“Plant nutrition”的意思、由来-开放百科全书

Plant nutrition is the study of the chemical elements and compounds necessary for plant growth, plant metabolism and their external supply. In 1972, Emanuel Epstein defined two criteria for an element to be essential for plant growth:

This is in accordance with Justus von Liebig's law of the minimum. The essential plant nutrients include carbon, oxygen and hydrogen which are absorbed from the air, whereas other nutrients including nitrogen are typically obtained from the soil (exceptions include some parasitic or carnivorous plants).

There are seventeen most important nutrients for plants. Plants must obtain the following mineral nutrients from their growing medium:-^[2]

These elements stay beneath soil as salts, so plants consume these elements as ions. The macronutrients are consumed in larger quantities; hydrogen, oxygen, nitrogen and carbon contribute to over 95% of a plants' entire biomass on a dry matter weight basis. Micronutrients are present in plant tissue in quantities measured in parts per million, ranging from 0.1^[3] to 200 ppm, or less than 0.02% dry weight.^[4]

Most soil conditions across the world can provide plants adapted to that climate and soil with sufficient nutrition for a complete life cycle, without the addition of nutrients as fertilizer. However, if the soil is cropped it is necessary to artificially modify soil fertility through the addition of fertilizer to promote vigorous growth and increase or sustain yield. This is done because, even with adequate water and light, nutrient deficiency can limit growth and crop yield.

Processes

Plants take up essential elements from the soil through their roots and from the air (mainly consisting of nitrogen and oxygen) through their leaves. Nutrient uptake in the soil is achieved by cation exchange, wherein root hairs pump hydrogen ions (H⁺) into the soil through proton pumps. These hydrogen ions displace cations attached to negatively charged soil particles so that the cations are available for uptake by the root. In the leaves, stomata open to take in carbon dioxide and expel oxygen. The carbon dioxide molecules are used as the carbon source in photosynthesis.

The root, especially the root hair, is the essential organ for the uptake of nutrients. The structure and architecture of the root can alter the rate of nutrient uptake. Nutrient ions are transported to the center of the root, the stele, in order for the nutrients to reach the conducting tissues, xylem and phloem.^[5] The Casparian strip, a cell wall outside the stele but within the root, prevents passive flow of water and nutrients, helping to regulate the uptake of nutrients and water.^[5] Xylem moves water and mineral ions within the plant and phloem accounts for organic molecule transportation. Water potential plays a key role in a plant's nutrient uptake. If the water potential is more negative within the plant than the surrounding soils, the nutrients will move from the region of higher solute concentration—in the soil—to the area of lower solute concentration - in the plant.

Nutrients can be moved within plants to where they are most needed. For example, a plant will try to supply more nutrients to its younger leaves than to its older ones. When nutrients are mobile within the plant, symptoms of any deficiency become apparent first on the older leaves. However, not all nutrients are equally mobile. Nitrogen, phosphorus, and potassium are mobile nutrients while the others have varying degrees of mobility. When a less-mobile nutrient is deficient, the younger leaves suffer because the nutrient does not move up to them but stays in the older leaves. This phenomenon is helpful in determining which nutrients a plant may be lacking.

Many plants engage in symbiosis with microorganisms. Two important types of these relationship are

Though nitrogen is plentiful in the Earth's atmosphere, relatively few plants harbour nitrogen-fixing bacteria, so most plants rely on nitrogen compounds present in the soil to support their growth. These can be supplied by mineralization of soil organic matter or added plant residues, nitrogen fixing bacteria, animal waste, through the breaking of triple bonded N₂ molecules by lightning strikes or through the application of fertilizers.

Functions of nutrients

At least 17 elements are known to be essential nutrients for plants. In relatively large amounts, the soil supplies nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur; these are often called the macronutrients. In relatively small amounts, the soil supplies iron, manganese, boron, molybdenum, copper, zinc, chlorine, and cobalt, the so-called micronutrients. Nutrients must be available not only in sufficient amounts but also in appropriate ratios.

Plant nutrition is a difficult subject to understand completely, partially because of the variation between different plants and even between different species or individuals of a given clone. Elements present at low levels may cause deficiency symptoms, and toxicity is possible at levels that are too high. Furthermore, deficiency of one element may present as symptoms of toxicity from another element, and vice versa. An abundance of one nutrient may cause a deficiency of another nutrient. For example, K⁺ uptake can be influenced by the amount of NH{{su|b=4|p=+}} available.^[5]

Although nitrogen is plentiful in the Earth's atmosphere, relatively few plants engage in nitrogen fixation (conversion of atmospheric nitrogen to a biologically useful form). Most plants, therefore, require nitrogen compounds to be present in the soil in which they grow.

