In addition to light, carbon dioxide and water, plants need mineral nutrients to support the myriad biochemical processes that occur in their cells. In nature and in traditional agriculture, plants get these nutrients from the soil through their roots (although nutrients can be supplied through leaf feeding). Plants under hydroponic culture also absorb essential minerals through their roots, not from the soil, but from an aqueous solution.
It is convenient to discuss the mineral requirements of plants in terms of essential elements. Although there are 92 naturally occurring elements, there is some disagreement as to the exact definition of essential as well as some variation in the techniques and apparatus used in experiments to test for mineral requirements. Hydroponics has been crucial in allowing scientists to determine which elements are required and which are not. In practice, it is difficult to completely eliminate an element from an experimental system, making mineral nutrient research a challenging and exacting field. Also, keep in mind that just because a sensitive chemical analysis of plant tissue reveals the presence of an element, it doesn’t mean that element is really required by the plant. Plants will absorb and store plutonium—but plutonium is certainly not an essential element.
For an element to be considered essential in plants, the following four criteria should be met: (1) It must be required for the completion of the lifecycle of the plant; (2) it cannot be replaced by some other element; (3) it must play a direct and specific role in plant metabolism; and (4) it must required by a substantial number of plant species, not just a single species or two. Clearly, this fourth “rule” can lead to disagreement among scientists about which minerals are truly essential.
Typical chlorosis on older leaves of tobacco plants caused by nitrogen deficiency. (R.J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company, Bugwood.org)
For the purposes of this article, we’ll discuss the 16 main elements that are accepted by most as essential. Some unusual plants do require more or can get by on fewer, but most plant physiologists wouldn’t quibble with the general requirement for the “standard 16.” For most of these elements, there is no question of their essential nature (carbon, hydrogen, nitrogen and oxygen for example). The trouble arises when considering the roles of elements that may be required in extremely small amounts. The elements required by virtually all plants for normal growth and development are (in order of usual concentration found in dried tissue): carbon (C), oxygen (O), hydrogen (H), nitrogen (N), potassium (K), calcium (Ca), magnesium (Mg), phosphorus (P), sulfur (S), chlorine (Cl), iron (Fe), manganese (Mn), boron (B), zinc (Zn), copper (Cu) and molybdenum (Mo). Some scientists would insist that nickel (Ni) and perhaps cobalt (Co) be added to this list, but since there is no universal agreement on this, they’ll be left out.
The 16 essential elements play diverse roles in plant metabolism. Some are structural components of biomolecules such as enzymes and nucleic acids; others play a regulatory role; still others carry electrical charges that influence plant functions and responses. Most essential elements are metals but carbon, oxygen, nitrogen, chlorine, sulfur and phosphorus are non-metals. Although hydrogen is on the same side of the periodic table as metals, it is only metallic under extreme pressure. Boron is a metalloid, with properties similar in some ways to both metals and non-metals.
This article, the first of two, discusses the six so-called “macronutrients”—nitrogen, phosphorus, potassium, magnesium, calcium and sulfur. Macronutrients are found in plant tissues in notably higher concentrations than the other class of nutrients—the “micronutrients.” Carbon, hydrogen and oxygen, which are not usually considered “mineral” elements will not be discussed.
Role in Plants. Nitrogen (N) is present in plants in the largest concentration of any of the mineral nutrients. It is a component of many organic molecules of great importance, including chlorophyll and the energy transfer molecules adenosine triphosphate (ATP) and adenosine diphosphate (ADP). Nitrogen is a component of amino acids, which are the molecular subunits from which proteins are synthesized. The nucleic acids DNA and RNA also contain nitrogenous bases—the As, Ts, Cs and Gs that make up the genetic coding sequences.
