Low sodium in soils may never be recognized as limiting soil fertility because so much of it exists in native rocks and in rainwater. Animals satisfy their desire for common salt from salt blocks in the field.

IODINE AND FLUORINE are needed by animals, but as far as we know they are not needed for the growth and development of any form of plantlife.

Deficiencies of iodine in people and livestock have been serious in the past, but goiter, which is associated with iodine deficiency, is easily recognized and easily treated with iodized salt. It is doubtful whether any good purpose would be served in trying to introduce iodine into foodstuffs through soil fertilization procedures. It would be expensive and perhaps less effective than the prophylactic measures now in use.

IODINE is known to almost everyone as a blackish-gray, volatile, solid dissolved in alcohol (tincture of iodine). It always occurs in plant materials, but sometimes may be too low to meet animal requirements for the iodine needed to maintain activity of the thyroid gland. Foods containing less than 0.01 p.p.m. can give rise to goiter in livestock. The general syndrome of goiter is complicated by other substances, besides iodine, that can be absorbed by the thyroid tissues in competition with iodine. Greater amounts of iodine consequently may be required in diets that contain such substances.

Suggestions that iodine is a plant nutrient have never been confirmed. Iodine is toxic to plants in concentrations of 0.1 p.p.m. in culture solutions. For soil applications, as little as 3 pounds of iodine to the acre have been toxic to plants. We know of no authenticated instances that fertilization with iodine has increased plant yields. The fact that plant toxicity is produced from such low amounts suggests high reactivity of iodine in some plant biochemical systems. Iodine therefore remains an element of active interest for investigation by plant physiologists.

FLUORINE, like iodine, is always contained in soil-grown plants. Both are indispensable in animal nutrition, but neither has been shown to be required by plants.

Fluorine also is a common component of rock phosphates. Some deposits having several parts per hundred of fluorine are roasted at high temperatures to remove fluorine before they are marketed as phosphatic fertilizer.

But fluorine, however, is required for animal life at least by animals that have bone skeletons. It is an important constituent of apatite, the calcium phosphate mineral of which bones and teeth are composed.

Skeletons cannot form in the normal way without small amounts of fluorine. Bones of rats fed low-fluorine diets do not form properly; the lower jawbone is particularly deformed. The fluoride ion is slightly smaller than the hydroxyl (OH) ions found in the crystal structure of apatite and therefore readily exchanges for hydroxyl ion if present in small amounts.

Administered orally, the fluorine from fluoride salts is absorbed by bones and teeth. A small amount of fluoride substituted in the apatite structure hardens the mineral--an advantage, especially for teeth. The crystal structure is warped and weakened without some fluorine. There is a critical point, however, above which the structure is again weakened by having too many fluorine atoms for the most compact crystalline structure.

Fluorine concentrations above 2 p.p.m. in drinking waters are considered excessive and can be removed by passing the water through filters made of ground bone. Difficulties have been reported with livestock that drink water containing 10 to 20 P.P.M. of fluorine; their tooth enamel was so soft that rapid erosion resulted and made it difficult or impossible for the animals to chew.

SILICON AND ALUMINUM We mention just for the sake of completeness. They dominate the soil mass and are present in large quantities. They are always found in plant materials, sometimes in high concentrations. An example is silica in cereal straw.

The two elements provide an example of the general proposition that plants adsorb materials from soils whether they are needed in their physiological processes or not. We know that silicon is necessary for normal development of diatoms small one-celled organisms whose siliceous skeletons have given rise to huge deposits of "diatomaceous earth."

Studies with culture solutions, especially directed toward discovering whether silicon and aluminum are of use to plants, generally have resulted in indefinite or negative conclusions: It is quite possible to grow plants in solutions as nearly free from these chemical elements as has been possible to attain by refined purification procedures without interfering seriously with plant growth.

In researches with culture solutions, particularly when cereals or beets are grown, the addition of silica to the solutions seems to give rise to sturdier plants. Whether silica is necessary for their growth and continuation of life cycles is not known. There have been many reports of beneficial results from the application of sodium silicate to field crops, but the reasons for the response are obscure. The assumption usually has been that the additions of silicon have resulted in a better phosphate status of the plants fertilized with silicon because soil-adsorbed phosphates were released by alkaline silicate salts.

Whatever the place of silicon and aluminum in the nutrition of plants and animals, we can rest assured that if they are needed by plants their requirements are in micro amounts. It seems quite unlikely, moreover, that the two elements will ever become of importance from the point of view of soil fertility for the reason that they constitute the dominant portion of the soil mass and always appear in soil solutions and plants.