ROTATION OF CROPS. 



ttese substances taken up by each plant is determined by 

 analyzing its asbes. The amount of each of these materials 

 in healthy plants of the same species at the same stage 

 of growth is pretty constant. 



In some species no soda at all is found, in others no 

 magnesia, in others very little lime, and the proportion of 

 acids as well as alkalies, varies in different plants. The 

 object of special manures is to supply those substances 

 which are most likely not to exist in the soil in sufficient 

 quantity for the wants of the plant under cultivation. The 

 salts most important to the growth of plants in which soils 

 are most apt to be deficient are lime, phosphate of lime, 

 and potash. An application of the two latter is almost 

 invariably beneficial. Other substances also when taken 

 up in large quantities will require to be supplied, such as 

 soda, sulphuric acid, and chlorine. The analysis of a plant 

 however does not always indicate truly what the plant 

 most needs in the soil. Many plants have more of the 

 phosphate in their composition than the turnip but no one 

 is so much benefited by the application of phosphate and 

 superphosphate of lime. 



EoTATioN OF Crops. — As different plants appropriate 

 different substances, the rotation of crops has considerable 

 influence in retaining the fertility of a soil. If the same 

 kind of plants are continued upon the same soil, only a 

 portion of the properties of the manure applied is used, 

 while by a judicious rotation everything in the soil or in the 

 manure suitable for vegetable food is taken np and appro- 

 priated by the crop. Some vegetables as onions and carrots 

 are very exhausting to the soil while lettuce is very slightly 

 prejudicial. Hence, however plentiful manure may be, a 

 succession of exhausting crops should not be grown upon 

 the same bed, not only because abundance is no excuse foi 



