Plant nutrition
Essential nutrient elements
Plants require 16 (Resh 2013) or according to other sources 17 (Bittszansky et al. 2016) essential nutrient elements without which they are unable to complete a normal life cycle. Plants require essential nutrients for normal functioning and growth. A plant’s sufficiency range is the range of nutrient amount necessary to meet the plant’s nutritional needs and maximize growth. The width of this range depends on individual plant species and the particular nutrient. Nutrient levels outside of a plant’s sufficiency range cause overall crop growth and health to decline due to either a deficiency or toxicity.
Plants normally obtain their water and mineral needs from the soil. In hydroponics they still need to be supplied with water and minerals. In aquaponics, the situation is complicated by the fact, that the system water contains a highly complex mixture of organic and inorganic compounds originating from fish waste and fish food. There are two major categories of nutrients: macronutrients and micronutrients (Figure 8). Both types are essential, but in differing amounts. Much larger quantities of the six macronutrients are needed compared with the micronutrients, which are only needed in trace amounts (Jones & Olson-Rutz 2016).
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Figure 8: Classification of essential elements (nutrients) that are needed for the plant growth
Macronutrients are divided into three groups. The terms ‘primary’ and ‘secondary’ refer to the quantity, and not to the importance of a nutrient. A lack of a secondary nutrient is just as detrimental to plant growth as a deficiency of any one of the three primary nutrients, or a deficiency of micronutrients. A basic understanding of the function of each nutrient is important in order to appreciate how they affect plant growth (Table 6). A good orientation of how much of particular nutrient is required gives the elemental composition of plant material (Figure 9). If nutrient deficiencies occur, it is important to be able to identify which element is lacking in the system and adjust it accordingly by adding supplemental fertilizer or increasing mineralization (see also Chapters 6 and 9).
Figure 9: Representation of nutrient amounts in dried plant material
Table 6: Essential elements and their role in plants (adapted after Resh 2013)
| Element | Role |
|---|---|
| Carbon (C) | C forms the backbone of most biomolecules, including proteins, starches and cellulose. Photosynthesis converts CO2 from the air or water into carbohydrates which are used to store and transport energy within the plant. |
| Hydrogen (H) | H is constituent of all organic compounds of which carbon is a constituent. It is obtained almost entirely from water. It is important in cation exchange in plant–soil relations. H+ ions are required to drive the electron transport chain in photosynthesis and in respiration. |
| Oxygen (O) | O is a component of many organic and inorganic compounds in plants. Only a few organic compounds, such as carotene, do not contain O. It can be acquired in many forms: O2 and- - 2−CO2, H2O, NO3 , H2PO4 and SO4 . It is also involved in anion exchange between roots andthe external medium. Plants produce O2 during photosynthesis but then require O2 to undergo aerobic respiration and break down this glucose to produce ATP. |
| Nitrogen (N) | N is part of a large number of organic compounds, including amino acids, proteins, coenzymes, nucleic acids, and chlorophyll. It is essential for photosynthesis, cell growth, and metabolic processes. Usually, dissolved N is in the form of nitrate, but plants can utilize moderate quantities of ammonia and even free amino acids. |
| Phosphorus (P) | P is part of the phospholipid backbone of nucleic acids (such as DNA, deoxyribonucleic acid), and adenosine triphosphate (ATP, the molecule that stores energy in the cells), and is contained in certain coenzymes. It is essential for photosynthesis, as well as the formation of oils and sugars, and encourages germination and root development in seedlings. As young tissues require more energy, it is particularly important for juveniles. |
| Potassium (K) | K acts as a coenzyme or activator for many enzymes. Protein synthesis requires high potassium levels. It is used for cell signalling via controlled ion flow through membranes. K also controls the opening of the stomata, and is involved in the development of flowers and fruit. It is also involved in the production and transportation of sugars, water uptake, disease resistance, and the ripening of fruits. K does not form a stable structural part of any molecules inside plant cells. |
| Calcium (Ca) | Ca is found in cell walls as calcium pectate, which cements together primary walls of adjacent cells. It is involved in strengthening stems, and contributes to root development. Required to maintain membrane integrity and is part of the enzyme α-amylase. It precipitates as crystals of calcium oxalate in vacuoles. Sometimes interferes with the ability of magnesium to activate enzymes. |
| Magnesium (Mg) | Mg is an essential part of the chlorophyll molecule. Without Mg, chlorophyll cannot capture the solar energy needed for photosynthesis. Mg is also required for activation of many enzymes needed for growth. It is essential to maintain ribosome structure, thus contributing to protein synthesis. |
