The calcium concentration in plants used for human foods is of more interest than the phosphorus content because of the greater possibility of deficiencies of calcium in the diet. Consequently, increasing the calcium concentration in such crops has been considered by several investigators. For example, the workers in some experiment stations in the Southeast organized a broad cooperative project to study the relationships between soils and the composition of certain vegetables, particularly the leafy vegetables used for greens. They noted some interesting effects of calcium and nitrogen fertilizers on the calcium content of turnip greens. For example, very small increases ( 0.06 percent of calcium.) in turnip greens resulted from applications of gypsum to the soil. On the other hand, nitrogen applied to the soil brought a relatively large decrease in calcium ( 0.36 percent of calcium). Thus, more than six times as much calcium was lost as a result of fertilization of the crop with nitrogen than was gained through fertilization with gypsum. Also, in the experiments, it was shown that soil type had a greater effect on calcium and phosphorus than did minerals applied as fertilizers.

The application of nitrogenous manures has generally been associated with a reduced calcium concentration in the plant. There is some evidence that ammonium sulfate as a nitrogen carrier may be more effective in this respect than sodium nitrate. Effects of nitrogen on the calcium concentration in mixed forages and pasture herbage may be associated with changes in botanical composition, as well as with changes in the chemical composition of individual species. This is due to the effect of heavy applications of nitrogen in encouraging growth of grasses at the expense of legumes. Potassium or magnesium applied as a fertilizer may also be associated with a reduction of the calcium concentration in plants. The effect of magnesium may be small and is ordinarily of little practical importance except in the case of a plant, such as timothy, that normally has a low level of calcium.

Applications of most of the micronutrient elements to soils even in very small quantities will be followed by an increased absorption of the element by the plant. One exception may be iron, and reported changes in copper concentration in the plant have been rather small. Relatively large increases in cobalt and manganese concentration in plants can be obtained by adding these elements to the soil. In New Zealand and elsewhere, cobalt deficiency is often corrected by the use of cobaltized superphosphate, limestone high in cobalt, or simply crude cobalt salts.

Interest has grown in the effect of certain micronutrient elements like boron, on the absorption of other elements, particularly calcium and nitrogen. It appears from work published by R. Q. Parks, C. B. Lyon, and S. L. Hood of the United States Plant, Soil, and Nutrition Laboratory in Ithaca, N. Y., that boron exerts a profound effect on the absorption of nearly every other plant nutrient. At normal levels of boron supply, an increase in the boron content of the nutrient solution caused an increased concentration of boron, organic nitrogen, magnesium, manganese, calcium, iron, cobalt, and sulfur in tomato plants grown in a greenhouse. The concentration of molybdenum, phosphorus, zinc, copper, and potassium decreased in the same plants. The work has not as yet had practical application, but it is of considerable fundamental importance in studies of ion absorption by plants.

Overliming is a common experience on a soil with a low buffer capacity. The troubles arising from too much limestone are often due to its effect on some of the micronutrient elements like boron and manganese in the soil. Symptoms characteristic of deficiencies of these elements appear in the plant as a result either of chemical reactions that make the compounds of these elements unavailable to plants or through adverse ratios of calcium to these elements in the soil or the plant.

These visual symptoms appear only in cases of acute deficiency. In less serious deficiencies, retarded growth in various degrees may be the only symptoms. Under subacute conditions only the amount of the micro-nutrient element absorbed by the plant may be limited. It is the latter condition that may be of greatest importance to the nutritionist. We have no clear evidence yet as to the lowest level that elements such as iron, manganese, and copper can reach in the plant before deficiency symptoms appear. It is certain that the content of iron and manganese can vary tremendously and that copper can vary within a limited range before their absence becomes evident. Apparently cobalt can be entirely absent without any visual physiological disorder being produced in the plant.

The addition of liming materials, consequently, is a practice that farmers and technicians should study carefully, particularly in areas that are subject to overliming troubles. Evidence is at hand that liming will reduce materially the manganese content of forage without reducing yields. It has long been known that the iron content can be lowered, and it is reasonable to assume that cobalt would be lowered under some conditions. The evidence with respect to cobalt is still incomplete.

The micronutrient element content of plants ( especially manganese and cobalt) may also be lowered through the excessive use of nitrogenous fertilizers. That is to be expected because of the effects of nitrogen in producing more rapid and greater vegetative growth. If such growth occurs in soils containing minimum quantities of an element, such as cobalt, that is not essential to the growth of the plant, a low concentration of the element in the plant will result.