Nutrients Essential for Crop Growth | Botany

For convenience the nutrients essential for crop growth can be grouped into four categories related to their primary role in plant nutrition: 1. Basic Structure 2. Energy Storage and Transfer Energy Bonding 3. Charge Balance 4. Enzyme Activation and Electron Transport.

1. Basic Structure: Carbon, Hydrogen and Oxygen:

Carbohydrates (CH₂O)n make up the skeleton or structure of plants and are a source of metabolic energy. Carbohydrates include numerous organic acids, simple and complex sugars, and polymers of sugars, such as starch, cellulose, and hemicellulose.

Organic acids are the precursors of amino acids, which polymerize with the peptide linkage to form proteins. By weight approximately 45, 6, and 43% of a plant is composed of C, H, and O, respectively. Therefore, over 90% of plant dry weight or crop yield is derived from air and water.

2. Energy Storage and Transfer Energy Bonding:

i. Nitrogen:

Nitrogen constitutes 79% of the atmosphere, and even more N is in the soil as organic sediments. Unfortunately neither dinitrogen (N₂) of the atmosphere or combined N in soil sediments is available for plant growth. Only the oxidized (NO₃) or reduced (NH₄) forms (ions) are available.

Bonding to hydrogen, which reduces N, can be accomplished by lightning, by nitrogen- fixing organisms, or commercially by the Haber-Bosch process. Ammonia is oxidized to nitrate by the nitrifying bacteria- NH₄ ⇌ NO₂ ⇌ NO₃. Such N transformations are biological and therefore sensitive to soil pH, temperature, and moisture.

Soil temperatures of 25°C or higher were most favorable for nitrification, while ammonification was less temperature sensitive. Virtually no nitrification occurs in the winter months in temperate climates. During spring, while soils are still cold and wet, nitrification is minimal and usually inadequate for healthy plant growth. Growth of grass plants is stunted and yellow unless an available form of N is applied.

Nitrification was also greatly inhibited in forests and grass climax vegeta­tion due to the presence of natural inhibitors, such as tannins and phenols. On the other hand, clearing and cultivation greatly enhance nitrification, due to degradation and disappearance of inhibitors.

It is of interest to note that N deficiencies in wheat on Oklahoma prairie soils were not widely recognized as a problem until about 1957. Since then these soils have been responsive to N application, necessary because of continued loss of organic matter and N release from cultivation. These applica­tions of N fertilizers, however, have resulted in soil acidity and increased lime requirements.

Denitrification is favored by warm temperatures and reducing conditions, such as waterlogging. Warm, aerated soils favor nitrification and nitrate loss by leaching. To guard against nitrate losses, commercial inhibitors such as nitropyrin (2-chloro-6-trichloromethylpyridine) are used to retain the N as ammonium (NH₄), which is adsorbed by soil particles and therefore less subject to leaching.

Grain yields of maize were increased significantly and stalk rot incidence was reduced by the use of nitropyrin with fall-applied ammonia (NH₃). Nitrification inhibitors can also be used to reduce nitrification in the soil and nitrate uptake and accumulation in leafy vegetables, such as spinach.

The N content of plants averages 2 to 4% and may be as high as 6%. Crop plants can take up NO^–₃ or NH⁺₄ ions and assimilate them. The NO^–₃ form is absorbed, primarily due to the rapid conversion of NH₄ to NO^–₃ in the soil. However, maize absorbed NH⁺₄ and NO^–₃ at the same rate; uptake was linear above the 21 µmol concentration of N and declined below this threshold to a steady state at 4 µmol.

In lima bean, dry matter accumulation was consistently higher when NO^–₃ was 75% or more of the total available N, which illustrates genetic differences in ion preference. An interaction with K was demonstrated in maize; yields were depressed with NH^–₄N and the N-K ratios were elevated, compared with NO^–₃N.

High tissue levels of NH⁺₄ can retard growth and cause elevated levels of CI in tobacco, but NO^–₃ is not harmful at high levels. The form used by the plant depends in part on rainfall and pH; acid soils favored NO^–₃ uptake and depressed NH⁺₄ uptake. Because of the above fac­tors, NO^–₃ is the predominant ion absorbed by crop plants other than rice.

Assimilation of N into organic molecules was dependent on the reduction of NO^–₃ by the nitrate reductase enzyme in plant tissue. Nitrate reduction, which must occur before amino acids and other chemical combinations of N can be produced, requires electrons; the primary donors are nicotinamide adenine dinucleotide (NADH) or nico­tinamide adenine dinucleotide phosphate (NADPH), which are products of photosynthesis. It follows that high light and photosynthetic rates were found conducive to nitrate reductase activity.