Carbon and oxygen are absorbed from the air while other nutrients are absorbed from the soil. Green plants obtain their carbohydrate supply from the carbon dioxide in the air by the process of photosynthesis. Each of these nutrients is used in a different place for a different essential function.^[10]

Macronutrients (derived from air and water)

Carbon

Carbon forms the backbone of most plant biomolecules, including proteins, starches and cellulose. Carbon is fixed through photosynthesis; this converts carbon dioxide from the air into carbohydrates which are used to store and transport energy within the plant.

Hydrogen

Oxygen

Oxygen is a component of many organic and inorganic molecules within the plant, and is acquired in many forms. These include: O₂ and CO₂ (mainly from the air via leaves) and H₂O, NO{{su|b=3|p=−}}, H₂PO{{su|b=4|p=−}} and SO{{su|b=4|p=2−}} (mainly from the soil water via roots). Plants produce oxygen gas (O₂) along with glucose during photosynthesis but then require O₂ to undergo aerobic cellular respiration and break down this glucose to produce ATP.

Macronutrients (primary)

Nitrogen

There is an abundant supply of nitrogen in the earth's atmosphere — N₂ gas comprises nearly 79% of air. However, N₂ is unavailable for use by most organisms because there is a triple bond between the two nitrogen atoms in the molecule, making it almost inert. In order for nitrogen to be used for growth it must be “fixed” (combined) in the form of ammonium (NH{{su|b=4|p=+}}) or nitrate (NO{{su|b=3|p=−}}) ions. The weathering of rocks releases these ions so slowly that it has a negligible effect on the availability of fixed nitrogen. Therefore, nitrogen is often the limiting factor for growth and biomass production in all environments where there is a suitable climate and availability of water to support life.

Nitrogen enters the plant largely through the roots. A “pool” of soluble nitrogen accumulates. Its composition within a species varies widely depending on several factors, including day length, time of day, night temperatures, nutrient deficiencies, and nutrient imbalance. Short day length promotes asparagine formation, whereas glutamine is produced under long day regimes. Darkness favors protein breakdown accompanied by high asparagine accumulation. Night temperature modifies the effects due to night length, and soluble nitrogen tends to accumulate owing to retarded synthesis and breakdown of proteins. Low night temperature conserves glutamine; high night temperature increases accumulation of asparagine because of breakdown. Deficiency of K accentuates differences between long- and short-day plants. The pool of soluble nitrogen is much smaller than in well-nourished plants when N and P are deficient since uptake of nitrate and further reduction and conversion of N to organic forms is restricted more than is protein synthesis. Deficiencies of Ca, K, and S affect the conversion of organic N to protein more than uptake and reduction. The size of the pool of soluble N is no guide per se to growth rate, but the size of the pool in relation to total N might be a useful ratio in this regard. Nitrogen availability in the rooting medium also affects the size and structure of tracheids formed in the long lateral roots of white spruce (Krasowski and Owens 1999).^[18]

Microorganisms have a central role in almost all aspects of nitrogen availability, and therefore for life support on earth. Some bacteria can convert N₂ into ammonia by the process termed nitrogen fixation; these bacteria are either free-living or form symbiotic associations with plants or other organisms (e.g., termites, protozoa), while other bacteria bring about transformations of ammonia to nitrate, and of nitrate to N₂ or other nitrogen gases. Many bacteria and fungi degrade organic matter, releasing fixed nitrogen for reuse by other organisms. All these processes contribute to the nitrogen cycle.

Phosphorus

Like nitrogen, phosphorus is involved with many vital plant processes. Within a plant, it is present mainly as a structural component of the nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), as well as a constituent of fatty phospholipids, that are important in membrane development and function. It is present in both organic and inorganic forms, both of which are readily translocated within the plant. All energy transfers in the cell are critically dependent on phosphorus. As with all living things, phosphorus is part of the Adenosine triphosphate (ATP), which is of immediate use in all processes that require energy with the cells. Phosphorus can also be used to modify the activity of various enzymes by phosphorylation, and is used for cell signaling. Phosphorus is concentrated at the most actively growing points of a plant and stored within seeds in anticipation of their germination. Phosphorus is most commonly found in the soil in the form of polyprotic phosphoric acid (H₃PO₄), but is taken up most readily in the form of H₂PO{{su|b=4|p=−}}. Phosphorus is available to plants in limited quantities in most soils because it is released very slowly from insoluble phosphates and is rapidly fixed once again. Under most environmental conditions it is the element that limits growth because of this constriction and due to its high demand by plants and microorganisms. Plants can increase phosphorus uptake by a mutualism with mycorrhiza.^[5]

A Phosphorus deficiency in plants is characterized by an intense green coloration or reddening in leaves due to lack of chlorophyll. If the plant is experiencing high phosphorus deficiencies the leaves may become denatured and show signs of death. Occasionally the leaves may appear purple from an accumulation of anthocyanin. Because phosphorus is a mobile nutrient, older leaves will show the first signs of deficiency.

