26 AGRICULTURE FOR BEGINNERS 



bones. The rocks or bones are usually treated with sul- 

 phuric acid. This treatment changes the phosphoric acid 

 into a form available for plant use. 



These three kinds of plant food are ordinarily all that 

 we need to supply. In some cases, however, lime has to 

 be added. Besides being a plant food itself, lime acts 

 beneficially on most soils by improving their physical 

 structure ; by sweetening the soil, thereby aiding the 

 little living germs called bacteria ; by hastening the decay 

 of organic matter; and by liberating the potash that is 

 locked up in the soil. 



