ROTATION OF CROPS. 21 



ROTATION OF CROPS. 



When a certain crop has been grown for a number 

 of years in the same field, it often occurs that the yield 

 decreases with each successive harvest, until finally an 

 amount scarcely more than the seed used is returned. 

 When a different crop is planted on such land it usu- 

 ally yields a paying crop, and after a number of years 

 the original crop can be again grown with a profit. 

 This phenomenon has given rise to the belief by some 

 people that the first crop put something in the soil 

 that was detrimental to itself; others held that there 

 was something taken out of the soil that was after- 

 wards restored. The latter, we have seen, were nearer 

 the truth than the former. There are crops, however, 

 that grow "tired" of a certain piece of land, or 

 rather, the land grows "tired" of a certain crop. 

 Some of these instances cannot be explained by the 

 exhaustion of certain elements, but something else 

 seems to be the cause. Certain pieces of land in Ger- 

 many grew tired of growing beets, and it was called 

 ruben-mude (b -et-tired) ; after growing certain other 

 crops on this land, it would again produce beets in the 

 same quantity. Later investigation showed that this 

 " beet tired " was due to the presence of a microscopic 

 worm closely related to the one that causes root-knot 

 on our vegetables. 



Certain crops are able to grow repeatedly on the 

 same land and not cause any falling off in quantity or 

 quality of the yield. For example, in an onion rais- 

 ing district a certain piece of land has grown more 

 than thirty crops of onions, and the plot is preferred 

 to-day to any of the surrounding land that was just as 

 good formerly. Doctors Lawes and Gilbert grew 