Hydroponic Source. Nitrogen in hydroponic nutrient solutions is supplied in the form of nitrogen salts containing nitrate (NO3-) or ammonium (NH4+). A combination of both of these offers some desirable pH buffering in the solution. Nitrates of potassium (KNO3), calcium [Ca(NO3)2] and ammonium (NH4NO3), or ammonium phosphate [(NH4)3PO4] are commonly used to formulate hydroponic nutrient solutions. In cases where the addition of no other nutrient elements is desired, nitric acid is an option (HNO3).
Deficiency Symptoms. Nitrogen deficiency in plants is characterized by chlorosis, which is a yellowing of the leaves. Nitrogen is a mobile element in plants, and can be moved around as needed; thus, older leaves tend to be the first plant parts to show signs of nitrogen deficiency (as nitrogen is transported to support new growth). Chlorosis is often evident when other minerals are deficient as well. Plants grown in poor nitrogen conditions tend to have stunted growth, and abnormally thin shoots.
Excess Symptoms. Too much of any element is detrimental. Excessive nitrogen in plants is evident as abnormally dark green leaves. Plants that have absorbed too much nitrogen are not as hardy, and more susceptible to attack from insects, bacteria and fungi.
Role in Plants. Phosphorus is a part of the previously mentioned energy molecules ATP and ADP and is found in the backbone portion of both DNA and RNA. Many organic molecules contain a phosphate group and amino acids incorporated into proteins may be phosphorylated after protein synthesis. The phosphorylation and dephosphorylation of proteins is an important mechanism in regulating the activity of proteins. Cell membranes are rich in phosphate groups that are part of the “head” regions of the phospholipid molecules that form the membrane bilayers.
Hydroponic Source. Phosphorus is usually provided in hydroponics solutions as mono- or dihydrogen phosphate (HPO42- or H2PO4-). In either case, the actual species in solution is pH dependent, with monohydrogen phosphate favored near neutral (pH about 7.0) and dihydrogen phosphate favored at more acid pH. Phosphorus is provided in conjunction with either potassium or nitrogen in the form of ammonium [(NH4)2HPO4 or NH4H2PO4] or potassium [K2HPO4 or KH2PO4] phosphates. In instances where there is already enough of either potassium or nitrogen in the solution, phosphoric acid (H3PO4) is an option.
Deficiency Symptoms. Phosphorus (P) deficiency can be difficult to diagnose. Since phosphorus is important in many metabolic functions as well as in the synthesis of new cell membranes, phosphorus deficiency results in slow growth. In a moderate case, the leaves become a darker green than normal with perhaps a blue or purple tint, which can give the illusion of a healthy plant. In addition to darker leaf color, plants will exhibit stunted growth and thin stems, with longer than normal distance between the branches. In severe cases of phosphorus deficiency chlorosis appears followed by leaf curl and drop.
Excess Symptoms. Although it is relatively uncommon in soil-cultured plants, the possibility of phosphorus toxicity is somewhat greater in hydroponically grown plants. The problem likely arises, not directly from the affect of too much phosphorus, but from its interaction and interference with other elements such as iron, magnesium and especially zinc. For this reason, the symptoms of toxicity, if they are detectable at all, will likely present as symptoms associated with one of these other elements.
“Hydroponics has been crucial in allowing scientists to determine which elements are required and which are not.”
Role in Plants. Potassium (K) does its job as a small monatomic ion (K+) in plants. It has diverse roles, including acting as a cofactor for the function of many enzymes important in energy production and carbohydrate metabolism and catabolism. Potassium ions are a critical part of the system that controls the movement of water in and out of leaves. It also facilitates water uptake by roots, which is probably a more important function in soil-grown plants.
Hydroponic Source. Potassium is supplied as potassium nitrate (KNO3) or sulfate (K2SO4). If no other source of chloride is available for the solution, potassium chloride (KCl) is an option.
