Nutrients

Of the ninety-two naturally occurring elements, only about twenty are indispensable or essential for the growth of plants. Plants, however, absorb many more mineral elements than that from the soil in which they grow. Which of these elements are the essential ones? The best way to answer that is to withhold the element in question from the plants. If then the plants grow poorly or die while plants supplied with the element thrive, the element has been shown to be essential.

Such an experiment cannot be done with soil-grown plants. Soils contain most of the elements in the periodic table of elements. No element can be removed from soil so thoroughly as to deprive plants of that element; the chemical means for doing that would destroy the soil.

Therefore scientists devised a simplified method for growing plants, called solution culture, or hydroponics. In this technique the roots of the plants are not in soil but in water, which contains the dissolved salts of those elements considered to be essential. That way, scientists can control and monitor the chemical composition of the medium in which the plants grow.

Failure of the plants in such an experiment suggests that some essential element is missing, and by trial and error scientists then determine which element cures the deficiency. By this method most of the elements known now to be essential have been identified. Those elements needed in relatively large amounts are called macronutrients; those needed in only small or very small amounts are micronutrients.

By the latter half of the nineteenth century, all the macronutrient mineral elements (see accompanying table) and one micronutrient, iron, had been identified. But throughout the twentieth century additional elements were shown to be micronutrients. It took so long to identify them because early on the water and the nutrient salts used for supplying the macronutrient elements contained substantial impurities, some of which were micronutrients. Investigators therefore supplied, without knowing it, several micronutrients

MINERAL ELEMENTS IN CROP PLANTS
Element	Range of Concentrations
Macronutrients
Nitrogen (N)	0.5-6%*
Phosphorus (P)	0.15-0.5%
Sulfur (S)	0.1-1.5%
Potassium (K)	0.8-8%
Calcium (Ca)	0.1-6%
Magnesium (Mg)	0.05-1%
Micronutients
Iron (Fe)	20-600 ppm†
Manganese (Mn)	10-600 ppm
Zinc (Zn)	10-250 ppm
Copper (Cu)	2-50 ppm
Molybdenum (Mo)	0.1-10 ppm
Chlorine (Cl)	10-80,000 ppm
Boron (B)	0.2-800 ppm
Nickel (Ni)	0.05-5 ppm
Other Elements
Sodium (Na; essential for some plants)	0.001-8%
Silicon (Si; quasi-essential for some plants)	0.1-10%
Cobalt (Co; essential in all nitrogen-fixing systems)	0.05-10 ppm
* Percent of dry matter.
† Micrograms per gram dry matter (or parts per million).
SOURCE: Data collected from various sources.

to their experimental plants. Once this was understood, plant biologists developed ever more refined methods for purifying water and nutrient salts and, little by little, several additional elements were shown to be essential.

When determining the chemical composition of plants, plant nutritionists usually dry the plant first, keeping it at about 70°C (158°F) for forty-eight hours. Fresh plant material is mostly water (H₂ O) so that its dry weight is only around 10 to 20 percent of the initial fresh weight. Carbon and oxygen each make up about 45 percent of the dry matter, and hydrogen 6 percent. These elements can be removed by careful digestion. The inorganic nutrients together make up only about 4 percent of dry plant matter and are left in the digest.

Essential Elements

The table above lists the elements known to be essential to plants, in addition to carbon, oxygen, and hydrogen, and also includes a quantitative indication of their prevalence in plant tissues. For the macronutrient elements, these values are expressed as percent of the dry matter, and for the micronutrients, as micrograms per gram dry matter, or parts per million. The reason for giving a range of values rather than a single one for each element is that these values differ considerably, depending on the kind of plant, the soil in which it grows, and other factors. Three of these elements, sodium, silicon, and cobalt, cannot unequivocally be called nutrients, as explained below.

