270 



AMERICAN AGRICULTURE. 



Modern agricultural science has detected, in part at least, 

 the true theory of the necessity for rotation. It has been 

 discovered, that every crop robs the soil of a part of its ele- 

 ments, (fifteen or sixteen elementary substances combined in 

 various forms and proportions), and that no two dissimilar 

 crops abstract these elements or their compounds from the 

 soil, in the same proportions. Thus, if we consider the 

 amount of the salts taken out of the soil by a crop of turneps, 

 amounting to five tons of roots per acre ; of barley, 38 

 bushels ; one ton each of dry clover or rye grass ; and of 

 wheat, 25 bushels,*we shall find the great disproportions of 

 the various elements, which the different vegetables have 

 appropriated. As given by Johnston they will be in pounds 

 as follows : 



Besides the elements above noted, all crops absorb oxide 

 of iron, and nearly all oxide of manganese and iodine ; and 

 of the organic elements associated in various combinations, 

 they appropriate about 97 per cent, of their entire dried 

 weight. Now, it is not only necessary that all the above 

 materials exist in the soil, but that they are also to be found 

 in a form precisely adapted to the wants of the growing 

 plant. That they exist in every soil, in some condition, to 

 an amount large enough to afford the quantity required by 

 the crop, can hardly be doubted ; but that they are all in a 

 form to supply the full demands of a luxuriant crop, is pro- 

 bably true of such only as are found, under favorable cir- 

 cumstances of season and climate, to have produced the 

 largest burthens. 



If a succession of any given crops are gathered and 

 carried off the land, without the occasional addition of 

 manures, they will be found gradually to diminish in quan- 

 tity, till they reach a point when they will scarcely pay the 

 expenses of cultivation. I mean to be understood as affirm- 

 ing this of all crops and all soils, however naturally fertile 

 * This is exclusive of the turnep tops. 