Accumula­tion of NO^–₃ to levels toxic to animals may occur in forages during cloudy conditions. A favorable temperature is also necessary for nitrate reduction, with considerable differences among species. Reduction of NO^–₃ is not without an energy cost to the plant.

Woody species essentially limit nitrate reduction to roots, whereas in crop plants reduction occurred in both roots and leaves. Certain vegetable crops, such as spinach and members of the family Chenopodiaceae, are evidently missing nitrate reductase capacity in the roots and may accumulate high amounts of nitrate in leaves.

A positive association between nitrate reductase activity and grain yield and protein has been found in corn, wheat, and sorghum by various workers, but in another study it was not highly correlated to high yield and protein in wheat. In two spring wheat nitrate reductase was ample at senescence, but N was not.

Nitrogen is a constituent of amino acids, amides, N bases such as purine, and proteins and nucleoproteins. Enzymes contain long-chain, complex protein molecules plus a non-protein reactive group, which is generally a micronutrient.

Proteins are polymers from 20 amino acids joined by the peptide linkage in a myriad of combinations, resulting in a high molecular weight. Amino acids have amino-N attached at the a-carbon position and may also have N in the ring as with tryptophan.

Glutamine has N in an amide group, and adenine is a purine base with N in the ring. Adenine is a part of many nucleotides and nucleoproteins, such as DNA and RNA. Nitrogen is also a constituent of a host of compounds termed alkaloids whose function is not well understood and which evidently are not essential metabolites. They are believed to serve as compounds of N storage.

A deficiency of N limits cell division and expansion, chloroplast develop­ment, chlorophyll concentration, and enzyme activity. Deficiency symptoms include general stunting and yellowing, particularly of the older plant parts. The reduction in plant growth can cause accumulation of sugars and in some species, especially maize, cause the basal tissues to turn purple due to anthocyanin formation.

Nitrogen is highly mobile in the plant. Younger leaves and developing organs with strong sink demands, such as fruits and seeds, may draw heavily on N in the older or lower leaves. The result of such redistribution when N uptake is limited is firing (yellowing and senescence) of the lower leaves.

Non-modulated, N-starved soybean senesced earlier and had 60% of total N in the seeds, compared with a control group of plants with later senescence and only 20% of total N in the seeds. Such firing of lower leaves formerly was erroneously attributed to moisture deficiency since it tended to become obvious in midsummer.

In summary, N is an essential constituent of amino acids, amides, nu­cleotides, and nucleoproteins and is essential to cell division, expansion, and therefore growth. It is mobile in the plant; N moves to young tissues so a deficiency is first visible in older leaves. A deficiency interrupts growth proc­esses, causing stunting, yellowing, and reduced dry matter yields.

ii. Sulfur:

Sulfur is derived from soil organic matter and also from inorganic salts, such as calcium sulfate and magnesium sulfate. The atmosphere contains gas­eous S (SO₂), as does rainwater (acid rain). The atmosphere of geographic areas distant from industrial cities or the sea/i.e., certain areas of Africa, the United States, Australia, and New Zealand), has little S so crop deficiencies are common.

Mineralization of S and formation of sulfate ions (transformation) from organic matter are quite similar to N transformations from organic matter. In anaerobic conditions H₂S (reduced) may form and accumulate in toxic concentrations. In poorly aerated conditions this compound is oxidized to elemental S by photosynthetic and chemotrophic bacteria; further oxidation produces sulfuric acid (H₂SO₄) and soil acidity.

Sulfur, absorbed primarily as the SO^2-₄ ion, is actively and passively translocated. Leaves can absorb appreciable quantities of SO₂ gas. Like N, all oxidized forms must first be reduced enzymatically before assimilation by the plant.

Sulfur, like N, is involved in low-energy bonding and protein synthesis. It forms thiol bonds analogous, energetically, to the N peptide bonds. Sulfhydryl groups (SH) are thought to be important in the hardening of protoplasm to cold and drought. In energy transfer sulfur can function in a manner similar to P.

Sulfur is a constituent of the amino acids cystine, cysteine, and methionine. It also activates certain proteolytic enzymes and is a constituent of coenzyme A, glutathione, and certain Vitamins.

Members of the Cruciferae family may contain over 1% S, and legumes are also relatively high in S. Maximum alfalfa hay yields were obtained when the S content of leaves was 0.15 to 0.20%. For maximum yield, a N-S ratio of 10 to 15 was optimum for sugar cane, whereas the N-S ratios on soils well supplied with S were 15 to 16 for maize, 20 for soybean, and 8 to 9 for cotton and okra.

