82 AGRICULTURE. 



is so essential to plant growth, it must be sup- 

 plied in some other way. This phase of the 

 subject will be further discussed under " Fertil- 

 izers." 



All of the food elements obtained from the 

 air, except nitrogen, are directly available from 

 that source, so need no further mention. 



2. Soil-derived Elements. 



Of the ten elements obtained from the soil, 

 all except phosphorus, potassium, and lime are 

 present in sufficient quantities, and in such form 

 as to supply the needs of plants, except in 

 special cases. 



(i) PHOSPHORUS. It has been proven by re- 

 peated experiments that phosphorus in the form 

 of phosphates * is essential to the healthy de- 

 velopment of plants. Growth cannot take place 

 without the presence, of phosphorus in the 

 nucleus of the cells. It helps in the assimila- 

 tion of other food, induces seed-formation and 

 the maturity of the plant, and assists in trans- 

 ferring the albuminoids to the seed. 



The presence of phosphorus in an available 

 form, if uncounteracted, is manifested by early 

 maturity and plump, well-filled seeds. Ordinary 

 soils are in time impoverished of the natural 

 supply of available phosphates unless a portion 



" It has been well established that the salts of phosphoric 

 acid OK phosphates are the only source from which phosphorus 

 of plants can be derived." Bulletin 94, Maryland Agricultural 

 Experiment Station. 



