10. THE ROTATION OF CROPS. 



Illustrative material: Reproduce Figures 20, 21, and 22 on the 

 blackboard, using different colored crayons to represent the 

 nitrogen, phosphoric acid and potash. 



Plant Foods Prepared Slowly. We learned in Lesson 

 8 that cropping the farm tends to make the soil " poor." 

 Another process, however, tends to keep the soil fertile 

 in spite of the cropping. We learned in Lesson 4 that 

 roots can take in plant food only as it is dissolved in 

 water. Most soils contain phosphoric acid and potash 

 that are not yet dissolved in water, and so are not in 

 condition to be used by plants. These undissolved food 

 materials are slowly dissolved by the action of carbon 

 dioxide in the soil; hence the soluble phosphoric acid and 

 potash tend to increase slowly in uncropped soils. Some 

 ammonia also is washed down from the atmosphere by 

 rains and snows, and this tends to increase the nitrogen 

 in the soil. But these influences do not, in themselves, 

 furnish enough plant food to produce a good crop every 

 year. 



Rotation of Crops. Some products, as wheat and to- 

 bacco, remove much fertility from the soil; others, as 

 butter, remove very little. If we raise only those prod- 

 ucts that remove much fertility, our soil will, of course, 

 grow " poor " faster than if we grew a part of the time 

 those that remove only a little. In the latter case, the 

 fertility furnished by the soil and atmosphere may be 



47 



