222 



AMERICAN AGRICULTURE. 



accustomed useful crops. This system, it will be perceived, 

 implies the loss of the income of the soil for a certain portion 

 of the time, and it can be tolerated only where there is more 

 land than can be cultivated. 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 portion of its elements, (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 5 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 dispropor- 

 tions 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 ex- 

 ist in the soil, but that they are also to be found in a form pre- 

 cisely adapted to the, wants of the growing plant. That they 

 exist in every soil, in some conditions, 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 probably true of such only as 

 are found, under favorable circumstances of season and cli- 

 mate, to have produced the largest burthens. If a succession 

 of any given crops are gathered and carried off the land, 



* This is exclusive of the turnep tops. 