On some soils, the phosphorus nutrition of some conifers, including the spruces, depends on the ability of mycorrhizae to take up, and make soil phosphorus available to the tree, hitherto unobtainable to the non-mycorrhizal root. Seedling white spruce, greenhouse-grown in sand testing negative for phosphorus, were very small and purple for many months until spontaneous mycorrhizal inoculation, the effect of which was manifested by a greening of foliage and the development of vigorous shoot growth.

It is useful to apply a high phosphorus content fertilizer, such as bone meal, to perennials to help with successful root formation.^[5]

Potassium

Unlike other major elements, potassium does not enter into the composition of any of the important plant constituents involved in metabolism,^[12] but it does occur in all parts of plants in substantial amounts. It seems to be of particular importance in leaves and at growing points. Potassium is outstanding among the nutrient elements for its mobility and solubility within plant tissues. Processes involving potassium include the formation of carbohydrates and proteins, the regulation of internal plant moisture, as a catalyst and condensing agent of complex substances, as an accelerator of enzyme action, and as contributor to photosynthesis, especially under low light intensity.

Potassium regulates the opening and closing of the stomata by a potassium ion pump. Since stomata are important in water regulation, potassium regulates water loss from the leaves and increases drought tolerance. Potassium deficiency may cause necrosis or interveinal chlorosis. The potassium ion (K⁺) is highly mobile and can aid in balancing the anion (negative) charges within the plant. Potassium helps in fruit coloration, shape and also increases its brix. Hence, quality fruits are produced in potassium-rich soils. Potassium serves as an activator of enzymes used in photosynthesis and respiration.^[5] Potassium is used to build cellulose and aids in photosynthesis by the formation of a chlorophyll precursor. Potassium deficiency may result in higher risk of pathogens, wilting, chlorosis, brown spotting, and higher chances of damage from frost and heat.

When soil-potassium levels are high, plants take up more potassium than needed for healthy growth. The term luxury consumption has been applied to this. When potassium is moderately deficient, the effects first appear in the older tissues, and from there progress towards the growing points. Acute deficiency severely affects growing points, and die-back commonly occurs. Symptoms of potassium deficiency in white spruce include: browning and death of needles (chlorosis); reduced growth in height and diameter; impaired retention of needles; and reduced needle length.^[28] A relationship between potassium nutrition and cold resistance has been found in several tree species, including two species of spruce.^[29]

Macronutrients (secondary and tertiary)

Sulfur

In plants, sulfur cannot be mobilized from older leaves for new growth, so deficiency symptoms are seen in the youngest tissues first.^[3] Symptoms of deficiency include yellowing of leaves and stunted growth.^[4]

Calcium

Another common symptom of calcium deficiency in leaves is the curling of the leaf towards the veins or center of the leaf. Many times this can also have a blackened appearance^[5] Calcium has been found to have a positive effect in combating salinity in soils. It has been shown to ameliorate the negative effects that salinity has such as reduced water usage of plants.^[6] Calcium in plants occurs chiefly in the leaves, with lower concentrations in seeds, fruits, and roots. A major function is as a constituent of cell walls. When coupled with certain acidic compounds of the jelly-like pectins of the middle lamella, calcium forms an insoluble salt. It is also intimately involved in meristems, and is particularly important in root development, with roles in cell division, cell elongation, and the detoxification of hydrogen ions. Other functions attributed to calcium are; the neutralization of organic acids; inhibition of some potassium-activated ions; and a role in nitrogen absorption. A notable feature of calcium-deficient plants is a defective root system.^[21] Roots are usually affected before above-ground parts.^[39]

Magnesium

The outstanding role of magnesium in plant nutrition is as a constituent of the chlorophyll molecule. As a carrier, it is also involved in numerous enzyme reactions as an effective activator, in which it is closely associated with energy-supplying phosphorus compounds. Magnesium is very mobile in plants, and, like potassium, when deficient is translocated from older to younger tissues, so that signs of deficiency appear first on the oldest tissues and then spread progressively to younger tissues.

