Essential Elements for Plant Growth

Essential Elements for Plant Growth

List of Essential Elements

---

The essential mineral elements are:

Nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, iron, manganese, zinc, copper, molybdenum, and nickel.

In addition to the essential mineral elements are the beneficial elements, elements which promote plant growth in many plant species but are not absolutely necessary for completion of the plant life cycle, or fail to meet Arnon and Stout's criteria on other grounds. Recognized beneficial elements are:

Silicon, sodium, cobalt, and selenium

Other elements that have been proposed as candidates for essential or beneficial elements include chromium, vanadium, and titanium, although strong evidence is lacking at this time.
Another group is the essential nonmineral elements, elements taken up as gas or water, which are:

Hydrogen, oxygen, and carbon

Out of all of the many natural elements, essential mineral elements, essential nonmineral elements, and beneficial elements are not randomly scattered, but instead cluster in several groups on the periodic chart.

Various classification schemes for essential elements include

Plant concentrations of essential elements may exceed the critical concentrations, the minimum concentrations required for growth, and may vary somewhat from species to species. Nonetheless, the following table gives the general requirements of plants:

Typical concentrations sufficient for plant growth. After E. Epstein. 1965. "Mineral metabolism" pp. 438-466. in: Plant Biochemistry (J.Bonner and J.E. Varner, eds.) Academic Press, London.

Element	Symbol	mg/kg	percent	Relative number of atoms
Nitrogen	N	15,000	1.5	1,000,000
Potassium	K	10,000	1.0	250,000
Calcium	Ca	5,000	0.5	125,000
Magnesium	Mg	2,000	0.2	80,000
Phosphorus	P	2,000	0.2	60,000
Sulfur	S	1,000	0.1	30,000
Chlorine	Cl	100	--	3,000
Iron	Fe	100	--	2,000
Boron	B	20	--	2,000
Manganese	Mn	50	--	1,000
Zinc	Zn	20	--	300
Copper	Cu	6	--	100
Molybdenum	Mo	0.1	--	1
Nickel	Ni	0.1	--	1

Please note that concentrations, whether in mg/kg (=ppm, parts per million) or Percent (%), are always based on the weight of dry matter, instead of the fresh weight. Fresh weight includes both the weight of the dry matter and the weight of the water in the tissue. Since the percentage of water can vary greatly, by convention, all concentrations of elements are based on dry matter weights.
Somewhat arbitrarily, a dividing line is drawn between those nutrients required in greater quantities, macronutrients, and those elements required in smaller quantities, micronutrients. This division does not mean that one nutrient element is more important than another, just that they are required in different quantities and concentrations. On the table above, the dividing line is typically drawn between S and Cl, meaning that:
Macronutrients: N, K, Ca, Mg, P, and S, and
Micronutrients: Cl, Fe, B, Mn, Zn, Cu, Mo, and Ni
The prefix "micro" is well-understood from its use in terms such as "microscope". The term "macro" is somewhat less common, but indicates objects of a somewhat large size. Intermediate sizes are sometimes indicated by "meso". For example, the fauna (animal life) of soil may be divided into macrofauna (moles, mice, etc.), mesofauna (earthworms, burrowing insects, etc.), and microfauna (nematodes, etc.)

Essential Elements for Plant Growth

Primary and Secondary Nutrients

---

All essential elements are by definition required for plant growth and completion of the plant life cycle from seed to seed. Some essential elements are needed in large quantities and others in much smaller quantities. However, from a practical standpoint, three of the six essential macronutrients are most often "managed" by the addition of fertilizers to soils, while the others are most often found in sufficient quantities in most soils and no soil amendments are required to supply adequate supplies.
From a management perspective only, the primary nutrients are N, P, and K, because they are most often limiting from a crop production standpoint. All of the other essential macronutrient elements are secondary nutrients because they are rarely limiting, and more rarely added to soils as fertilizers.
The ability of soils to supply secondary nutrients to plants indefinitely is is subject to the law of conservation of matter and is therefore dependent upon nutrient cycling. Continued crop removal of Ca, Mg, and S requires replentishment just as surely as primary nutrients, but most likely less frequently. Calcium and magnesium are often supplied by mineral weathering, either of natural soil materials or of aglime, ground limestone added to correct soil acidity. Sulfur is often added to soil as either atmospheric deposition (associated with air pollution) or as impurities in fertilizers, particularly common P fertilizers.
To demonstrate that this classification is more responsive to soil ability to supply nutrients than plant requirements, it should be noted that plant requirements for Ca, a secondary nutrient element, is greater than for P. Calcium is found as a principle exchangeable cation in most soils and an important soluble cation in the soil solution. Phosphorus, on the other hand, is only slightly soluble in most soils, and many soils (particularly acid soils and alkaline soils) have the potential for causing phosphorus deficiencies.
Whether a macronutrient or micronutrient, or whether a primary or secondary nutrient, the Law of the Minimum holds: the most growth-limiting nutrient will limit growth, no matter how favorable the nutrient supply of other elements. For example, a deficiency of Fe or Mn (most common in soils containing calcium carbonate) can severely limit plant growth in spite of adequate N, P, and K.

