The straw of cereals such as wheat, and the stinging hairs on nettles, are stiffened with silica. Otherwise, if plants need aluminum and silicon at all it must be in small quantities. Cobalt, iodine, fluorine and vanadium, although apparently of no benefit to green plants, are classified now as micronutrients because, in tiny amounts, they have been proven essential for animals. Cattle and sheep, grazing luxuriant plants on land seriously deficient in cobalt, waste away and die unless the soil is treated with small amounts of cobalt salts. Similarly, an iodine deficiency causes goiter in man and other mammals because that element is needed to stimulate the thyroid gland Fluorine is necessary for the development and health of bones and teeth.
Selenium is most notorious and troublesome.
It is a hazard to livestock where some soils, in a semiarid belt extending from Canada to Mexico, contain concentrations of this sulfur-like mineral. Certain forage plants absorb enough of it so that browsing animals are seriously affected and frequently die. On the other hand, some soils are so deficient in micronutrients such as zinc, boron and molybdenum that, unless treated with small amounts of those minerals, plants and especially animals which eat them become diseased. Land which has been robbed of its fertility by over-cropping, without any applications of fertilizers, may be seriously deficient in phosphorus, potassium, calcium, sulfur and magnesium. A bunch of carrots, grown on such soils, may not have much more food value than a toothpick.