Plant Nutrition

A plant nutrient is a chemical element that is essential for plant growth and reproduction. Essential element is a term often used to identify a plant nutrient. The term nutrient implies essentiality, so it is redundant to call these elements essential nutrients. Commonly, for an element to be a nutrient, it must fit certain criteria. The principal criterion is that the element must be required for a plant to complete its life cycle. The second criterion is that no other element substitutes fully for the element being considered as a nutrient. The third criterion is that all plants require the element. All the elements that have been identified as plant nutrients, however, do not fully meet these criteria, so, some debate occurs regarding the standards for classifying an element as a plant nutrient. Issues related to the identification of new nutrients are addressed in some of the topics.

The first criterion, that the element is essential for a plant to complete its life cycle, has historically been the one with which essentiality is established (1). This criterion includes the property that the element has a direct effect on plant growth and reproduction. In the absence of the essential element or with severe deficiency, the plant will die before it completes the cycle from seed to seed. This requirement acknowledges that the element has a function in plant metabolism; that with short supply of the nutrient, abnormal growth or symptoms of deficiency will develop as a result of the disrupted metabolism; and that the plant may be able to complete its life cycle with restricted growth and abnormal appearance. This criterion also notes that the occurrence of an element in a plant is not evidence of essentiality. Plants will accumulate elements that are in solution without regard to the elements having any essential role in plant metabolism or physiology.

The second criterion states that the role of the element must be unique in plant metabolism or physiology, meaning that no other element will substitute fully for this function. A partial substitution might be possible. For example, a substitution of manganese for magnesium in enzymatic reactions may occur, but no other element will substitute for magnesium in its role as a constituent of chlorophyll (2). Some scientists believe that this criterion is included in the context of the first criterion (3).

The third criterion requires that the essentiality is universal among plants. Elements can affect plant growth without being considered as essential elements (3,4). Enhancement of growth is not a defining characteristic of a plant nutrient, since although growth might be stimulated by an element, the element is not absolutely required for the plant to complete its life cycle. Some plants may respond to certain elements by exhibiting enhanced growth or higher yields, such as that which occurs with the supply of sodium to some crops (5,6). Also, some elements may appear to be required by some plants because the elements have functions in metabolic processes in the plants, such as in the case of cobalt being required for nitrogen-fixing plants (7). Nitrogen fixation, however, is not vital for these plants since they will grow well on mineral or inorganic supplies of nitrogen. Also, plants that do not fix nitrogen do not have any known need for cobalt (3). Elements that might enhance growth or that have a function in some plants but not in all plants are referred to as beneficial elements.

Seventeen elements are considered to have met the criteria for designation as plant nutrients. Carbon, hydrogen, and oxygen are derived from air or water. The other 14 are obtained from soil or nutrient solutions (Table 1.1). It is difficult to assign a precise date or a specific researcher to the discovery of the essentiality of an element. For all the nutrients, their roles in agriculture were the subjects of careful investigations long before the elements were accepted as nutrients. Many individuals contributed to the discovery of the essentiality of elements in plant nutrition. Much of the early research focused on the beneficial effects or sometimes on the toxic effects of the elements. Generally, an element was accepted as a plant nutrient after the body of evidence suggested that the element was essential for plant growth and reproduction, leading to the assignment of certain times and individuals to the discovery of its essentiality (Table 1.1).

TABLE 1.1
Listing of Essential Elements, Their Date of Acceptance as Essential, and Discoverers of Essentiality

Element	Date of Essentialitya	Researchera
Nitrogen	1804	de Saussureb
	1851–1855	Boussingaultb
Phosphorus	1839	Liebigc
	1861	Villeb
Potassium	1866	Birner & Lucanusb
Calcium	1862	Stohmannb
Magnesium	1875	Boehmb
Sulfur	1866	Birner & Lucanusb
Iron	1843	Grisc
Manganese	1922	McHarguec
Copper	1925	McHarguec
Boron	1926	Sommer & Lipmanc
Zinc	1926	Sommer & Lipmanc
Molybdenum	1939	Arnon & Stoutc
Chlorine	1954	Broyer, Carlton, Johnson, & Stoutc
Nickel	1987	Brown, Welch, & Cary (11)

^aThe dates and researchers that are listed are those on which published articles amassed enough information to convince other researchers that the elements were plant nutrients. Earlier work preceding the dates and other researchers may have suggested that the elements were nutrients.
^bCited by Reed (22).
^cCited by Chapman (13).

Techniques of hydroponics (8,9) initiated in the mid-1800s and improved in the 1900s enabled experimenters to grow plants in defined media purged of elements. Elements that are required in considerable quantities (macronutrients), generally accumulating to 0.1% and upward of the dry mass in plant tissues, were shown to be nutrients in the mid-1800s. Most of the elements required in small quantities in plants (micronutrients), generally accumulating to amounts less than 0.01% of the dry mass of plant tissues, were shown to be essential only after techniques were improved to ensure that the water, reagents, media, atmosphere, and seeds did not contain sufficient amounts of nutrients to meet the needs of the plants. Except for iron, the essentiality of micronutrients was demonstrated in the 1900s.

Beneficial elements may stimulate growth or may be required by only certain plants. Silicon, cobalt, and sodium are notable beneficial elements. Selenium, aluminum, vanadium, and other elements have been suggested to enhance growth of plants (3,10). Some of the beneficial elements may be classified in the future as essential elements as developments in chemical analysis and methods of minimizing contamination during growth show that plants will not complete their life cycles if the concentrations of elements in plant tissues are diminished sufficiently. Nickel is an example of an element that was classified as beneficial but recently has been shown to be essential (11).

Studies of the roles of nutrients in plants have involved several diagnostic criteria that address the accumulation of nutrients and their roles in plants. These criteria include visual diagnosis, plant analysis, biochemical tests, and soil tests.