Oils of some plants, particularly those of Cruciferae and onion, are rich in S. Sulfur fertilization has been shown to increase the seed oil content of crops such as flax and soybeans.

Sulfur deficiency, like N, is expressed as stunting and general plant yellow­ing; stems are thin. Although S is mobile in the plant, redistribution from older to younger leaves is not as pronounced with S as with N and firing of lower leaves does not commonly occur. Bouma (1967) found redistribution occurred from roots and petioles of subterranean clover but not appreciably from the leaves.

In summary S is a part of certain amino acids, glutathione, coenzyme A, and certain vitamins. Sulfur physiology is similar to that of N with regard to mineralization, uptake, reduction, energy bonding, incorporation, and stunted and yellow deficiency symptoms. Redistribution is not as great as that of N and so does not cause firing of lower leaves as does N deficiency.

iii. Phosphorus:

Phosphorus is derived from the soil organic and inorganic frac­tions as follows:

(1) Soil solution containing extremely minor amounts of soluble P, such as orthophosphate (HPO^2-₄ or H₂PO^–₄);

(2) P-containing minerals such as the apatites and the Ca-, Mg-, Fe-, and aluminum (AO- phosphates;

(3) The labile pool consisting of P adsorbed on soil colloids and of Fe- and Al-phosphates in equilibrium with phosphate in solution. The amount of P in solution is extremely low relative to the labile fraction. For this reason, P is generally second to N as the most limiting nutrient for plant growth.

Phosphorus is absorbed primarily as the monovalent ion H₂PO^–₄ and less as the divalent ion HPO^2-₄ which is more prevalent of the two at neutral pH or above. Roots actively absorb P from very low concentrations in soil solution and hold it in the plant at concentrations of up to 1,000-fold. The P absorption capacity of soybean root depended on age; the absorption of 18-day-old roots was four times that at 73 days.

Phosphorus is mobile in the plant, redistributed from older to younger parts. Young leaves or developing fruit can be nourished from the labile P of older plant tissues even though the soil source is interrupted. Depending on geographic location, the critical level in maize was 0.18 to 0.25% in the leaf subtending the ear. Sufficiency levels were 0.25 to 0.41%.

Phosphorus is a structural component of a number of vital compounds: energy transfer molecules ADP and ATP (adenosine di- and triphosphate), NAD, NADPH, and genetic information system compounds DNA and RNA (desoxyribo- and ribonucleic acid). Phosphate esters are formed with sugars, alcohols, acids, or other phosphates (polyphosphates).

Phytic acid is an important phosphate storage compound commonly found in seeds. This stored form of P is mobilized to support the high rate of metabolism during seed germination.

Phosphorus is also a constituent of phospholipids such as lecithin and choline, which play an important role in membrane integrity. Lecithin, an important by-product in soybean oil extraction, has numerous food and com­mercial uses.

The visible P-deficiency symptoms, somewhat opposite those of N or S deficiency, are dark green to blue-green leaves rather than yellow. Plants are stunted. Ryegrass root number and length were found to be reduced. In P-deficient plants sugar accumulates, reflected by anthocyanin pigmentation in the base of stems and veins, particularly in maize. As with N deficiencies, older leaves show P deficiencies first, due to redistribution of P to young tissues.

In summary, P is usually present in extremely low concentrations in the soil solution. It is an essential component of the energy transfer compounds (ATP and other nucleoproteins), the genetic information system (DNA and RNA), cell membranes (phospholipids), and phosphoproteins. Phosphorus is mobile and is redistributed from old to young tissues, so older leaves first show deficiency symptoms.

3. Charge Balance:

i. Potassium:

Potassium is derived from primary minerals and from secondary minerals such as clays. Generally, soils high in clay content tend to be relatively high in K, while organic and sandy soils are generally low. The main source of K for plants comes from weathering of K-containing minerals.

Soil K exists in three fractions:

(1) Chemically bound in primary and secondary soil minerals;

(2) Exchangeable, adsorbed to soil particles; and

(3) In the soil solution.

In mineral soils (e.g., soils high in montmorillonite), most of the K is in the mineral lattices. Only about 1 to 3% of the total is adsorbed or exchange­able and even a smaller fraction is in the soil solution. Exchangeable and soil solution K are in equilibrium. Uptake was primarily from the soil solution, but K can come, to some extent, from nonexchangeable forms. Most soils are highly buffered for K; fluctuations from year to year are minor.

