384 ROTATION OF CROPS, 



Prof. James F. W. Johnston says :* " Two practical rules 

 are suggested by the fact that different plants require differ- 

 ent substances to abound in a soil in which they shall be 

 capable of flourishing. 



" 1. To grow alternately as many different classes or 

 families of plants as possible, repeating each class at the 

 greatest convenient distance of time. In this country 

 (England) we grow, chiefly, root crops corn plants refined 

 for seed leguminous plants, sometimes for seed (peas and 

 beans), and sometimes for hay or fodder (clover and tares), 

 and grasses ; and these in alternate years. 



" Every four, five, or six years, therefore, the same class 

 of plants comes round again, and a demand is made upon 

 the soil for the same kinds of food in the same proportion. 

 * * * * * ^ perfect rotation would include all those 

 classes of plants which the soil, climate, and other circum- 

 stances allow to be cultivated with a profit. 



" 2. A second rule is, to repeat the same species of plants 

 at the greatest convenient distance of time. * * * * * 



" Instead, therefore, of a constant repetition of the turnip 

 every four years, theory says, make the carrot or the potato 

 take its place now and then, and instead of perpetual clover, 

 let tares, or peas, or beans occasionally succeed to your crops 

 of corn.f 



* Agricultural Chemistry, page 493. 



f " Corn, in English agricultural writing, is a general term corresponding to 

 our grain." 