Micro-nutrients

Plants are able sufficiently to accumulate most trace elements. Some plants are sensitive indicators of the chemical environment in which they grow (Dunn 1991),^[40] and some plants have barrier mechanisms that exclude or limit the uptake of a particular element or ion species, e.g., alder twigs commonly accumulate molybdenum but not arsenic, whereas the reverse is true of spruce bark (Dunn 1991).^[40] Otherwise, a plant can integrate the geochemical signature of the soil mass permeated by its root system together with the contained groundwaters. Sampling is facilitated by the tendency of many elements to accumulate in tissues at the plant's extremities.

Iron

Iron is necessary for photosynthesis and is present as an enzyme cofactor in plants. Iron deficiency can result in interveinal chlorosis and necrosis.

Iron is not a structural part of chlorophyll but very much essential for its synthesis. Copper deficiency can be responsible for promoting an iron deficiency.^[42]

Molybdenum

Molybdenum is a cofactor to enzymes important in building amino acids and is involved in nitrogen metabolism. Molybdenum is part of the nitrate reductase enzyme (needed for the reduction of nitrate) and the nitrogenase enzyme (required for biological nitrogen fixation).^[7] Reduced productivity as a result of molybdenum deficiency is usually associated with the reduced activity of one or more of these enzymes.

Boron

Boron is absorbed by plants in the form of the anion BO{{su|b=3|p=3−}}. It is available to plants in moderately soluble mineral forms of Ca, Mg and Na borates and the highly soluble form of organic compounds. It is available to plants over a range of pH, from 5.0 to 7.5. It is mobile in the soil, hence, it is prone to leaching. Leaching removes substantial amounts of boron in sandy soil, but little in fine silt or clay soil. Boron's fixation to those minerals at high pH can render boron unavailable, while low pH frees the fixed boron, leaving it prone to leaching in wet climates. It precipitates with other minerals in the form of borax in which form it was first used over 400 years ago as a soil supplement. Decomposition of organic material causes boron to be deposited in the topmost soil layer. When soil dries it can cause a precipitous drop in the availability of boron to plants as the plants cannot draw nutrients from that desiccated layer. Hence, boron deficiency diseases appear in dry weather.

Boron has many functions within a plant: it affects flowering and fruiting, pollen germination, cell division, and active salt absorption. The metabolism of amino acids and proteins, carbohydrates, calcium, and water are strongly affected by boron. Many of those listed functions may be embodied by its function in moving the highly polar sugars through cell membranes by reducing their polarity and hence the energy needed to pass the sugar. If sugar cannot pass to the fastest growing parts rapidly enough, those parts die.

Boron is not relocatable in the plant via the phloem. It must be supplied to the growing parts via the xylem. Foliar sprays affect only those parts sprayed, which may be insufficient for the fastest growing parts, and is very temporary.

Boron is essential for the proper forming and strengthening of cell walls. Lack of boron results in short thick cells producing stunted fruiting bodies and roots. Calcium to boron ratio must be maintained in a narrow range for normal plant growth. For alfalfa, that calcium to boron ratio must be from 80:1 to 600:1. Boron deficiency appears at 800:1 and higher. Boron levels within plants differ with plant species and range from 2.3 mg/kg for barley to 94.7 mg/kg for poppy. Lack of boron causes failure of calcium metabolism which produces hollow heart in beets and peanuts.

Inadequate amounts of boron affect many agricultural crops, legume forage crops most strongly. Of the micronutrients, boron deficiencies are second most common after zinc. Deficiency results in the death of the terminal growing points and stunted growth.

Boron supplements derive from dry lake bed deposits such as those in Death Valley, USA, in the form of sodium tetraborate (borax), from which less soluble calcium borate is made. Foliar sprays are used on fruit crop trees in soils of high alkalinity. Boron is often applied to fields as a contaminant in other soil amendments but is not generally adequate to make up the rate of loss by cropping. The rates of application of borate to produce an adequate alfalfa crop range from 15 pounds per acre for a sandy-silt, acidic soil of low organic matter, to 60 pounds per acre for a soil with high organic matter, high cation exchange capacity and high pH.

Boron concentration in soil water solution higher than one ppm is toxic to most plants. Toxic concentrations within plants are 10 to 50 ppm for small grains and 200 ppm in boron-tolerant crops such as sugar beets, rutabaga, cucumbers, and conifers. Toxic soil conditions are generally limited to arid regions or can be caused by underground borax deposits in contact with water or volcanic gases dissolved in percolating water. Application rates should be limited to a few pounds per acre in a test plot to determine if boron is needed generally. Otherwise, testing for boron levels in plant material is required to determine remedies. Excess boron can be removed by irrigation and assisted by application of elemental sulfur to lower the pH and increase boron solubility.

