CROPPING AND MANURING. 189 



Von Thaer, who has not, perhaps, his superior in the 

 practical and scientific business of farming, any where, 

 has turned his attention, for several years, to a series of 

 experiments and observations, with a view to ascertain 

 the degree of diminution or augmentation of fertility, 

 which soils ordinarily experience from the culture of the 

 principal farm-crops ; and has combined the results of his 

 observations in a series of tables. Although these do 

 not possess perfect accuracy, (for any thing like this would 

 be impossible from the nature of the inquiry,) they never- 

 theless serve as useful data to farmers who are anxious to 

 preserve or to increase the fertility of their soils, by ju- 

 dicious rotations, and by applying all the means of fertility 

 which the farm affords. 



" The vegetative power," says ' British Husbandry,' 

 "•is supposed to be in proportion to the quantity of liu- 

 mus^ (or soluble vegetable matter,) or mould, which is 

 contained in the soil; and its consumption has been found 

 to be regulated according to the amount of nutritive mat- 

 ter consumed by the crops which are grown upon it. 

 The degrees of exhaustion thus occasioned, have only 

 been fixed by naturalists with any degree of certainty, in 

 so far as regards the usual species of cultivated grain and 

 pulse ; for, as to the other products of the earth, although 

 they have doubtless similar effects when similarly repeat- 

 ed, yet those which consist of vegetable roots and grasses, 

 and which are drawn from the land before they have per- 

 fected their seed, are nevertheless — whether from the in- 

 fluence attributed to their shade upon the soil, from 

 sustenance drawn from the air and water, or from other 

 causes with which we are not acquainted — only viewed 

 as ameliorating crops. Corn crops are, however, consid- 

 ered respectively to exhaust in proportions which render 

 the proportion of about 4J bushels of wheat equal to that 

 of 6 bushels of rye, SJ of barley, and 12 of oats.'' 



"According to all the experiments which have been 

 made, there is reason to suppose, however, that upon a 

 soil of moderate fertility, an average crop of wheat em- 

 poverishes the land to the extent of 40 per cent., while 

 one of rye only produces that effect as far as 30. Al- 