Law of the Minimum

---

Justus von Liebig's Law of the Minimum states that yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be. From this, it may be inferred that if the deficient nutrient is supplied, yields may be improved to the point that some other nutrient is needed in greater quantity than the soil can provide, and the Law of the Minimum would apply in turn to that nutrient.
Liebig made great contributions to the science of plant nutrition and soil fertility. As a result of millenia of practical experience of farmers manuring fields to improve fertility, many early chemists thought that the "principle of vegetation", the essential nutrients needed for plant growth, were organic in nature rather than mineral. Liebig essentially debunked the humus theory and made a scientific case for plant requirements for mineral elements from the soil, carbon from CO₂ in the air, and H and O₂ from water. Liebig thought that plants derived most of their nitrogen content from the air as well, which is somewhat correct for legumes, but not true for other plants. Liebig developed the first mineral fertilizers applied to replentish nutrients removed from soils by crops and clearly saw mineral fertilizers as part of sustainable agricultural practices.
Justus von Liebig (1803-1873) was a German chemist who spent the early part of his accomplished career as a pioneer in organic chemistry. He turned to what is now called biochemistry about 1838, and first published on agricultural chemistry in 1840, and made numerous significant advances and engaged in extensive, fruitful debate with other researchers in the field.
Recent scholarship, beginning in 1950, has discovered the significance of the German agronomist Carl Sprengel (1787-1859) who conducted pioneering research that could be considered the start of agricultural chemistry, including disproving the humus theory and formulating the Law of the Minimum. His publications on these subjects predated Liebig's 1840 publication and therefore he has precedence for these discoveries. It is uncertain that Liebig was unaware of Sprengel's work, so Liebig may be considered a propagandist and promulgator of these discoveries and, by virtue of his greater reputation, may have been unduly credited for the discoveries themselves. The Association of German Agricultural Experimental Stations regularly acknowledges outstanding service or achievement to agricultural with the Sprengel-Liebig Medal, thereby honoring both scientists.
For more information on Liebig, see:

• C.C. Gillispie (ed.-in-chief). 1981-1990. Dictionary of Scientific Biography. Vol 7. Scribner, N.Y.
• C.A. Brown. 1942. "Justus von Liebig--Man and teacher." and "Liebig and the Law of the Minimum"in: Liebig and After Liebig: A century of progress in agricultural chemistry. Am. Assoc. Adv. Sci. The Science Press Printing Co., Lancaster, PA.
• van der Ploeg, R.R., W. Böhm, and M.B. Kirkham. 1999. "On the origin of the theory of mineral nutrition of plants and the Law of the Minimum." Soil Sci. Soc. Am. J. 63-1055-1062.

Ionic Charge

---

All but one of the essential elements is absorbed by plants from either soil solutions or nutrient solutions as either a cation, a positively-charged ion, or an anion, a negatively-charged ion. Cations and anions may differ not only in the sign of the ionic charge (positive/negative), but also its magnitude (1, 2, or 3).
In the table below, the essential elements are listed in their approximate order of critical concentrations, from greatest to least, from top to bottom. Interestingly, several elements are absorbed as oxyanions, ions composed of the element surrounded by three or four oxygen atoms, giving a net negative charge. Also of interest is the fact that several elements are taken up in different forms.

Cation(3+)	Cation(2+)	Cation(+)	Neutral(0)	Anion(-1)	Anion(-2)
Macronutrients:
	Ca2+ Mg2+	K+ NH4+		NO3- H2PO4-	HPO42- SO42-
Micronutrients:
Fe3+	Fe2+ Mn2+ Zn2+ Cu2+ Ni2+		B(OH)3	Cl-	MoO42-

From this table, a number of points can be made:

• Almost all elements are taken up as charged ions, with charges of +2, +1, -1, and -2.
• Absent are all organic compounds of the nutrient elements, indicating that all organic fertilizers must undergo mineralization of nutrients to become available to plants.
• Nitrogen may be taken up by plants as either an anion NO₃^- (nitrate) or a cation NH₄⁺ (ammonium). (Plant preferences for nitrate or ammonium are species-specific and are usually related to the natural environment of the species, with plants adapted to bogs, marshes, cold, or acid conditions often preferring ammonium-N, and most other dryland species preferring nitrate-N.)
• The ionic charge on the orthophosphate ion, whether H₂PO₄^- or HPO₄^2-, is dependent upon pH conditions prevailing in solution.
• Iron can be found as either Fe³⁺ or Fe²⁺, with a difference in redox state as well as ionic charge. Evidence exists that mandatory reduction of Fe³⁺ to Fe²⁺ is an essential part of the uptake of Fe in many plants.