THE MAJOR ESSENTIAL ELEMENTS REQUIRED BY PLANTS

by J. Benton Jones, Jr.

Those elements that must be present in the plant at specific concentrations in order for a plant to function and grow have been divided into two categories — the major elements and micronutrients, based strictly on their concentration found in the plant. In the highest concentrations in the plant’s dry matter are nine major elements: carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). In concentrations less than 0.1 per cent of the plant’s dry matter are seven micronutrients: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). In this discussion we will talk about the major elements; in a following article in this series we will talk about the micronutrients.

DISCOVERY AND DISCOVERERS

The plant’s requirement for these nine major elements has been known for over 150 years. Their designation as being “essential” was determined by four plant scientists: DeSaussure for the elements C, H, O, and N in 1804; Ville for P; and von Sachs and Knop for K, Ca, and Mg in 1860, and for S in 1865.

What is interesting is that the criteria for essentiality recognized today by plant scientists weren’t established until 1939. Two University of California plant physiologists, Arnon and Stout, set three requirements that an element must meet in order to be considered “essential”:

1. Omission of the element in question must result in abnormal growth, failure to complete the life cycle, or premature death of the plant.

2. The element must be specific and not replaceable by another.

3. The element must exert its effect directly on growth or metabolism and not by some indirect effect, such as by antagonizing another element present at a toxic level.

Some plant physiologists feel these three criteria may have inadvertently fixed the number of essential elements at the current 16, and for the foreseeable future no additional elements will be found that meet them. However, another category, beneficial elements, has been established by some plant physiologists, which identifies elements that do not meet all three of Arnon and Stout’s criteria but by their presence in the plant enhances growth. This category of elements will be discussed in a future article. The essential elements, their form for uptake, and functions in the plant are provided in Table 1.

TABLE 1. The Essential Elements, Their Form for Uptake, and Functions in the Plant
Essential Element	Form for Uptake	Functions in the Plant
C, H, O, N, S	Ions in solution (HCO3-, NO3-, NH4+, SO42-) (O2. N2, SO2)	Major constituents of organic material; essential elements of, or gases in, the atmosphere atomic groups involved in enzymatic processes; assimilation by oxidation-reduction reactions
P, B	Ions in solution (PO43-. SO42-)	Esterification with native alcohol groups; the phosphate esters are involved in energy transfer reactions
K, Mg, Ca, Cl	Ions in solution (K+. Mg2+, Ca2+, Cl-)	Nonspecific functions establishing osmotic potentials; more specific reactions in which the ion brings about optimum conformation of an enzyme protein (enzyme activation); bridging of the reaction partners; balancing anions; controlling membrane permeability and electropotentials

NOMENCLATURE

Plant mineral nutrition literature over the years has developed its own jargon, as it does with most subjects. There have been name changes over the years that make the reading of some publications confusing. The term “plant nutrition” would relate to the study of the nutritional characteristics of plants, although I prefer to use the words “plant mineral nutrition” to be more specific. The word “nutrient” is commonly used to designate those elements that are essential for plants, whereas I prefer to use the words “essential nutrient elements,” or “essential mineral elements,” depending on the context of the discussion.

I have designated C, H, and O as the “structural elements” — elements found in those compounds that constitute 90 to 95 per cent of the dry weight of the plant. The remaining major elements, N, P, K, Ca, Mg, and S, make up most of the remaining 5 to 10 per cent of the dry weight. The elements, N, P, and K have been called the “fertilizer elements” because they are the primary constituents found in most mixed fertilizers. To add to the confusion, however, the content for P and K in fertilizer is provided as their oxides, P2O5 and K2O, while N is provided as an element.

In the past, Ca, Mg, and S were referred to as the “secondary elements,” a term no longer used in most plant mineral nutrition literature today. They are today identified as major elements.

THE STRUCTURAL ELEMENTS

When chlorophyll-containing plant tissue (mainly leaves) is in the presence of light, three of the essential elements —C, H, and O — are combined in the process called “photosynthesis” to form a carbohydrate. Carbon dioxide (CO2) from the air and water (H2O) taken up through the roots, then delivered to the leaves, are the sources for the C, H, and O. In the photosynthetic process, which takes place mainly around the stomata on the upper leaf surface, a water molecule is split and combined with a molecule of CO2 to form a carbohydrate, while a molecule of oxygen (O2) is released. The simplified formula being

6CO2 + 6H2O

in the presence of light and a chlorophyll molecule

= C6H12O6 + 6O2

The first product of photosynthesis will be either a 3- or 4-carbon–containing carbohydrate, depending on whether a C3 or C4 plant species.

