108 The Potato 



with grass, clover and other hay crops which have a money 

 value as food for animals, and also leave large amounts 

 of organic matter in the soil from their roots and stubble, 

 as well as that in the manure from the hay fed. Legumes 

 are able to draw part of their nitrogen from the air to 

 increase the soil supply. Some nitrogen is also obtained 

 from the air in other ways, as by the non-symbiotic 

 bacteria. This is shown by the great fertility of the wild 

 prairie soils which have had few leguminous plants. 

 (6) Different plants draw on the plant-food in the soil 

 in varying proportions. Coasequently a rotation of CTops 

 enables each to secure its plant-food easier than it would 

 if grown continuously. (7) Though the potato sends its 

 roots deeper than some crops, there are others, like clover 

 and alfalfa, which root much deeper. These bring up 

 fertility from the subsoil, and the roots of potatoes follow- 

 ing are able to grow lower, even in hard clay soils.^ (8) It 

 is believed that plants throw off in the soil in their growth 

 toxic substances which by accumulation become injurioas 

 to succeeding crops of the same plant but not to others. 

 (9) The physical condition of the soil for succeeding crops 

 is improved by the thorough culture given the potato. 

 Instances of this are the use of wheat following potatoes 

 without plowing in fall, and oats without plowing in 

 spring. 



The net return of the rotation as a whole must be the 

 deciding factor in the choice of crops. The potato gives 

 such large cash returns to the acre and responds so well 

 to the ase of fertilizers that it is usually the most important 

 crop of the rotation in which it is grown. Wherever this 

 is true, the choice of the other crops to go with it is largely 

 governed by their effect on the soil for the production of 

 • Colo. Bui, 216. 