Uptake of K is in the form of the monovalent cation K₊. Uptake was active and translocation can be against strong electrical and chemical gradients. Soil temperature affects absorption; the optimum for most species was about 25°C, but species vary. For example, sudangrass absorbed K at 30 to 35°C, whereas pea lost K at 35°C.

Soybean lost K from the roots at low temperatures (i.e., 5°, 13°, and 15°C). Hall and Baker (1972) showed that X constituted 80% of the cations found in the phloem. Transport is primarily acropetal (upward), and K enhanced transport of nitrate. Redistribution of K from older organs to younger is the rule; K is the most mobile of the plant nutrients.

While K is essential to all higher and lowers plants, it is not a part of any known plant constituent. It is stored in large quantities in the vacuoles. It did not form ligands (complex organic molecules), serving primarily as an enzyme activator or cofactor for some 46 enzymes.

Cofactor use only partially explains the high requirement for K as, in addition to K, micronutrients and Mg act as enzyme activators for certain enzymes. This aspect is of particular interest, considering that neither enzyme nor cofactor is used up in the chemical reaction; a little should go a long way.

Potassium also aided in the maintenance of osmotic potential and water uptake. Plants well supplied with K lost less water since K increased osmotic potential and had a positive influence on stomatal closure as well. Potassium also serves to balance the charges of anions and influence their uptake and transport. It was reported to have reduced the incidence of certain diseases and associated lodging in maize for physiological reasons yet to be explained. For example, K significantly decreased the presence of Verticillium wilt on cotton.

Potassium was found to serve a vital role in photo­synthesis by directly increasing growth and leaf area index and stomatal open­ing (regulation of osmotic potential), and hence photosynthesis and outward translocation of photosynthate. The latter appears to result from formation of more ATP, essential for loading assimilate into the phloem. Sodium can partially substitute for K in a number of crops, especially in sugar beet and cotton; substitution is minimally effective in others, such as maize and sorghum.

The critical level of K in plant tissue is relatively high, usually about 1.0% or 4-fold that of P. Nearly all the K is absorbed during vegetative growth; little is transferred to the fruits or grains. An application of K to wheat during the reproductive stage had little effect on grain yield.

A deficiency of K resulted in increased root and stalk lodging in maize, indicating a probable association with diseases. The num­ber of brace roots decreased and stalk parenchyma disintegrated when K fertil­izer was omitted, as in N-O-O or N-P-O treatments. Severe K deficiencies caused small necrotic spots between veins and the firing of leaf tips and margins of the older leaves of many species.

ii. Calcium:

Because of the number of Ca-bearing minerals, the earth’s crust is relatively high in Ca. Apatite (Ca-phosphates), calcite (CaCO₃), and dolomite (CaCO₃, MgCO₃) are especially common, but soils derived from these minerals under humid conditions may be leached and actually low in Ca. Young soils from marl, chalk, or limestone may contain over 10% Ca.

Miner­alization of N to nitrates and the formation of carbonic acid in humid regions leads in time to acid soils low in Ca and Mg and to a degraded soil structure due to the replacement of these adsorbed cations with Al³⁺ and H⁺ on the soil colloids. In modern agricultural practice, dolomitic limestone is used exten­sively as a soil amendment to raise the pH and to supply Ca and Mg as nutrients.

Calcium is absorbed as the bivalent cation Ca²⁺. It is the most immobile of the essential elements. Uptake and transport are passive; entry into the stele was via the free space, and upward movement was with the transpiration stream. Compared with other ions, there is little or no move­ment in the phloem.

Calcium is highly adsorbed on exchange sites of the free space, which is probably a limiting factor in Ca delivery to other plant organs. Peanut required a high Ca content in the pegging zone for normal pod devel­opment, which is absorbed directly by the pegs and fruits. It is believed that the transpiration stream to these underground fruits is negligible and therefore insufficient for the required Ca delivery.

Calcium is a component of the cell wall, particularly of the cementing substance, Ca-pectate. It is also found as Ca-oxalate and Ca-carbonate in the vacuoles; these salts supposedly immobilize the constituent organic acids to a nontoxic level. It is essential for cell division and elongation.

A Ca deficiency caused plant meristem (root, shoot, fruit, and nodule) malformation and dieback, presumably due to lack of phloem transport and immobility in the plant; growth of bunch bean (phaseolus vulgaris) stopped almost immediately when Ca was withdrawn from the nutrient culture. Cal­cium is also essential for the selective regulatory function of cell membranes.

The Ca status of a plant is highly related to pH, which affects more than Ca availability. As indicated previously, Ca affects availability of other nu­trients and growth of soil micro-flora, especially bacteria. Many legumes of a temperate climate origin have a high pH and presumably a high Ca requirement. If grown at a low pH, a legume such as alfalfa soon becomes stunted and chlorotic.