The formed carbohydrate becomes the building block for the formation of other organic compounds, some of which form the cellular structure of the plant. The elements N and S combine with some of the generated carbohydrate to form amino acids, proteins, etc., and some of these compounds are formed into enzymes and similar active organic compounds.

THE MINERAL MAJOR ELEMENTS

The 5 to 10 per cent of the dry weight of plants consists mostly of the four essential mineral elements K, P, Ca, and Mg. Normally N and S are also grouped in this category because their forms are also taken up by the plant through the roots as either cations or anions, as shown in Table 2. The relationships between and among these mineral elements can have a significant affect on the plant as well as the concentration of the element itself. The form of an element, whether that found in a nutrient solution or in the solution of a mineral soil or soilless media, can have a marked affect on its utilization and affect on the plant. The form of N can significantly affect the growth of plants. It can exist as either the nitrate (NO3-) anion or the ammonium (NH4+) cation in the rooting medium. The nitrate anion is readily taken up through the plant roots, while the ammonium cation is a competitive cation with the other cations, K+, Ca2+, and Mg2+.

TABLE 2. The Six Mineral Essential Elements and Their Form Taken Up by Plants Through Their Roots
Element	Symbol	Ionic Form
cations:
Ammonium	NH4	NH4+
Calcium	Ca	Ca2+
Potassium	K	K+
Magnesium	Mg	Mg2+
anions:
Nitrate	NO3	NO3-
Phosphorus	P	H2PO4-; HPO42-
Sulfur	S	SO42-

PLANT REQUIREMENTS

The plant requirement for the six major mineral elements varies with species and variety and also with stage of plant growth. Under- or over-fertilizing a plant can impair growth and reduce fruit development and quality. The key elements are the fertilizer elements N, P, and K, whose deficiency or excess can have a significant effect on the plant. In order to ensure that the plant is being adequately supplied with these elements, as well as the other three (Ca, Mg, and S), the rooting medium and plant itself should be periodically assayed by means of tissue tests or plant analyses. These procedures will be covered in greater detail in a later article in this series.

It has been well established that there are genetic factors that influence the utilization of some elements, Mg being the most commonly known for the major elements. Recent findings suggest that a genetic factor may also be important in the N nutrition of tomato plants. Therefore, the sensitivity and response to an applied nutrient mineral element may be more influenced by the plant itself rather than the available supply.

EXCESS AND BALANCE

Of increasing concern is the accumulation of elements by the plant in excess of what is required, the one major element falling into this category being P. If this element is in excess in the plant, it begins to interfere with the normal function of two micronutrients, zinc (Zn) and iron (Fe). High P in the rooting medium will also impede the uptake of Zn. What was considered a commonly occurring deficiency is now a major concern of excess. Most fertilizers and compounds used to make nutrient solutions have high P contents, therefore contributing to the potential for excess.

The other major concern is the balance among the major cations, K+, Ca2+, and Mg2+. Both reduced plant growth as well as reduced fruit yield and quality can occur if these cations are not in proper balance, the primary elemental deficiency being Mg because this cation is the least competitive of the three. Potassium is readily taken up by plants, whereas Ca and Mg are not. If there is an ample supply of nitrate in the rooting medium, K uptake is also enhanced in order to maintain ionic balance in the plant. “Luxury consumption” is the term used to identify very high K uptake.

If the form of N supplied to the plant is primarily ammonium (NH4+), this cation will effectively compete with both the Ca2+ and Mg2+ cations. What is termed “ammonium toxicity” is really a disorder affecting the normal plant function of Ca in maintaining cellular integrity. Vascular decay, stem lesions, and blossom-end-rot in fruit are examples of the effects of ammonium toxicity.

As mentioned earlier, there has been much published that is misleading regarding the role and function of the major elements in plants. Successful plant growing should be based on maintaining a constant supply of the major elements within their sufficiency range and in proper balance, particularly among those elements that exist and are taken up as cations from the rooting medium. One major element that can significantly affect plants is N; its amount is important as is its form (nitrate anion or ammonium cation). The other major element, P, is becoming an increasingly significant factor affecting plant growth. Growers need to understand how the major elements affect plants and then apply those procedures of management needed to keep sufficient major elements provided to them. MY