72 A CRICUL TURE. [en. 



the other pined away. He explained this by assum- 

 ing that plants threw off excrementitious matter in the 

 soil, which after a time became so very objectionable 

 to the crop, that it became unhealthy and died. The 

 excrement of one crop he considered to be even 

 desirable for another crop ; at any rate it was supposed 

 to be unobjectionable. He based his explanation 

 upon the assumption that plants treated the food they 

 received somewhat in the same way as animals " did, 

 viz., that after making use of such portions of the 

 food as were desirable, the residue was thrown off as 

 an excrement. De Candolle's theory has, however, 

 been generally set aside, as less satisfactory than that 

 advanced by Liebig. 



147. According to Liebig's views, the difficulty 

 arose from a want of proper food for the plant, 

 and he held that the plant became unhealthy and 

 pined away, simply because the plant needed food 

 which the soil could not supply. He supported this 

 by showing the number of different substances which 

 our cultivated crops required the soil to supply (41). 

 He showed that when a soil was nearly exhausted of 

 the materials which one crop required, it might still 

 contain an abundant supply of food for another kind 

 of crop. He was of opinion that whilst one crop 

 would have had a short supply of food, there was an 

 abundance in the soil for the other crop. The one 

 crop might therefore fail, whilst the other would 

 flourish. It has been generally accepted as one of the 

 rules which should regulate the rotation of crops, that 

 those plants which required the same kind 

 of food, should be kept as far apart as 

 possible, whilst those which require different sup- 

 plies might follow each other. 



148. The second rule is that plants of the 

 same habit of growth, and general character 

 should not follow each other. For instance, 

 some plants strike deeply into the soil, and obtain 



