THE ROTATION OF CROPS. 55 



tion. De Candolle and others then supposed that the 

 necessity for a rotation of crops was attributable to the 

 fact that plants, during growth, throw off excrenientitious 

 matters by their roots, which, they agreed, were injuri- 

 ous to a following crop of their own kind, but not to 

 others. Exceptional cases, as that of the onion, would 

 have to be accounted for upon the hypothesis that that 

 plant either did not excrete from the root, or if it did, 

 the matter eliminated was not detrimental to itself. 

 Others supposed that the decaying fibrous roots might be 

 injurious. Whether or not either of these theories alone 

 correctly explains the phenomena of rotation, the practice 

 of rotation remains the best possible method of keeping 

 the soil clean, mellow, and in the best attainable condi- 

 tion for the production of remunerative crops with the 

 least cost of manure. 



It is doubtless true here, as in many other matters per- 

 taining to agriculture, that circumstances may, to a cer- 

 tain extent, modify a general rule. In the treatment of 

 land of different kinds and textures the farmer should 

 be an empiric as little as the physician. The fact that 

 rotation may, for a limited number of years, be unneces- 

 sary on deep alluvial soils, or on heavy clays, with, or 

 without manure, may not suffice to upset a general prac- 

 tice as old as agriculture itself. 



Joseph Harris in his " Talks on Manures," while dis- 

 cussing the results of the interesting experiments of 

 Lawes and Gilbert on the heavy clay soil of "Botham- 

 sted," and probably basing his observations too upon ex- 

 perience with his own clays, says: " The old notion that 

 there is any real chemical necessity for a rotation of crops 

 is unfounded. Wheat can be grown after wheat, and 

 barley after barley, and corn after corn, provided we use 

 the necessary manures, and get the soil clean and in the 

 right mechanical condition." In other parts of his val- 

 uable book he says: "A light sandy soil will not pre- 



