of remarkably fertile soils, and finding 

 in most cases very large proportions of carbonate of lime, they have 

 absurdly inferred that these were the most proper proportions. 

 Hence, different chemists have indicated as the most suitable for 

 the highest fertility of soil, proportions of this earth varying from 

 2 to 30 per cent, of the whole mass of soil. They who advocated 

 the larger quantities were ignorant that perhaps nine-tenths of the 



