Ch. VII.J ON THE ROTATION OF CROPS. 91 



of each, and in the abundance of which consists its fertility. The precise 

 nature of those qualities, or rather the causes which influence their pecu- 

 liar effect on plants of different species, has not been ascertained ; indeed 

 has been only vaguely conjectured ; and all the researches of chemical 

 science on the subject have ended in proving little more than what was 

 already known by experience, — namely, that certain plants can only be 

 grown with advantage on certain soils ; and they can only rarely be con- 

 tinued without evident diminution of their amount*. 



The land, to use the farmer's phrase, " grows tired" of a repetition of 

 the same crops, and refuses to reproduce them in their former abundance, 

 though it will yield an ample return of others of a different species f. From 

 this it has been conjectured, that some particles of the soil are adapted to 

 the nourishment of one kind of vegetable, and others to another : though, 

 from their effect being more perceptible on the production of grain than of 

 leguminous plants, it has also been presumed that the exhausting pro- 

 perties of the former are partly to be attributed to the greater exposure of 

 the ground to the influence of the sun when bearing them ; and that the 

 meliorating consequences of the latter arise, in a great measure, from their 

 shade : as well as that the large-leaved vegetables derive a considerable 

 portion of their sustenance from the atmosphere, while corn seems to draw 

 its support entirely from the earth. The tap-rooted vegetables, too, pene- 

 trating deeper into the soil than those which are grown nearer to the sur- 

 face, derive their nutriment from a greater extent of earth, and may 

 therefore be presumed to be less exhausting to that portion of it which is 

 most appropriate to the production of corn. 



Whether these hypotheses be well or ill-founded, experience — that surest 

 guide in farming operations — has demonstrated that land, in the common 

 course of tillage, cannot be kept in heart without a frequent change ot 

 crops. An exception to that rule may, indeed, be found in the vicinity of 

 large towns which afford the command of unlimited quantities of manure, 

 which, to some crops, furnish u succession of that nutritive substance, or 

 vegetative mould 1, which supplies the want of peculiar matter necessary to 



* Although making this remark upou the application of chemisty, we are, however, 

 by no means insensible to the value of chemical research upon agriculture, as well as 

 upon every other subject to which it is at all applicable: thus De Candolle's ' Theory 

 of the Rotation of Crops,' though still only a vague idea, yet holds out a field for 

 research which, if followed up, as it doubtless will be by the chemists and naturalists of 

 every country, bids fair to solve the problem of the principle of vegetation. 



f It is not, in fact, solely by exhausting the soil that certain plants deteriorate, if 

 planted in the same ground year after year ; for, were this the case, manure would reno- 

 vate the ground ; but it fails to do so, and thus if peas or wheat, for example, be grown 

 repeatedly on a piece of land, the farmer may manure to whatever extent he chooses, his 

 crops will dwindle and become poorer and poorei*. This is remarkably the case in the 

 Isle of Thanet, where, to use a local term, if the land be " overpea'd'' it becomes, as it 

 were, poisoned, and if the peas be again planted, though they rise from the soil, they 

 soon turn yellow, are " foxed," and produce nothing of a crop. — Tower's Domestic Gard- 

 ener, p. 397. 



This, however, does not apply to all crops indiscriminately ; for potatoes, for instance, 

 are known to thrive year after year in the same ground if well manured: and there is a 

 remarkable instance related in Young's ' Annals of Agriculture,' of an experiment on 

 the estate of the Duke of Grafton, in Northamptonshire, on some cold land of which 

 beans and wheat were sown alternately during eight successive years, being only 

 •slightly dunged every third year, and yet continued to produce equally good crops 

 throughout. It is worthy of remark, that the crops of wheat which followed the dunged 

 beans, were finer than those for which the wheat was dunged. 



J This is known to chemists by the names of humi/i, h^trmc acid, and luamis, and is con- 

 sidered as that portion of the soil on which the fertility of the land entirely depends, as- — 

 with the exception of water — it is the only substance which furnishes nutriment to piant.s. 



