Most of the toxicity of fluorine to plants has been by way of the atmosphere, particularly near factories that process phosphate and aluminum. Atmospheric fluorine may also cause trouble in cities where large amounts of soft coal are burned and around ore refineries. Active volcanoes discharge large amounts of fluorine, which have caused serious injury to livestock grazing nearby.

Instances of injury from volatilized fluorine have occurred in Tennessee. In Maury County the fluorine was emitted in the thermal processing of phosphate rock, which normally contains about 4 percent of fluorine. During the high- temperature operations, as much as 90 percent of the fluorine is given off as silicofluoride. On coming in contact with moist air, silicofluoride yields hydrofluoric acid, a gas that is highly toxic to the leaves of plants. Only about one-third as much fluorine is liberated in the sulfuric acid method as in the thermal process of manufacturing available phosphates. In Blount County, Tenn., the source of the fluorine was the cryolite that was being used in the production of aluminum. About 7 pounds of fluorine per acre were being brought down annually in that area. Somewhat larger amounts were found in the rain around the phosphate rock-processing factories in Maury County. Still larger amounts fell on the city of Knoxville. Less than a pound of fluorine falls on an acre in rain in nonindustrial areas.

Animals are subject to injury from high fluorine intake by way of feed and water. Serious injury to livestock occurred in the places mentioned.

We have limited evidence as to the need of animals for fluorine. But fluorine seems to be useful because it is a constituent of teeth and bones. It helps reduce the tendency to tooth decay if taken in proper amounts. Excess amounts lead to disintegration of the teeth and bones. Such troubles are experienced in areas near deposits of phosphate rock where the water tends to contain more than normal amounts of fluorine.

Equally serious problems arise when pulverized phosphate rock is fed to livestock as a substitute for bonemeal--but that danger can be avoided by roasting the rock at high temperatures to drive off the fluorine.

A person takes in about 0.5 to 1.0 milligrams of fluorine daily. If larger amounts are consumed, the excess is excreted in the urine. The only known injury from moderate excess is to the teeth and bones. Evidence of deficiency, indicated by excessive tooth decay. appears to be much greater than that of toxicity from too high consumption.

In some cities enough fluoride is added to the drinking water to raise its fluorine level to 1 p.p.m. We have indications that the incidence of caries is reduced when that is done.

BORON TOXICITY came into prominence during the First World War, when the United States was forced to develop its own supplies of potash salts to replace those formerly purchased from German producers. The most easily developed source was that at Searles Lake in California. When mixed fertilizers containing potash from that source were first used for crops, widespread damage resulted, notably to potatoes and vegetables.

Investigations disclosed that boron was present in these fertilizers in toxic concentrations. Some shipments of Searles Lake potash salts contained as much as 20 percent of borax, of which more than 11 percent is boron. Steps were taken immediately to refine the salts, and the problem was solved.

Plants need boron, but the amount required is about 25 to 75 p.p.m. Many plants absorb much larger amounts if the boron is available, often to the point of readily apparent injury. Some plants will accumulate up to 200 p.p.m. Soybeans, sweetpotatoes, and sunflowers are among these accumulator plants.

Boron differs from the other essential trace elements in that the tolerance of the plant for more than the amount required is limited. Any excess results in the development of yellowish-brown spots around the edges of the leaves, particularly the oldest ones. The brown spots may be distributed over the entire leaf in severe cases.

The boron problem is not confined to the manufacture and use of fertilizers. It enters also in the development and use of irrigation waters in and regions. Boron often is present in and soils and in the irrigation water used on them as water-soluble sodium borate. Some water carries so much that it cannot be used for irrigation.

In rating irrigation waters for crop production, the boron limit for sensitive crops is usually set at about 0.66 p.p.m. For semitolerant crops, the limit is set at about 1.33 p.p.m., and for tolerant crops at 2 p.p.m.

Most of the beans and all of the nut and fruit trees are classed with the sensitive crops. Alfalfa, sugar beets, onions, carrots, and cabbages are among the most tolerant crops. The remaining crops are mostly semitolerant.

The water-soluble forms of boron in humid regions have long since been carried off in drainage waters. The boron that remains is in the form of highly insoluble minerals, notably tourmaline. The total boron in the soils of humid areas normally is 25 to 100 p.p.m., of which about 1 percent is soluble in hot water.

About 0.35 P.P.m. of hot-water-soluble boron in soil is adequate for most crops, although some require considerably higher soluble-boron levels. The difficulty lies in knowing the point at which toxicity may develop for a particular crop.

It has become standard practice in the more intensively farmed parts of the humid regions to add 5 pounds of borax to each ton of mixed fertilizer. For topdressing established alfalfa, as much as 100 pounds of borax may be added to each ton of fertilizer.

A broadcast application of 10 pounds of borax to the acre can be made in most cases without hurting the crop. If that amount is placed only along the row, however, plants may be injured. Applications of 50 to 100 pounds of borax to the acre have been broadcast successfully for cauliflower and beets in deficient heavy soils. Great care is needed in using borax for crops on very sandy soils; on them the less soluble mineral colemanite is preferred.

MANGANESE AND IRON are a pair of trace elements that may interfere with each other as catalysts in enzyme systems that perform necessary functions in living organisms. One that is present in excess may substitute for the other to the extent that plants are injured.

Studies with culture solutions have shown that a deficiency of iron evidenced by a characteristic yellowing of the leaves can be induced by limiting the supply of iron available to the plant and by raising the supply of manganese to abnormally high levels in relation to those of iron. Catalase, peroxidase, and cytochrome oxidase activities are depressed then.

That is proof that a balance is involved. The problem is to determine for each plant in its environment what constitutes the best ratio of available iron to manganese in the nutrient solution or soil. That may apply to the many other pairs of essential and nonessential trace elements and to the secondary and major elements.

The important point is that a deficiency of one element and an excess of another may produce virtually identical effects on a plant: The element in excess has a toxic effect on the plant.

Often or always we can define toxicity, in relation to mineral nutrition of plants and animals, in terms of balance. On that basis, any element may be toxic in excess, and deficiency and toxicity are opposites.

Most soils in humid regions are naturally acid or may become acid soon after they are put to agricultural use. The reaction of such soils tends to fall to about pH 5 unless lime is applied to them. The pH values may fall to 4 or lower in some places, or they may remain well above 5 depending on the nature of the rock material from which the soil was formed and the nature of any rock material that lies immediately beneath the soil.

The purpose of liming soil is to raise its pH value usually to around 6, such a value having been found to be favorable for best growth of most crops, notably the legumes. One of the- reasons is that availability of manganese in acid soils is often so high as to result in toxicity. Liming the soil causes a lowering in the solubility of manganese and presumably brings the iron and manganese into better balance in relation to plant needs.