Boron deficiencies can be detected by analysis of plant material to apply a correction before the obvious symptoms appear, after which it is too late to prevent crop loss. Strawberries deficient in boron will produce lumpy fruit; apricots will not blossom or, if they do, will not fruit or will drop their fruit depending on the level of boron deficit. Broadcast of boron supplements is effective and long term; a foliar spray is immediate but must be repeated.

Copper

Copper is important for photosynthesis. Symptoms for copper deficiency include chlorosis.It is involved in many enzyme processes; necessary for proper photosynthesis; involved in the manufacture of lignin (cell walls) and involved in grain production. It is also hard to find in some soil conditions.

Manganese

Manganese is necessary for photosynthesis,^[34] including the building of chloroplasts. Manganese deficiency may result in coloration abnormalities, such as discolored spots on the foliage.

Sodium

Zinc

Zinc is required in a large number of enzymes and plays an essential role in DNA transcription. A typical symptom of zinc deficiency is the stunted growth of leaves, commonly known as "little leaf" and is caused by the oxidative degradation of the growth hormone auxin.

Nickel

In higher plants, nickel is absorbed by plants in the form of Ni²⁺ ion. Nickel is essential for activation of urease, an enzyme involved with nitrogen metabolism that is required to process urea. Without nickel, toxic levels of urea accumulate, leading to the formation of necrotic lesions. In lower plants, nickel activates several enzymes involved in a variety of processes, and can substitute for zinc and iron as a cofactor in some enzymes.^[2]

Chlorine

Chlorine, as compounded chloride, is necessary for osmosis and ionic balance; it also plays a role in photosynthesis.

Cobalt

Aluminium

Aluminum is one of the few elements capable of making soil more acidic. This is achieved by aluminum taking hydroxide ions out of water, leaving hydrogen ions behind.^[8] As a result, the soil is more acidic, which makes it unlivable for many plants. Another consequence of aluminum in soils is aluminum toxicity, which inhibits root growth.^[9]

Silicon

In plants, silicon has been shown in experiments to strengthen cell walls, improve plant strength, health, and productivity.^[52] There have been studies showing evidence of silicon improving drought and frost resistance, decreasing lodging potential and boosting the plant's natural pest and disease fighting systems.^[53] Silicon has also been shown to improve plant vigor and physiology by improving root mass and density, and increasing above ground plant biomass and crop yields.^[52] Silicon is currently under consideration by the Association of American Plant Food Control Officials (AAPFCO) for elevation to the status of a "plant beneficial substance".^[55]^[56]

Vanadium

Vanadium may be required by some plants, but at very low concentrations. It may also be substituting for molybdenum.

Selenium

Selenium is, however, an essential mineral element for animal (including human) nutrition and selenium deficiencies are known to occur when food or animal feed is grown on selenium-deficient soils. The use of inorganic selenium fertilizers can increase selenium concentrations in edible crops and animal diets thereby improving animal health.^[57]

Nutrient deficiency

The effect of a nutrient deficiency can vary from a subtle depression of growth rate to obvious stunting, deformity, discoloration, distress, and even death. Visual symptoms distinctive enough to be useful in identifying a deficiency are rare. Most deficiencies are multiple and moderate. However, while a deficiency is seldom that of a single nutrient, nitrogen is commonly the nutrient in shortest supply.

Nutrient status of plants

Nutrient status (mineral nutrient and trace element composition, also called ionome and nutrient profile) of plants are commonly portrayed by tissue elementary analysis. Interpretation of the results of such studies, however, has been controversial.^[60] During the last decades the nearly two-century-old “law of minimum” or “Liebig's law” (that states that plant growth is controlled not by the total amount of resources available, but by the scarcest resource) has been replaced by several mathematical approaches that use different models in order to take the interactions between the individual nutrients into account. The latest developments in this field are based on the fact that the nutrient elements (and compounds) do not act independently from each other;^[60] Baxter, 2015,^[11] because there may be direct chemical interactions between them or they may influence each other's uptake, translocation, and biological action via a number of mechanisms as exemplified for the case of ammonia.^[12]

Plant nutrition in agricultural systems

Hydroponics

Hydroponics is a method for growing plants in a water-nutrient solution without the use of nutrient-rich soil. It allows researchers and home gardeners to grow their plants in a controlled environment. The most common solution is the Hoagland solution, developed by D. R. Hoagland in 1933. The solution consists of all the essential nutrients in the correct proportions necessary for most plant growth.^[5] An aerator is used to prevent an anoxic event or hypoxia. Hypoxia can affect nutrient uptake of a plant because, without oxygen present, respiration becomes inhibited within the root cells. The nutrient film technique is a hydroponic technique in which the roots are not fully submerged. This allows for adequate aeration of the roots, while a "film" thin layer of nutrient-rich water is pumped through the system to provide nutrients and water to the plant.