| Sulphur (S) | S is incorporated into several organic compounds including amino acids (methionine and cysteine) and proteins (like photosynthetic enzymes). Coenzyme A and the vitamins thiamine and biotin also contain S. |
| Boron (B) | B is one of the less understood nutrients. It is used with Ca in cell wall synthesis and is essential for cell division. B increases the rate of transport of sugars from mature plant leaves to actively growing regions (growing point, roots, root nodules in legumes) and also to developing fruits. B requirements are much higher for reproductive growth as it helps with pollination, and fruit and seed development. Other functions include N metabolism, formation of certain proteins, regulation of hormone levels and transportation of K to stomata (which helps regulate internal water balance). |
| Chlorine (Cl) | Cl is classified as a micronutrient however plants may take up as much Cl as they do secondary elements such as S. Cl is important in the opening and closing of stomata. It is required for photosynthesis, where it acts as an enzyme activator during the production of oxygen from water. It functions in cation balance and transport within the plant. It is involved in disease resistance and tolerance. Cl competes with nitrate uptake, tending to promote the use of ammonium nitrogen. Lowering nitrate uptake may be a factor inchlorine’s role in disease suppression, since high plant nitrates have been associated with disease severity. |
| Copper (Cu) | Cu activates some enzymes which are involved in lignin synthesis and it is essential in several enzyme systems. It is also required in photosynthesis, plant respiration, and assists in plant metabolism of carbohydrates and proteins. Cu also serves to intensify flavour and colour in vegetables, and colour in flowers. |
| Iron (Fe) | Fe is required for the synthesis of chlorophyll and some other pigments and is an essential part of ferredoxins. Ferredoxins are small proteins containing Fe and S atoms that act as electron carriers in photosynthesis and respiration. Fe is also part of the nitrate reductase and activates certain other enzymes. |
| Manganese (Mn) | Mn activates one or more enzymes in fatty acid synthesis, the enzymes responsible for DNA and RNA formation, and the enzymes involved in respiration. It participates directly in the photosynthetic production of O2 from H2O and is involved in chloroplast formation, nitrogen assimilation and synthesis of some enzymes. It plays role in pollen germination, pollen tube growth, root cell elongation, and resistance to root pathogens. |
| Molybdenum (Mo) | Mo acts as an electron carrier in the conversion of nitrate to ammonium before it is used to synthesize amino acids within the plant. It is essential for nitrogen fixation. Within the plant, Mo is used in conversion of inorganic phosphorus into organic forms. |
| Nickel (Ni) | Ni is the metal cofactor of urease-enzymes: without it they are inactive (Polacco et al. 2013). Ureases are present in bacteria, fungi, algae, and plants - but they are absent from fish and other animals. Urease enzymes are responsible for the catabolic detoxification of urea, potentially phytotoxic waste excreted by the fish. |
| Zinc (Zn) | Zn activates a series of enzymes that are responsible for the synthesis of certain proteins, including some important enzymes like alcohol dehydrogenase, lactic acid dehydrogenase etc. It is used in the formation of chlorophyll and some carbohydrates, conversion of starches to sugars and its presence in plant tissue helps the plant to withstand cold temperatures. Zn is required for the formation of auxins, which are hormones which help with growth regulation and stem elongation. |
Nutrient availability and pH
Nutrients exist both as complex, insoluble compounds and as simple forms that are usually water soluble and readily available to plants. The insoluble forms must be broken down to available forms in order to benefit the plant. These available forms are summarized in Table 7.
Table 7: Absorbed nutrient forms and approximate concentrations in dry plant tissue (adapted from Jones & Olson-Rutz 2016)
| Element | Form absorbed | Concentration range in dry plant tissue (%) |
|---|---|---|
| Nitrogen (N) | NO3 - (nitrate) / NH4+ (ammonium) | 1 - 5 |
| Phosphorus (P) | H2PO4- , HPO42- (phosphate) | 0.1 – 0.5 |
| Potassium (K) | K+ | 0.5 – 0.8 |
| Calcium (Ca) | Ca2+ | 0.2 - 1.0 |
| Magnesium (Mg) | Mg2+ | 0.1 – 0.4 |
| Sulphur (S) | SO42- (sulfate) | 0.1 – 0.4 |
| Boron (B) | H3BO3 (boric acid) / H2BO3 -(borate) | 0.0006 – 0.006 |
| Chlorine (Cl) | Cl- (chloride) | 0.1 – 1.0 |
| Copper (Cu) | Cu2+ | 0.0005 – 0.002 |
| Iron (Fe) | Fe2+, Fe3+ | 0.005 – 0.025 |
| Manganese (Mn) | Mn2+ | 0.002 – 0.02 |
| Molybdenum (Mo) | MoO42- (molybdate) | 0.000005 - 0.00002 |
| Nickel (Ni) | Ni2+ | 0.00001 – 0.0001 |
| Zinc (Zn) | Zn2+ | 0.0025 – 0.015 |
The pH of the solution determines the availability of the various elements to the plant (Figure 10). The pH value is a measure of acidity. A solution is acidic if the pH is less than 7, neutral if the pH is at 7, and alkaline if the pH is above 7. Since pH is a logarithmic function, a one-unit change in pH means a 10-fold change in H+ concentration. Therefore, any small change in pH can have a large effect on the ion availability to plants. Most plants prefer a pH between 6.0 and 7.0 for optimum nutrient uptake.
