THE "ELEMENTS NEEDED BY PLANTS 99 



Calcium and magnesium. Calcium and magnesium, which 

 are much alike, are most familiar to us in limestone and dolo- 

 mite. In addition to being useful in forming compounds with 

 nitrogen that the plant can use, these elements form unions 

 with various acids in the plant which would otherwise be 

 harmful. Calcium is an important part of the chlorophyll and 

 nucleus, and promotes the hardiness of plants. On soils con- 

 taining much calcium or lime, plants endure drought and frost 

 much better than in soils in which it is lacking. Certain plants, 

 such as alfalfa, clover, peas, and beans, are often known as 

 lime plants because they cannot exist in soils deficient in this 

 element. Spinach, beets, lettuce, and many others cannot grow 

 without lime. On the other hand, many plants of sandy and 

 boggy soils are so sensitive to lime that they cannot endure 

 even small amounts in the soil water. Magnesium is important 

 in forming seeds, and its absence may not be noticed until 

 flowers and fruits fail to develop. While absolutely essential 

 to plant growth, magnesium in the absence of lime acts like 

 a poison. Calcium is usually more abundant than magnesium 

 in leaves and stems, but in seeds the ratio is usually reversed. 



Potassium and phosphorus. Potassium is supposed to aid in 

 the production and transportation of the carbohydrates and 

 to reduce the acidity of cell sap. It increases the turgidity of 

 the cell and hastens the ripening of wood and fruit. It also 

 increases the plant's resistance to frost and is reputed to deepen 

 the color of flowers and fruits. Certain plants contain so much 

 potassium or potash that they are called potash plants. Phos- 

 phorus is associated with the production of proteins, and its lack 

 prevents the development of seeds. Fleshy roots may also con- 

 tain much potassium. When soils are deficient in this element 

 the addition of sodium is followed by renewed plant growth. 



Sulphur and iron. Sulphur and iron, so frequently found 

 together in nature, are necessary constituents of protoplasm. 

 Iron is also essential to the formation of chlorophyll in plants. 



