26 Elementary Principles of Agriculture 



42. Protoplasm, or living substance, has the power 

 to combine simple compounds to form the complex 

 ones that compose the plant or animal body. Living 

 green plants absorb water and mineral matter from the 

 soil and carbon dioxid from the air, and with these form 

 the complex plant substances. Light is needed by the 

 leaves in making these combinations. 



43. Elements Necessary for Plant Growth. There 

 are about eighty different elements known, but only 

 about a dozen are actually used by plants. The follow- 

 ing elements are necessary for the healthy growth of 

 plants: (1) Carbon, absorbed by the leaves from the 

 air as carbon dioxid; (2) oxygen and (3) hydrogen taken 

 in as water; and the following, all taken in by the roots 

 from the soil solutions as soluble salts: (4) nitrogen, 

 (5) phosphorus, (6) potassium, (7) calcium, (8) magne- 

 sium, (9) sulphur, (10) iron, and (11) chlorine. Other 

 elements are often found in plants, but only the ones 

 named above are really essential. If any one of these 

 essential elements is withheld from the plant, the normal 

 growth is impaired. The importance of the mineral 

 substances to the welfare of plants will be discussed 

 later. (See Chapters XII and XIII.) 



44. Non-essential Elements in Plants. Besides the 

 essential elements named, plants usually contain other 

 elements that are really not necessary for their normal 

 growth. The most common ones are sodium (the prin- 

 cipal element in common salt), and silicon, a constitu- 

 ent of sand. 



45. The Amounts of the Elements in the Plant Body , 

 About half of the plant substance is carbon. It is a 

 part of practically all compounds found in plants. 

 Oxygen and hydrogen, too, are parts of nearly all 