See also

References

1. ^Swan, H.S.D. 1962. The scientific use of fertilizers in forestry. p. 13-24 in La Fertilisation Forestière au Canada. Fonds de Recherches Forestières, Laval Univ., Quebec QC, Bull. 5
2. ^{{cite book|last1=Haneklaus|first1=Silvia|last2=Bloem|first2=Elke|last3=Schnug|first3=Ewald|last4=de Kok|first4=Luit J.|last5=Stulen|first5=Ineke|editor1-last=Barker|editor1-first=Allen V.|editor2-last=Pilbeam|editor2-first=David J.|title=Handbook of plant nutrition|date=2007|publisher=CRC Press|isbn=978-0-8247-5904-9|pages=183–238|chapter-url=https://books.google.co.za/books?id=ZWjLBQAAQBAJ&q=sulfur#v=snippet&q=sulfur&f=false|accessdate=12 June 2017|chapter=Sulfur}}
3. ^{{cite web|title=Plant Nutrition|url=http://www.fao.org/ag/agp/agpc/doc/publicat/faobul4/faobul4/b402.htm|website=www.fao.org|accessdate=12 June 2017}}
4. ^{{cite web|title=Diagnosing sulphur deficiency in cereals|url=https://www.agric.wa.gov.au/mycrop/diagnosing-sulphur-deficiency-cereals|website=www.agric.wa.gov.au|accessdate=12 June 2017|language=en}}
5. ^{{Cite journal|last=Simon|first=E. W.|date=1978-01-01|title=The Symptoms of Calcium Deficiency in Plants|jstor=2431629|journal=The New Phytologist|volume=80|issue=1|pages=1–15|doi=10.1111/j.1469-8137.1978.tb02259.x}}
6. ^{{Cite journal|last=Kaya|first=C|last2=Kirnak|first2=H|last3=Higgs|first3=D|last4=Saltali|first4=K|date=2002-02-28|title=Supplementary calcium enhances plant growth and fruit yield in strawberry cultivars grown at high (NaCl) salinity|journal=Scientia Horticulturae|volume=93|issue=1|pages=65–74|doi=10.1016/S0304-4238(01)00313-2}}
7. ^¹²{{cite book|last1=Roy|first1=R.N.|last2=Finck|first2=A.|last3=Blair|first3=G.J.|last4=Tandon|first4=H.L.S.|title=Plant nutrition for food security: a guide for integrated nutrient management|date=2006|publisher=Food and Agriculture Organization of the United Nations|location=Rome|isbn=978-92-5-105490-1|pages=25–42|chapter-url=ftp://ftp.fao.org/agl/agll/docs/fpnb16.pdf|accessdate=20 June 2016|chapter=Chapter 3: Plant nutrients and basics of plant nutrition}}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}
8. ^{{Cite journal|last=Mossor-Pietraszewska|first=Teresa|year=2001|title=Effect of aluminium on plant growth and metabolism|url=http://www.actabp.pl/pdf/3_2001/673-686s.pdf|journal=Acta Biochimica Polonica|volume=48|pages=|via=}}
9. ^{{Cite journal|last=Delhaize|first=Emmanuel|year=1995|title=Aluminum Toxicity and Tolerance in Plants|journal=Update on Environmental Stress|volume=107|issue=2|pages=315–321|pmc=157131|pmid=12228360|doi=10.1104/pp.107.2.315}}
10. ^{{Cite web|url=http://canadianwollastonite.com/soil-amendments/silicon-plant-health/|title=Soil Amendments: Silicon and plant health|website=canadianwollastonite.com|language=en-US|access-date=2017-04-20|archive-url=https://web.archive.org/web/20170421092532/http://canadianwollastonite.com/soil-amendments/silicon-plant-health/|archive-date=2017-04-21|dead-url=yes|df=}}
11. ^{{Cite journal |doi=10.1093/jxb/erv040|pmid=25711709|pmc=4986723|title=Should we treat the ionome as a combination of individual elements, or should we be deriving novel combined traits?|journal=Journal of Experimental Botany|volume=66|issue=8|pages=2127–2131|year=2015|last1=Baxter|first1=Ivan}}