Living plants use up much water in transpiration. Water is also their main constituent. Carbon, oxygen, and hydrogen are the elements that make up carbohydrates. Plant cells have walls composed mostly of cellulose and related carbohydrate polymers. These three elements make up a high percentage of plant dry matter because quantitatively most of it is cell wall. In addition, it is mainly in the form of sugars (i.e., carbohydrates) that carbon initially assimilated by leaves through photosynthesis is translocated to the rest of the plant body, including the roots.

Nitrogen is a component of all amino acids, and as proteins are amino acid polymers, of all proteins. Nucleic acids and other essential compounds also contain nitrogen.
Phosphorus is part of several compounds essential for energy transfer, of which adenosine triphosphate (ATP), the "energy currency" of cells, is the best known. Nucleic acids and several other classes of biochemical entities also contain phosphorus as an integral component.
Three sulfur -containing amino acids and other compounds needed in metabolism account for the essentiality of sulfur.
Potassium is not an integral part of any compound that can be chemically isolated from plants. However, it activates some seventy enzymes, and along with other solutes regulates the water relations of plants.
Calcium is part of the middle lamella, the layer between the cell walls of adjacent cells. Another function is maintenance of the integrity of cell membranes. Calcium is also a cofactor (nonprotein part) of several enzymes. It functions to signal environmental changes in plant cells.
Magnesium is a constituent of the chlorophyll molecule and activates numerous enzymes.
Iron is a part of many metabolites, including those primarily involved in energy acquisition (photosynthesis), utilization (respiration), and nitrogen fixation.
Manganese activates a number of enzymes and is part of the protein complex that causes the evolution of oxygen, O₂, in Photosystem II of photosynthesis.
Zinc is a constituent of several enzymes.
Copper is also a constituent of several enzymes.
Nickel, the element required in the least amount, is a constituent of the enzyme urease. A deficiency of it causes an excessive accumulation of urea.
Boron has several functions in plant growth; severe boron deficiency causes the growing tips of both roots and shoots to die.
Chlorine (in the form of chloride ion) is required in Photosystem II of photosynthesis. Severely chlorine-deficient plants wilt, suggesting some unknown function in water relations.
Molybdenum is a constituent of enzymes active in the acquisition of nitrogen.
Cobalt is required by the symbiotic nitrogen-fixing bacteria associated with the root nodules of legumes and some other plants.
Sodium is prominent in many soils of arid and semiarid regions, and native wild plants growing on these saline soils grow best with an ample supply of it. Crops, however, often suffer under saline conditions. Plants with the C₄ photosynthetic pathway require sodium as a micronutrient.

Silicon is essential for plants of the family Equisetaceae, the horse-tails or scouring rushes. Although apparently not absolutely essential for plants in general it has nevertheless many beneficial effects; it has been called quasi-essential.

Deficiency and Toxicity Symptoms

When some element is deficient or present in such high concentration as to be toxic, plants often have symptoms somewhat characteristic of the particular condition afflicting them. For example, yellowing of leaves, or chlorosis, often indicates a deficiency of nitrogen. Nevertheless, visual identification of deficiencies or toxicities is not a reliable procedure. For example, sulfur deficiency may result in symptoms very similar to those of nitrogen deficiency. Therefore even experts check their visual impression by analyzing the tissue to find out whether its content of the suspected element is in fact below the value deemed adequate for that particular crop or present in excess. Often, such unrelated conditions as diseases caused by fungi or bacteria may result in the development of symptoms that mimic those of nutrient disorders.

Emanuel Epstein

Bibliography

Bennett, W. F., ed. Nutrient Deficiencies and Toxicities in Crop Plants. St. Paul, MN: American Phytopathological Society, 1993.

Epstein, Emanuel. Mineral Nutrition of Plants: Principles and Perspectives. New York: John Wiley & Sons, 1971.

——. "Silicon." Annual Review of Plant Physiology and Plant Molecular Biology 50 (1999): 641-64.

Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, 1988.