Figure 10: The effect of pH on the availability of plant nutrients (from Roques et al. 2013)
Nutritional disorders in plants
A nutritional disorder is caused by either excess or deficiency of a certain nutrient (Resh 2013). It is important to detect nutritional disorders as soon as possible, to prevent spreading of the symptoms and eventual death of the plant. However, the precise diagnosis of nutrient disorders is not easy, because many deficiencies have overlapping symptoms. To make things more complicated, there are also plant diseases that can cause similar symptoms. The only way to be able to distinguish these symptoms from one another is to acquire knowledge through practice. Observe your plants, note the different symptoms, and relate these to the results of the water quality analysis. Also, a beginner should always consult an expert.
One aspect of the diagnosis is the distinction between mobile (Mg, P, K, Zn, N) and immobile elements (Ca, Fe, S, B, Cu, Mn). All nutrients move relatively easily from the root to the growing portion of the plant through the xylem. However, mobile elements can also be repositioned from older leaves to the actively growing region of the plant (younger leaves), when the deficiency occurs. As a result, the deficiency symptoms first appear on the older leaves. Conversely, immobile elements, once incorporated into the various structures, cannot be disassembled from these structures and re-transported through the plant. Deficiency symptoms first appear on the upper young leaves of the plant. Other aspects of diagnosis and their terminology are summarized in Table
8. Descriptions of deficiency and toxicity symptoms for essential elements are presented in Table 9.
Table 8: Terminology used for the description of symptoms of nutritional disorders (adapted from Resh 2013)
| Term | Description |
|---|---|
| Generalized | Symptoms spread over entire plant or leaf |
| Localized | Symptoms limited to one area of plant or leaf |
| Drying | Necrosis—scorched, dry, papery appearance |
| Marginal | Chlorosis or necrosis—on margins of leaves; usually spreads inward as symptom progresses |
| Interveinal chlorosis | Chlorosis (yellowing) between veins of leaves |
| Mottling | Irregular blotchy pattern of indistinct light (chlorosis) and dark areas; often associated with virus diseases |
| Spots | Discoloured area with distinct boundaries adjacent to normal tissue |
| Colour of leaf undersides | Often a particular coloration occurs on the lower surface of the leaves, for example, phosphorus deficiency—purple coloration of leaf undersides |
| Cupping | Leaf margins or tips may cup or bend upward or downward |
| Checkered (reticulate) | Pattern of small veins of leaves remaining green while interveinal tissue yellows— manganese deficiency |
| Brittle tissue | Leaves, petioles, stems may lack flexibility, break off easily when touched— calcium or boron deficiency |
| Soft tissue | Leaves very soft, easily damaged—nitrogen excess |
| Dieback | Leaves or growing point dies rapidly and dries out—boron or calcium deficiencies |
| Stunting | Plant shorter than normal |
| Spindly | Growth of stem and leaf petioles very thin and succulent |
Table 9: Deficiency and toxicity symptoms for essential elements (adapted from Resh 2013)
| Element | Deficiency | Toxicity |
|---|---|---|
| Nitrogen (N) | Reduction in protein results in stunted growth and dormant lateral buds. Stems, petioles, and lower leaf surfaces of corn and tomato can turn purple. The chlorophyll content of leaves is reduced, resulting in general pale yellow colour, especially older leaves. Flowering, fruiting, protein and starch contents are reduced. | Plants usually dark green in colour with abundant foliage but usually with a restricted root system. Can cause difficulties in flower and fruit set. |
| Phosphorus (P) | Poor root development, stunted growth. Reddening of the leaves. Dark green leaves (may be confused with excessive N supply, as it also leads to darker green leaves). Delayed maturity. The tips of plant leaves may also appear burnt. Deficiency symptoms occur first in mature leaves. | No primary symptoms yet noted. Sometimes Cu and Zn deficiencies occur in the presence of excess P. |
| Potassium (K) | Deficiency will cause lower water uptake and will impair disease resistance. Symptoms first visible on older leaves. Margins of leaves curl inward. In dicots, these leaves are initially chlorotic but soon scattered burnt spots (dead areas) develop. In monocots, the tips and margins of the leaves die first. | Usually not excessively absorbed by plants. Excess K may lead to Mg, and possibly Mn, Zn or Fe deficiency. |
| Calcium (Ca) | Signs of deficiencies include tip burn on leafy plants and roots, blossom end rot on fruity plants, and improper growth of tomatoes. Young leaves are affected before old leaves. | No consistent visible symptoms. |
