Ch. VII.] ON THE ROTATION OF CROPS. 93 



but as tlie sole view of the farmer is to obtain from it tlie greatest amount 

 of net product of which it is capable, that end, under proper management, 

 is, in either case, equally attainable. 



Under the old system of farming, the land was cropped with corn until 

 it was partly exhausted, and was then left to recruit itself under natural 

 pasture ; but after the introduction of turnips, and the culture of other 

 roots as field-crops, the alternation of grain and vegetables was adopted 

 from Flanders, where it is the invariable method to carry an alternate crop 

 for man and beast ; and it was found that the land was both preserved from 

 the baneful effects of over-cropping, did not demand such frequent periods 

 of rest, and, through the means thus obtained of feeding live-stock, could be 

 more abundantly supplied with manure. Experience, however, proved that 

 although this change has laid the foundation of our agricultural prosperity, 

 yet it had only made one stride towards its completion ; for it was found 

 that the succession of green crops to corn, still left undecided the serious 

 question, whether the particular crop selected was the most proper to follow 

 that by which it was preceded ? Various trials were therefore made, and 

 are still continued, in order to ascertain the best succession of crops on 

 different soils* ; and these several series are termed the rotation of crops. 



In cultivating the ground, two objects ought always to be kept in view : 

 first, to obtain from it the greatest quantity of the most valuable produce; 

 secondly, that this produce be obtained in such a manner as may least 

 exhaust the earth ; or, in other words, that the care of the farmer, while 

 prompted by regard to his immediate benefit, be so guided by discretion as 

 not to dry up the sources of future and more lasting advantage. 



The general principles upon which every separate rotation should be 



* Particular attention has been lately attracted to the investigation of this subject by 

 the writings of some foreign naturalists, whose experiments on vegetable physiology- 

 have led them to the conclusion that the roots of plants contain secretions, formed by 

 their sap, which being thrown off in the nature of excrement, render the land unfit for 

 the re-production of similar crops ; in like manner as no animal can subsist upon its own 

 fffices. When the soil has become much impregnated with the exudations of any parti- 

 cular vegetable, they are therefore supposed to poison it, so far as to be injurious to the 

 future vegetation of the same species of "plant ; and the land is then said to be over- 

 cropped, though they may assist the nourishment of other jilants. 



Many curious experiments have been tried to ])rove the truth of the hypothesis ; the 

 most interesting of which, to farmers, are those upon beans, potatoes, and grain. 



Tlie bean exists in pure water, which continues nearly clear, though it assumes a yel- 

 low tinge, occasioned, as it was found, by the discharge from the plant of a gmnmy mat- 

 ter and a little carbonate of lime. Fresh plants of beans did not live well ni it : but in 

 order to ascertain whether this was occasioned by the presence of the exuded matter, 

 plants of wheat were placed in the water, and they throve perfectly ; the colour of the 

 water became lighter, the sediment was reduced, and it was evident that the plants of 

 wheat had absorbed a portion of the matter discharged by the beans : from which it may 

 be inferred, that the eJ^periment justifies the practice of sowing u'heat after beans. 



The potato also puts forth its leaves in pure water, which becomes scarcely dis- 

 coloured, leaves but little sediment, affords but a slight taste, and leads to the supposition, 

 that it is one of those plants whose roots secrete little of a decided character : this experi- 

 ment., therefore, goes far to prove the truth of the well-knov<n fact, that the potato, unless 

 aided by a large quantity of manure, is a bad preparative for corn. 



Wheat, rye, and barley do not live well in pure water : that in which they vegetated 

 was without colour, smell, or taste ; but contained some salts, alkali, and a very small 

 portion of gum. As gummy matter appears — as in the experiment on beans — to be a 

 good preparation for wheat, and grain seems to afford but little of that substance, this 

 proves that corn-crops should not succeed each other, and the experiment shows the neces- 

 sity of preparing the land by the culture of green crops. — See the experiments of M. 

 Macaire as stated in the Transactions of the ' Societe de Phys. et d'Hist., Nat. de 

 Geneve,' vol. v. ; and the ' Edinburgh New Phil. Journ.,' No.'xxviii. ; also the ' Field 

 Naturalist,' No. ix. ; and various Papers oa the subject in NoSi xxi., xxiii., aad xxiv., of 

 the * Quart. Journ, of Agric,' 