12. ^¹²{{Cite journal |doi=10.3389/fpls.2013.00039|pmid=23526060|pmc=3605521|title=The Plant Ionome Revisited by the Nutrient Balance Concept|journal=Frontiers in Plant Science|volume=4|pages=39|year=2013|last1=Parent|first1=Serge-Étienne|last2=Parent|first2=Léon Etienne|last3=Egozcue|first3=Juan José|last4=Rozane|first4=Danilo-Eduardo|last5=Hernandes|first5=Amanda|last6=Lapointe|first6=Line|last7=Hébert-Gentile|first7=Valérie|last8=Naess|first8=Kristine|last9=Marchand|first9=Sébastien|last10=Lafond|first10=Jean|last11=Mattos|first11=Dirceu|last12=Barlow|first12=Philip|last13=Natale|first13=William}}
13. ^{{Cite journal |doi=10.1016/j.plantsci.2014.12.005|pmid=25576003|title=Overcoming ammonium toxicity|journal=Plant Science|volume=231|pages=184–190|year=2015|last1=Bittsánszky|first1=András|last2=Pilinszky|first2=Katalin|last3=Gyulai|first3=Gábor|last4=Komives|first4=Tamas}}
14. ^¹²{{cite book|author1=Allen V. Barker|author2=D. J. Pilbeam|title=Handbook of plant nutrition|url=https://books.google.com/?id=ZWjLBQAAQBAJ|accessdate=17 August 2010|year=2007|publisher=CRC Press|isbn=978-0-8247-5904-9}}
15. ^¹²{{cite book|last1=Barker|first1=AV|last2=Pilbeam|first2=DJ|title=Handbook of Plant Nutrition.|date=2015|publisher=CRC Press|isbn=9781439881972|edition=2nd|url=https://books.google.com/?id=Ttw_CQAAQBAJ&pg=PP1&dq=cobalt+plant+nutrition#v=onepage&q=cobalt&f=false|accessdate=5 June 2016}}
16. ^¹{{cite web |url=http://aesl.ces.uga.edu/publications/plant/Nutrient.htm |title=Archived copy |accessdate=2010-02-10 |deadurl=yes |archiveurl=https://web.archive.org/web/20100219114737/http://aesl.ces.uga.edu/publications/plant/Nutrient.htm |archivedate=2010-02-19 |df= }} Retrieved Jan. 2010
17. ^¹{{cite book|editor1-last=Marschner|editor1-first=Petra|title=Marschner's mineral nutrition of higher plants|date=2012|publisher=Elsevier/Academic Press|location=Amsterdam|isbn=9780123849052|edition=3rd}}
18. ^¹²³⁴⁵⁶⁷⁸⁹¹⁰¹¹¹²¹³{{cite book|author1=Norman P. A. Huner|author2=William Hopkins|title=Introduction to Plant Physiology 4th Edition|publisher=John Wiley & Sons, Inc.|isbn=978-0-470-24766-2|chapter= 3 & 4|date=2008-11-07}}
19. ^¹Pages 68 and 69 Taiz and Zeiger Plant Physiology 3rd Edition 2002 {{ISBN|0-87893-823-0}}
20. ^¹Black, C.A. 1957. Soil-plant relationships. New York, Wiley and Sons. 332 p.
21. ^¹²Russell, E.W. 1961. Soil Conditions and Plant Growth, 9th ed. Longmans Green, London, U.K.. 688 p.
22. ^¹Benzian, B. 1965. Experiments on nutrition problems in forest nurseries. U.K. Forestry Commission, London, U.K., Bull. 37. 251 p. (Vol. I) and 265 p. (Vol II).
23. ^¹Swan, H.S.D. 1960b. The mineral nutrition of Canadian pulpwood species. Phase II. Fertilizer pellet field trials. Progress Rep. 1. Pulp Pap. Res. Instit. Can., Montreal QC, Woodlands Res. Index No. 115, Inst. Project IR-W133, Res. Note No. 10. 6 p.
24. ^¹Heiberg, S.O.; White, D.P. 1951. Potassium deficiency of reforested pine and spruce stands in northern New York. Soil Sci. Soc. Amer. Proc. 15:369–376.
25. ^¹Sato, Y.; Muto, K. 1951. (Factors affecting cold resistance of tree seedlings. II. On the effect of potassium salts.) Hokkaido Univ., Coll. Agric., Coll. Exp. Forests, Res. Bull. 15:81–96.
26. ^¹²Swan, H.S.D. 1971a. Relationships between nutrient supply, growth and nutrient concentrations in the foliage of white and red spruce. Pulp Pap. Res. Inst. Can., Woodlands Pap. WR/34. 27 p.