| Magnesium (Mg) | Without sufficient amounts of Mg, plants begin to degrade the chlorophyll in the old leaves. This causes interveinal chlorosis, the main symptom of Mg deficiency. Later, necrotic spots may occur in the chlorotic tissue. Growth is reduced. | No information. |
| Sulphur (S) | Not often encountered. S deficiency can be easily confused with lack of N. Symptoms, like delayed and stunted growth, are similar. However, general chlorosis occurs on younger leaves first, whereas N deficiency symptoms are first visible on older foliage. | Reduction in growth and leaf size. Sometimes interveinal yellowing or leaf burning. |
| Boron (B) | Symptoms vary with species and first appear on new leaves and the growing points (which often die. The branches and the roots are often short and swollen. Leaves show mottled chlorosis, thickening, brittleness, curling, wilting. Internal tissues sometimes disintegrate or discolour. Since B helps transport sugars, its deficiency causes a reduction ofexudates and sugars from plant roots, which can reduce the attraction and colonization of mycorrhizal fungi. | Yellowing of leaf tip followed by progressive necrosis starting on the leaf margin and progressing toward midrib. Unlike most nutrient deficiencies that typically exhibit symptoms uniformly across the crop, Bsymptoms can appear randomly within a crop (Mattson & Krug 2015). |
| Chlorine (Cl) | Wilting of leaves, often with stubby tips. Leaf mottling and leaflet blade tip wilting with chlorosis and necrosis. Roots become stunted and thickened near tips. Chlorine deficiency in cabbage is marked by an absence of the typical cabbage odour. | Excessive Cl can be as a major component of salinity stress and toxic to plants (Chen et al. 2010). Symptoms include scorched leaf margins, bronzing, yellowing, excessive abscission, reduced leaf size, lower growth rate. Cl accumulation is higher in older tissue. |
| Copper (Cu) | Natural deficiency is rare. Typically, the symptoms start as cupping of young leaves, with small necrotic spots on the leaf margins. As the symptoms progress, the newest leaves are smaller in size, lose their sheen and may wilt. The growth points (apical meristems) may become necrotic and die. Plants typically have a compact appearance as the stem length between the leaves shortens. Excess K, P or other micronutrients can indirectly cause Cu deficiency. | Reduced growth followed by symptoms of iron chlorosis, stunting, reduced branching, thickening, and abnormal darkening of rootlets. |
| Iron (Fe) | Pronounced interveinal chlorosis. Similar to Mg deficiency, but here chlorosis will start at the tips of younger leaves and will work its way to older leaves. Other signs, always be coupled with the leaf chlorosis, can include poor growth and leaf loss. | Not often evident in natural conditions. Has been observed after the application of sprays where it appears as necrotic spots. |
| Manganese (Mn) | Leaves turn yellow and there is also interveinal chlorosis, first on young leaves. Necrotic lesions and leaf shedding can develop later. Disorganization of chloroplast lamellae. Mn may be unavailable to plants where pH is high. This is why it often occurs together with Fe deficiency, and also has similar symptoms.The symptoms of Mn deficiency are also similar to Mg because Mn is also involved in photosynthesis. | Sometimes chlorosis, uneven chlorophyll distribution.Reduction in growth. |
| Molybdenum (Mo) | As Mo is closely linked to N, its deficiency can easily resemble N deficiency. Deficiency symptoms start on older or midstem leaves: interveinal chlorosis, in some crops the whole leaf turns pale; leaf marginal necrosis or cupping. Leaves can be misshapen. Crops that are most sensitive to Mo deficiency are crucifers (broccoli, cauliflower, cabbage), legumes (beans, peas, clovers), poinsettias and primula. | Rarely observed. Tomato leaves turn golden yellow. |
| Nickel (Ni) | Ni is part of enzymes that detoxify urea. Although urea is an excellent source of nitrogen for plants (Yang et al. 2015), at higher concentrations it is strongly toxic to plant tissues. Typical symptoms of urea toxicity, and potentially also of Ni deficiency, are leaf burn and chlorosis (Khemira et al. 2000). | Ni is strongly phytotoxic at higher concentration. In induces change in activity of antioxidant enzymes, and has a negative effect on photosynthesis and respiration. Excess Ni causes are chlorosis, necrosis and wilting. Cell division and plant growth are inhibited. High uptake of Ni induces a decrease in water content, which can act as an indicator for Ni toxicity in plants (Bhalerao et al. 2015). |
| Zinc (Zn) | Stunted growth, with shortened internodes and smaller leaves. Leaf margins are often distorted or puckered. Sometimes interveinal chlorosis. | Excess Zn commonly produces iron chlorosis in plants. |
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