Deficiency Symptoms. Like nitrogen, potassium is highly mobile in plants resulting in deficiency symptoms appearing in the oldest foliage first. Plants grown in potassium deficient conditions show a distinctive yellowing at the tips and edges of the oldest leaves. As the yellowing spreads, the leaves die and turn brown at their peripheries giving a burned appearance. Since K+ ions are important in maintaining water balance and turgor, potassium deficient plants are sensitive to water stress and wilt easily.
Excess Symptoms. Too much potassium is not directly toxic to a plant. However, the balance between the concentrations of potassium, calcium and magnesium ions is important, and too much K+ can be detrimental. An excessively high potassium to calcium ratio causes magnesium or calcium deficiency.
Role in Plants. Most of the calcium (Ca) in plants is found embedded in the cell walls where it has a structural role, and in membranes where it influences the flexibility of membranes. Changes in the normally low concentration of calcium ions (Ca2+) found in the cell cytoplasm are thought to be involved in signaling environmental stress.
Hydroponic Source. Calcium nitrate [Ca(NO3)2] is the reagent usually chosen to supply calcium in a nutrient solution. Many water supplies contain significant calcium and this should be taken into account before deciding how much calcium salt to include in a formulation.
Deficiency Symptoms. Newly emergent leaves are usually the first parts of a plant to show symptoms of calcium deficiency. The young leaves will be malformed, with ragged margins, and eventually turn brown. Roots also are affected and will turn brown (or translucent in some cases), and develop a swollen bulbous appearance accompanied by stunted growth. In fruit bearing plants, a lack of calcium interferes with normal cell division and elongation, causing blossom end rot on developing fruit.
Excess Symptoms. As mentioned, a proper balance between the concentrations of potassium, calcium and magnesium ions is important. Too much calcium can result in either magnesium or potassium deficiency.
Role in Plants. A magnesium ion (Mg2+) is found at the central position of every chlorophyll molecule, making it an important part of the photosynthetic apparatus. Magnesium ions also serve as enzyme cofactors, notably in ATP metabolism. Some magnesium is also present in cell walls.
Hydroponic Source. Magnesium sulfate (MgSO4) is the most common magnesium source for hydroponics. As is true for calcium, the amount of naturally occurring magnesium in the feed water should be determined before deciding how much reagent to use.
Deficiency Symptoms. Because of the importance of magnesium in photosynthesis, a lack has a profoundly negative effect and is difficult to recover from when it happens. Chlorosis between the veins of older leaves is a good indicator, with perhaps the addition of a red or orange tint. Growth is strongly inhibited. Magnesium has a complex relationship with Ca2+, K+ and NH4+ ions, and an imbalance among them can lead to magnesium deficiency.
Excess Symptoms. Excess magnesium is not harmful in and of itself. The main risk of having too much magnesium in solution is in creating an imbalance with respect to other ions, especially calcium. For this reason, the Ca and Mg concentrations should be about the same to avoid problems.
Role in Plants. Although only two amino acids contain sulfur (S), few proteins would function normally without them. Sulfur is found in plant hormones, and in molecules involved in chemical defense, odor and taste. Many organic molecules, intermediate compounds and proteins, contain or are modified with sulfate side-groups.
Hydroponic Source. Growers have a choice of several other nutrient ions to pair up with sulfur when designing nutrient solutions. Potassium (KSO4), magnesium (MgSO4) or ammonium [(NH4)2SO4] sulfates are all good options.
Deficiency Symptoms. Sulfur deficiency is difficult to distinguish from nitrogen deficiency. One difference between the two is that a lack of sulfur tends to cause more of an overall yellowing of a plant, rather than a yellowing of the older leaves first.
Excess Symptoms. There is little evidence that plants are harmed by high concentrations of sulfate ion.
- Barack, Phillip (1999), Essential Elements for Plant Growth, http://www.soils.wisc.edu/~barak/soilscience326/essentl.htm (accessed 09/06/09)
- Jones, J. Benton (2005), Hydroponics: A practical guide for the soilless grower, CRC Press, Boca Raton, FL
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