27. ^¹{{cite book|last=Lowenfels, Lewis|first=Jeff, Wayne|title=Teaming with microbes|year=2011|isbn=978-1-60469-113-9|pages=49, 110}}
28. ^¹{{cite journal | last1 = Krasowski | first1 = M.J. | last2 = Owens | first2 = J.N. | year = 1999 | title = Tracheids in white spruce seedling's long lateral roots in response to nitrogen availability | doi = 10.1023/A:1004610513572 | journal = Plant and Soil | volume = 217 | issue = 1/2| pages = 215–228 }}
29. ^¹²(2012). New Light Shined on Photosynthesis. http://www.newswise.com/articles/new-light-shined-on-photosynthesis University of Arizona
30. ^¹²University of Zurich (2011). Blossom end rot: Transport protein identified. http://phys.org/news/2011-11-blossom-protein.html
31. ^¹Chapman, H.D. (Ed.) 1966. Diagnostic Criteria for Plants and Soils. Univ. California, Office of Agric. Publ. 794 p.
32. ^¹²{{cite journal|title=Silicon nutrition in plants|journal=Plant Health Care, Inc.|date=12 December 2000|pages=1|url=http://excellerator.files.wordpress.com/2011/02/phc_silicon.pdf|archive-url=https://web.archive.org/web/20110419074023/http://excellerator.files.wordpress.com/2011/02/phc_silicon.pdf|dead-url=yes|archive-date=19 April 2011|accessdate=1 July 2011}}
33. ^¹{{cite journal |last=Prakash |first=Dr. N. B. |title=Evaluation of the calcium silicate as a source of silicon in aerobic and wet rice |publisher=University of Agricultural Science Bangalore |year=2007 |pages=1}}
34. ^¹{{cite web|title=AAPFCO Board of Directors 2006 Mid-Year Meeting|url=https://docs.google.com/viewer?a=v&q=cache:iOI8KNDnLWIJ:www.aapfco.org/MY06BODAgenda.pdf+aapfco+silicon&hl=en&gl=us&pid=bl&srcid=ADGEESjMlF3h06OX6FdbTRJOEdaajE2qOt3w4NSERgyku4mg6N0CkbhDSWZE3P31RoNP-BDM4Td8YajxqeqPrnCNY1vt01pOAMfTO85N4j4AXUhwbR2q1Wba3orzcMj6Bpr0yk55P_GZ&sig=AHIEtbS2-zE_UrT_3T9_gqKrxD9us-1_bA|publisher=Association of American Plant Food Control Officials|accessdate=18 July 2011}}
35. ^¹{{cite web |last=Miranda |first=Stephen R. |first2=Bruce |last2=Barker |title=Silicon: Summary of Extraction Methods |url=https://docs.google.com/viewer?a=v&q=cache:SzfW40-2DDcJ:www.aapfco.org/AM09/LSC_Si_Methods_DC.ppt+aapfco+siicon&hl=en&gl=us&pid=bl&srcid=ADGEESj4Jo-RFFj54kb6Sun3ikgJW9DMHzRAuUS045YkFErzE5NaSA084KvIyRxJp0IVX5ktDhaPPqcYLRx2hVu6K5YVWj95h2kgvkvDLQLyrxcJXXD3tQ3P5YLJ7J5F8rRYzenxznHp&sig=AHIEtbSPNk7BtSIpiRnvNI1F-2jSLN5LYA|publisher= Harsco Minerals |date=August 4, 2009 |accessdate=18 July 2011}}
36. ^¹²Dunn, C.E. 1991. Assessment of biogeochemical mapping at low sample density. Trans. Instit. Mining Metall., Vol. 100:B130–B133.
37. ^¹{{Cite web |url=http://esciencenews.com/articles/2012/04/13/nutrient.and.toxin.all.once.how.plants.absorb.perfect.quantity.minerals |title=Nutrient and toxin all at once: How plants absorb the perfect quantity of minerals |date=April 12, 2012 |website=esciencenews.com |access-date=2019-03-12}}
38. ^¹{{Cite journal |last=Ronco |first=F. |year=1970 |title=Chlorosis of planted Engelmann spruce seedlings unrelated to nitrogen content |journal=Can. J. Bot. |volume=48 |issue=5 |pages=851–853|doi=10.1139/b70-117 }}
39. ^¹²{{cite journal|last1=White|first1=Philip J.|title=Selenium accumulation by plants|journal=Annals of Botany|date=2016|volume=117|issue=2|pages=217–235|doi=10.1093/aob/mcv180|url=http://aob.oxfordjournals.org/content/117/2/217.full|accessdate=5 June 2016|pmc=4724052|pmid=26718221}}