THE ALTERNATION OF CROPS. 161 



weights of wheat and tobacco must be as 97-7 : 16. 

 This difference is very considerable. The roots of 

 tobacco, as well as those of wheat, extract the phos- 

 phates contained in the soil, but they restore them 

 again, because they are not essentially necessary to 

 the development of the plant. 



CHAPTER VIII. 



ON THE ALTERNATION OF CROPS. 



It has long since been found by experience, that 

 the growth of annual plants is rendered imperfect, 

 and their crops of fruit or herbs less abundant, by 

 cultivating them in successive years on the same 

 soil, and that, in spite of the loss of time, a greater 

 quantity of grain is obtained when a field is allowed 

 to lie uncultivated for a year. During this interval 

 of rest, the soil, in a great measure, regains its 

 original fertility. 



It has been further observed, that certain plants, 

 such as peas, clover, and flax, thrive on the same 

 soil only after a lapse of years ; whilst others, such 

 as hemp, tobacco, helianthus tuberosus, rye, and oats, 

 may be cultivated in close succession when proper 

 manure is used. It has also been found, that several 

 of these plants improve the soil, whilst others, and 

 these are the most numerous, impoverish or exhaust 

 it. Fallow turnips, cabbage, beet, spelt, summer 

 and winter barley, rye and oats, are considered to 

 belong to the class which impoverish a soil ; whilst 

 by wheat, hops, madder, late turnips, hemp, poppies, 

 teasel, flax, weld, and licorice, it is supposed to be 

 entirely exhausted. 



The excrements of man and animals have been 

 employed from the earliest times for the purpose of 

 increasing the fertility of soils ; and it is completely 



established by all experience, that they restore cer- 



14* 