The author (Gardner) clearly demonstrated that such plants were also non-modulated and N deficient, since they turned green with N topdressing.

The major sensitivity was apparently in the Rhizobium meliloti rather than in the alfalfa per se. It seems conclusive that a Ca deficiency in many legumes, indicated by stunting and yellowing, is primarily N deficiency result­ing from pH sensitivity of the bacterium symbiont. Sensitivity varies widely among rhizobia, temperate species being the most sensitive.

Deficiencies of Ca are first seen in the younger plant parts as deformed and chlorotic leaves, while deficiencies are seldom observed in older organs. Calcium is not redistributed to younger tissues; hence, young leaves and developing fruits are totally dependent on Ca delivery in the transpiration stream of the xylem.

In fruit growth, a Ca deficiency caused blossom-end rot in tomato, and brown heart in peanut. There is evidence that Florida citrus fruit size may be limited by Ca deficiency even though roots are well supplied.

iii. Magnesium:

Soil Mg is derived primarily from the weathering of primary minerals (e.g., biotite, serpentine, hornblende, dolomite, and olivine). It is also present in secondary minerals (e.g., montmorillonite, illite, and vermicu- lite). Arid soils are generally high in dolomite and MgSO₄.

As with other cations, Mg²⁺ is in the soil solution adsorbed to soil particles, and in primary and secondary minerals. Normally Mg constitutes about 4 to 20% of the CEC, compared with as much as 80% for Ca and 5% for K. As expected, in humid soils A1 may readily replace Mg²⁺.

Uptake of Mg²⁺ is both active and passive. Transport is primarily in the transpiration stream. However, Mg is more mobile in the plant than Ca; more Mg than Ca was demonstrated to be present in the phloem (active transport) by autoradiogram studies. Developing fruits and storage organs are dependent on Mg redistribution from older leaves via phloem transport, but deficiencies developed slowly compared with Ca.

Magnesium is the center of the chlorophyll molecule, a Mg-chelate in the chloroplast. It also chelates with ADP, ATP, and organic acids and hence is essential for hundreds of enzymatic reactions.

Magnesium forms a bridge between ATP and the enzyme molecule and is required for photophosphorylation in synthesis and breakdown reactions of photosynthesis and in oxidative phosphorylation in respiration. It was a cofactor for many enzymes activating phosphorylation in the glycolysis and in the tricarboxylic acid cycle.

Since it is required to activate RuBP carboxylase, it is rate limiting in the photosynthetic process. Nitrogen metabolism and protein syn­thesis are also dependent on the presence of Mg, and it is believed to enhance the integrity of the ribosomes.

Deficiencies of Mg are generally observed first as interveinal chlorosis in the older leaves but may progress to affect younger leaves. Like K, and unlike Ca, Mg is somewhat mobile in the plant. Older leaves are affected first. Deficiencies were shown to affect chloroplast substructure of bunch bean, causing a reduction in grana number and size. Chlorosis started at leaf margins and tips and progressed inward in the leaf parenchyma cells. The veins remained green. In severe cases leaf necrosis occurred and reproductive phases were delayed.

In summary, Mg is a part of the chlorophyll molecule and an activator of photosynthesis and respiration enzymes and necessary for protein synthesis. It is redistributed in the plant, so a deficiency shows first in older leaves as interveinal chlorosis.

4. Enzyme Activation and Electron Transport:

i. Iron:

Iron constitutes about 5% of the earth’s crust and is universally present in soils. It is derived from primary minerals of ferromagnesium silicates, which include olivine, augite, hornblende, and biotite. Iron oxides common in many soils include hematite (Fe₂O₃), magnetite (FeO₄), and siderite (FeCO₃). Iron may also be present in the lattices of secondary minerals (e.g., montmorillonite).

Highly weathered ferromagnesium minerals produce hydrous Fe-ox- ides in the soil; these oxides along with the clay and aluminum oxides are concentrated in lateritic soils (e.g., Oxosols), which generally predispose severe management problems. Probably all soils have an ample content of Fe, but the solubility, which is regulated primarily by pH, may be so low as to cause Fe deficiency, especially in Fe-inefficient species and cultivars.

Solubil­ity may decrease 1000-fold per unit change in pH, as indicated by the following equation:

In poorly drained soils, reduced or ferrous forms (Fe²⁺) of Fe predominate and increase Fe availability, even to the extent of toxicity (known as bronzing in paddy rice).

