BIOCHEMICALLY IMPORTANT MINERAL ELEMENTS 



195 



which, however, can be alleviated by feeding small amounts of soluble 

 copper salts. 



Although silicon is a normal constituent of many plants, no absolute 

 requirement for it has been well enough established to be generally 

 accepted. Silicon tends to be concentrated particularly in fibrous plant 

 tissues such as wheat straw, cereal brans, rice hulls, timothy hay, and 

 the like. Such plant materials have a high ash content (3-7 per cent 

 or more), and from one-fourth to two-thirds or more of the ash consists 

 of silicates and silicon dioxide. 



Courtesy of A. B. Burrell, Cornell University. Reproduced from Hunger Signs 

 in Crops, a publication of the American Society of Agronomy and the National 

 Fertilizer Association, Washington, D. C. 



Fig. 8-5. Oldenburg apples about five weeks after petal fall, showing 

 symptoms of both internal and external cork due to boron deficiency. 



Selenium. Although this element is not among those needed by living 

 things, it has attracted much attention because of its presence in the 

 plants grown in certain localities and its toxic effect on animals consuming 

 such plants. The so-called "alkali disease" of horses and cattle, known 

 since pioneer days in parts of Kansas, Nebraska, South Dakota, and 

 Wyoming, is a chronic form of selenium poisoning. The disease, which 

 also affects sheep, pigs, dogs, rats, and poultry, follows consumption of 

 a diet containing 5-10 ppm. or more of selenium. Symptoms include 

 soreness and sloughing of the hoofs. Fig. 8-6, loss of hair, stiff joints, and 

 atrophy of the heart and liver. Poor hatching of eggs is one of the 

 earliest signs in poultry. A more acute form of the disease known as 

 "blind staggers," resulting from higher selenium intake, is quickly fatal. 

 The most practical antidote or control measure for range cattle, sur- 



