154 AGRICULTURE. 



2. Prevents Loss by Exposure. The mate- 

 rials from the soil are not only taken up by the 

 plants, but continuous free and open cultivation 

 exposes the humus of the soil to the sun and to 

 the oxygen of the air, and more of its nitrogen 

 is made soluble than can be taken up by the 

 plants; hence, it is washed out and carried away 

 by the rains (Fig. 8). (See under "Cover 

 Crops," p. 159.) 



II. Increases Food Supply. 



1. Renders Plant-food Available. Repetition 

 of certain kinds of crops as, timothy or blue- 

 grass tends to use up the food faster than it is 

 rendered available, while change of crops and 

 consequent cultivation hastens the breaking up 

 of the chemical compounds in the soil, and thus 

 renders plant-food available. 



2. Brings Up Plant- food from the Subsoil. 

 The food supply may be further increased by 

 rotating clover, or any legume (all of which 

 have deep-feeding roots), with a crop of corn, or 

 wheat (Fig. 36), which has surface-feeding 

 roots. In this way the deep-feeding roots bring 

 up food elements from the subsoil, and when 

 these roots decay these food materials are ac- 

 cessible to the surface-feeding plants. 



3. Facilitates Fertilizing. Rotation not only 

 prevents the exhaustion of the fertility of the 

 soil, but may be useful in making artificial fer- 