The uptake of Fe is primarily as Fe²⁺, although Fe³⁺ and Fe-chelates are also present in the rhizosphere. Reduction is essential for absorption, the electron source probably being the cytochromes or flavins at the plasma mem­brane. Uptake is in competition with, and is influenced by, other cations. Good aeration, high pH, and Ca, phosphate, and nitrate ions depressed uptake.

Iron is a constituent of the electron transport enzymes, for example, the cytochromes and ferredoxin, which are active in photosynthesis and in mito­chondrial respiration. It is a constituent of the enzymes catalase and perox­idase, which catalyze the breakdown of H₂O₂ into H₂O and O₂, preventing H₂O₂ toxicity. Iron, along with Mo, is an element of the nitrite and nitrate reductase enzymes and of the N₂-fixation enzyme nitrogenase.

Although Fe is not a part of the chlorophyll molecule, it affects chlorophyll levels because it must be present for chloroplast ultrastructure formation. An Fe deficiency reduces the number and size of chloroplasts. Grana and lamella of the chloro­plast were reduced in Fe-deficient maize plants.

Iron is highly immobile in the plant and not redistributed. Brown (1961) recognized that plant genotypes varied widely in Fe uptake efficiency. Deficient plants develop chlorosis, which seemed to be a problem more of metabolism than of uptake. Species and cultivars that were high secretors of OH^– ions were Fe-inefficient. Cereals and grasses like wheat were high secretors of OH^–.

Iron-efficient tomato plants under Fe stress acidified the rooting medium and secreted reductants; one, caffeic acid, enhanced the solubility of Fe. Young leaves depend on current uptake. The addition of inorganic Fe compounds to the soil may have little or no value in correcting deficiencies except at high rates, since such compounds are rapidly converted to unavailable forms. Ferrous-Fe (FeSO₄) applied to leaves has been used with some success.

Fertiliza­tion with Fe-chelates as a soil amendment or foliar spray is more effective; Fe- EDDHA and Fe-montmorillonite clay were more effective than Fe-EDTA as a soil treatment in calcareous soils. Success of leaf application of FeCl₃ on a number of crop species was related to the number of stomata, application in light rather than at night, and use of surfactant.

ii. Manganese:

Ferromanganese minerals in soils supply Mn as well as Fe, the two being closely associated. Oxides of both are common. The Mn content of most soils ranges from 200 to 3,000 ppm, more than adequate in total but not necessarily adequate in available Mn. Manganese exists in soils as the divalent Mn²⁺ ion in the soil solution, as exchangeable Mn²⁺, and as Mn³⁺ and Mn⁴⁺ oxides in equilibrium with other Mn forms.

The Mn²⁺ ion predominates at low pHs, in natural chelates, and in reducing conditions such as waterlogging. Waterlogging, as in rice paddies and some soils low in pH, can produce soluble Mn to toxic levels. Uptake of Mn²⁺ is active; it may compete with other cations, particularly with NH⁺₄ and Fe²⁺. It is believed to be passively trans­ported. Manganese is an activator of several enzymes; especially those in­volved in fatty acid and nucleotide synthesis and are essential in respiration and photosynthesis.

In photosynthesis Mn²⁺ is oxidized to Mn³⁺ with the transfer of one electron from water to the chlorophyll molecule. Manganese can substi­tute for Mg in certain reactions; both ions are capable of bridging with certain enzymes (e.g., phosphokinase and phosphotransferase). It also activates in- doleacetic acid (IAA) oxidase, which results in less IAA concentration in tissues. Like Fe, Mn is relatively immobile and preferentially translocated to young or meristematic tissues.

These parts cannot depend on transfer from older leaves and hence are the first to show Mn deficiency as lesions on younger leaves. About 10 ppm was the critical level in young sorghum leaf tissue. Oats are prone to Mn deficiency, which appears as spotting, or grey specks, on younger leaves. Soybean, pea, and sugar beet are also susceptible to Mn deficiencies.

Soybean cultivars differ significantly in toler­ance to Mn deficiency; for example, ‘Bragg’ was resistant and ‘Forrest’ suscep­tible Deficiencies usually occur on calcareous peat soils, due to reduced availability at high pHs and immobilization by microorganisms. They can be corrected by Mn foliar sprays of MnSO₄ or chelated Mn (Mn-EDTA). On leached Podzol soils, which are naturally low in total Mn, a soil treatment with MnSO, is effective in correcting deficiencies.

iii. Zinc:

Zinc in soils is derived from the ferromagnesium minerals augite, hornblende, and biotite, which are found in basic igneous rocks. It is also present in the secondary minerals sphalerite (ZnFe) S, zincite (ZnO), and smithsonite (ZnCO₃). Zinc sulfide may be present under reducing conditions.

