1 68 AGRICULTURAL CHEMISTRY 



than calcium but in the seeds of grains it is stored up 

 three times more liberally. Magnesium is assimilated 

 more slowly than calcium ; in fact it is assimilated, as a 

 rule, more slowly than any other ash element. The 

 plant does not require magnesium until the approach of 

 seed formation, although a small amount is necessary for 

 perfect leaf action as it enters into the chemical compo- 

 sition of the chlorophyl. When plants are grown with an 

 incomplete supply of magnesium , the seeds are frequently 

 sterile. In culture experiments, the absence of magne- 

 sium is not observed so much in the first stages of growth 

 as when the time of seed formation approaches when its 

 absence is followed by restricted development. 



217. Aluminum is found in the ash of many plants, as 

 wheat, peas, beans, and rice, although it occurs in very 

 small amounts and, so far as known, is not essential for 

 plant growth. Most soils contain traces of soluble sili- 

 cates of aluminum, and hence plants cannot well be free 

 from it. 



218. Iron in small amounts is necessary for plant growth. 

 It occurs in about the smallest amount of any of the ash 

 elements, but is always present in plants. When plants 

 are unable to obtain their requisite supply of iron, the 

 production of chlorophyl does not take place and they 

 fail to develop a normal green color. 



The function of the iron is to assist in the formation of 

 chlorophyl, the coloring-matter of plants. It is not known 

 whether iron enters into the chemical composition of the 

 chlorophyl, or is simply organically associated with it 



