44 SCIENCE AND SOIL 



hydrochloric acid, a fact which indicates that the iron is a con- 

 stituent of the living matter of plants. Animals also have a small 

 but absolute requirement for iron. 



Aside from oxygen, iron is the most abundant essential plant- 

 food element, constituting about 5^ per cent of the solid crust of 

 the earth, although the amount required by plants is very insig- 

 nificant. Thus, the earth contains more than 50 times as much 

 iron as phosphorus, while the corn kernel contains nearly 40 times 

 as much phosphorus as iron, so that the supply of phosphorus would 

 be depleted as much by the removal of 100 crops as the supply 

 of iron would be by 200,000 crops. 



While a very small supply of calcium is of vital importance, 

 considerable amounts of that element are commonly taken up and 

 deposited in the coarser parts of plants, as in straw, cornstalks, 

 and hay, and large supplies of calcium are required for legumes, 

 especially for clover and alfalfa. This larger use of calcium appears 

 to be due, especially in grain crops, to its power as a base to unite 

 with organic acids that might otherwise injure the plant; and the 

 salts formed are commonly deposited, not in the seed or with stored 

 food materials, but in the older tissues as inert matter. 



The common use of certain calcium compounds, such as burned 

 lime and ground limestone, for correcting soil acidity should not 

 be confused with the essential need of the element calcium as plant 

 food. Even strongly acid soils often contain abundance of the ele- 

 ment calcium for plant food, not in the form of carbonates, but in 

 silicates, which, however, have no power to correct soil acidity. 



Aluminum, silicon, sodium, chlorin, and manganese. These 

 elements are not known to be essential to plant growth, but they 

 are commonly found in plants, although the amount of manganese 

 is very small and that of aluminum still smaller. 



The opinion that silicon was essential and gave stiffness to the 

 straw of cereals is not correct, and the report that manganese 

 exerts a marked stimulating action on plant growth has not been 

 verified upon more thorough investigation. Sodium is now known 

 to be a nonessential, but there is still a possible question regarding 

 chlorin. According to Pfeffer, " it remains for precise researches 

 to determine whether a minimal amount is essential, or whether 

 chlorin simply favors growth under special cultural conditions." 