As with other cations, Zn²⁺ and ZnOH⁺ may occupy exchange sites on soil colloids. Generally zinc levels are positively correlated with increasing organic matter and negatively correlated with increasing pH. Zinc interacts with organic matter to form Zn-organic complexes.

About 60% of these chelates are soluble and constitute a major source of Zn in soils. Zinc availability was negatively related to P solubility. Leggett experienced 30 to 50°7o reduction of Zn in maize plants fertilized with approximately 300 kg P. ha^-2. Similar results were obtained with citrus in California and maize in Nebraska.

Uptake of Zn is primarily from the divalent Zn²⁺ ion, but probably some is from ZnCl⁺ and ZnOH⁺. Iron and Mn were found to be antagonistic to Zn uptake.

Zinc was found to be essential for the enzymes in the synthesis of trypto­phan, the precursor to IAA. Plants deficient in Zn are low in tryptophan and IAA and exhibit small leaves and early abscis­sion. Zinc is also a constituent of carbonic anhydrase, which catalyzes the reaction H₂CO₃ – H₂O + CO₂. Zinc and Cu are both constituents of superox­ide dismutase, which can split molecular O₂.

In soybean a Zn level below the critical 12µg. g^-1 in leaf 3 (from the top) reduced photosynthesis and reduced the carbonic anhydrase activity. A deficiency of Zn caused a reduction of RNA synthesis and ribosome stability. In soybean small leaf size was the first visible symptom, followed by chlorosis in the youngest leaves.

Acute deficiencies showed initial chlorosis in the interveinous parenchyma, then retarded leaf growth, and finally dieback. A Zn deficiency can be corrected by foliar sprays or soil treatment with Zn-chelates, preferably Zn-EDDHA on calcareous soils, since Ca replaces the Zn in the complex. A ZnSO₄ application of 4 to 5 kg. ha^-1 every 5 to 8 years has been used with success.

Soil treatment around individual fruit trees such as pecan and orange is the usual practice. Liming corrected a Zn deficiency in alfalfa but induced a B deficiency. Cultivars of maize varied in Zn response, and soy­bean was more Zn efficient than maize. Zinc deficiency did not relate to Zn uptake but rather to the P-Zn ratio, evidently because of competition with Zn in metabolism.

iv. Boron:

Boron is derived from primary minerals, such as the borosilicates. It is in the soil solution at very low levels as boric acid or borate (HBO₃) and adsorbed to soil particles as borate. Soils derived from sedimentary rock such as shales may have 100 ppm B, compared with about 15 ppm in soils from igneous rocks. Boron deficiency was found to be the most wide­spread of micronutrient deficiencies.

Adsorption of borate decreases with increasing soil pH, and availability is low in alkaline soils (pH 7-9). Heavy liming, as for alfalfa, frequently induces a B deficiency.

Uptake is believed to be as undisassociated boric acid; it seems to be primarily passive, based on B-polysaccharide complexes observed in the free space, although a small amount of active transport of B has been demon­strated. Passive transport is via the transpiration stream. It follows that B is relatively immobile in the plant, and young organs are dependent on current uptake.

Boron is believed to influence cell development by control of sugar trans­port and polysaccharide formation. Another function attributed to this ele­ment is combining with the active site of phosphorylation to inhibit starch formation, which prevents excessive sugar polymerization at the sites of sugar synthesis. Further, it appears that B may determine whether sugars are decom­posed for energy release via the glycolytic pathway or the pentose phosphate shunt, the two alternate pathways of sugar decomposition to pyruvic acid.

Requirements for B and Ca often go hand in hand; this has suggested that B, like Ca, may be needed for cell wall formation and for the metabolism of pectic compounds. It is of interest that numerous physiological (nonpatho­genic) diseases, such as brown heart of turnip, heart rot of sugar beet, and leaf roll of potato (all indicating cell wall integrity problems), have been traced to B deficiency.

A wide range of B fertility levels had no effect on the vegetative growth of maize, but tassels from deficient plants had no viable pollen; also, silks were not receptive to the pollen taken from high-B-fertility plants. In fenugreek, a B deficiency caused flowering and reproductive failure, rosetting of terminal buds, small leaves, and chlorosis. Deficiencies were usually corrected by application of a borated fertilizer mixture at 0.5 to 3 kg. ha^-1 broadcast, 0.1 to 0.5 kg. ha^-1 leaf spray, or 2 kg. ha^-1 row application.

v. Copper:

Copper is found in primary and secondary minerals but occurs pri­marily in organic complexes. It is present as an exchange ion on soil particles and minutely in the soil solution. Adsorbed Cu is held tightly, replaceable to some extent by other cations. For this reason, correction of a Cu deficiency is best accomplished by Cu-chelates. Total Cu in soils is usually less than 50 ppm, and leached, sandy soils are inherently low in Cu.

In Florida, Cu deficiencies have normally been associated with organic soils; however, cucumber yields from a fine sandy loam were nearly doubled by the addition of 2.24 kg . ha^-1 and increased further by 8.96 kg . ha^-1 of CuSO₄. Liming induced a Cu deficiency in alfalfa on an acid soil (pH 5.3), which was corrected by adding Cu. The Cu content of most plants varies from 2 to 20 ppm.

Copper plays a role in photosynthesis, as it is a part of the chloroplast enzyme plastocyanin in the electron transport system between photosystems I and II. Most of the Cu in plants is found in the organelles. Copper is a part of several oxidases, such as ascorbic acid oxidase and polyphenol oxidase. Cu and Zn are found in superoxide dismutase, which can split O₂ in aerobic organisms. It is a cofactor for the synthesis of certain enzymes.

Cereal crops, such as oat, are more prone to Cu deficiency. At the tillering stage, leaf tips become white and twisted and give an overall bushy appear­ance. Panicles may fail to develop and form grain. In fruit trees, terminal shoots are stunted and may suffer summer dieback. Species and cultivars differ in tolerance; soybean is highly tolerant to Cu deficiencies.

Copper may become toxic in soils with a history of Cu sprays, such as Bordeaux mixture. However, most soils are strongly buffered against free Cu in quantities sufficient to be toxic by their strong Cu adsorption. Deficiencies can be corrected by chelates such as Cu-DTPA (diethyltriaminepentacetate).

vi.Molybdenum:

Molybdenum may be derived from weathering of a number of minerals that include MoS₂ (reduced), oxycomplexes such as Ca-MoO₄, and hydrated forms. Absorbed primarily as a divalent anion (MoO^2-₄), Mo oc­curred in the soil solution at low concentrations, 2 × 10^-8 M to 8 × 10^-8 M. The average content for agricultural soils was 2 ppm. Deficiencies occur at concentrations below 1 ppm in the Ivory Coast, and the critical level in leaf 17 of oil palm is 0.1 to 1 ppm.

Molybdenum availability increases with increasing pH, so lim­ing increases availability. The saying attributed to Australians, “An ounce of molly is worth a ton of lime,” refers to the light or aerial application of Mo fertilizer to rangelands that is as effective in promoting clover growth as tons of lime per hectare. Microbes in high-organic soils immobilize Mo, and highly leached, sandy Podzols may be deficient.

The only known use of Mo is in the enzymes nitrite reductase and nitrate reductase, where it acts as an electron carrier between oxidized and reduced states. Mo-deficiency symptoms include whiptail disease and dieback on cauliflower and broccoli. Interveinal chlorosis often occurs. Visible deficiency symptoms could not be demonstrated on oil palm in nutrient culture. Deficiencies of Mo can generally be corrected by adding lime to the soil or by adding Na₂MoO₄.

vii. Chlorine:

Chlorine is the most common anion in nature and may be super optimal in areas near the sea, old lake beds, and slightly leached soils in arid areas and in arid irrigated lands. The ocean sodium chloride content is high enough to preclude life for higher plants. Plants may derive sufficient chlorine from the chlorine gas of the atmosphere. This source increases in importance near the sea.

In soil, chlorine is adsorbed by colloids as the CI^– anion. Hoagland (1944) demonstrated that plants can take up CI at a concentration many times that of the external solution, which indicated active uptake. Normal accumulation is in the vacuole, and the tonoplast membrane becomes rate limiting. Uptake is in competition with other an­ions, especially NO^–₃. Chlorine is not mobile in the plant and accumulates in the older parts.

Chlorine is not a constituent of any known plant metabolite but was found essential for the evolution of oxygen in photosystem II. Deficiency symptoms appear first as wilted leaves that subse­quently can become chlorotic and bronze colored.

The contribution of CI to reduction of lodging has been a frequent question, since observations of re­duced lodging were made when the fertilizer KCl was used. Chlorine had no benefit on lodging of maize when KCl and NH₄Cl were compared, and it was concluded that the reduction in lodging was due to the benefits from K.

Correction of CI deficiency is seldom, if ever, needed, since adequate amounts can come from the air, rain, and from animal urine and perspiration.