Micronutrient Deficiency in Soils and Plants

By the term ‘micronutrient availability’ we mean the total micronutrient forms in soils, which are available for plants (i.e., all the soluble forms that can be taken up by plants). Availability depends on solubility in soils, i.e., the solubility of micronutrients in soils determines their availability for plants (thus their uptake) and their downward mobility. Understanding the factors controlling trace element solubility allows the selection of soil amendments that promote or reduce their availability and of course the selection of the suitable plant species (those that are optimal for the desired goal of managing trace element influx in the soil-plant system). There are many soil factors influencing micronutrient solubility and availability for plants. The most important of these factors are pH, organic matter content, CaCO3, soil texture, cation exchange capacity (C.E.C.), erosion, soil moisture and temperature etc. Generally, trace element solubility in soils depends on a plethora of physical (e.g., water retention capacity), chemical (e.g., pH), microbial (e.g., mycorrhiza) and plant factors, as well as on the properties of each nutrient, which are fully analyzed below. There are many extractants (like DTPA, EDTA, Mehlich-1, Mehlich-3) used to determine available for plants quantities of micronutrients in soils; since great differences exist in the extractability of these chemical solutions, in order to estimate plant available micronutrient concentrations it is absolutely necessary to study micronutrient extractable concentrations in soils, in relation to their uptake by plants. All the topics concerning micronutrient solubility in soils, together with the factors influencing micronutrient availability and uptake by plants, are analyzed in detail in this chapter. In addition, a comparison between the extractant solutions used to estimate plant available quantities of micronutrients in soils is included in the second part of this chapter.

Despite the fact that micronutrients are required in very low concentrations by plants, they are as essential for plant metabolism and growth as macronutrients. Foliar analysis is a valuable tool in order to detect micronutrient deficiencies before macroscopic symptoms appear in plants; for that purpose, critical micronutrient concentrations have been established. Other diagnostic tools used to assess micronutrient deficiencies are soil analysis, plant-growth response (in annual plants), and visual observation of symptoms. Recently, more and more biochemical indicators, as early detectors of micronutrient deficiencies, are used. Fertilization (soil or foliar application) should be included in the cultivation program in order to improve the low nutritional status of plants. However, before fertilization, it is absolutely necessary to have a deep knowledge of the physiological roles of the necessary micronutrients for normal plant growth. So, the physiological roles of micronutrients, as well as some critical deficiency concentrations in soils and plants, are presented in detail and fully discussed in this chapter.

Despite the fact that globally Fe is in great abundance and total Fe content in soils is high, many times plants suffer from Fe chlorosis. This happens because the greatest part of this content exists in insoluble forms (oxides and hydroxides of Fe, phosphate substances of Fe etc.), thus it can not be taken up by plants. Iron solubility and uptake depends on many soil (pH, soil humidity, C.E.C., organic matter, CaCO₃ content etc.) and non-soil factors (such as root exudates, plant-microbial interactions, production of phytosiderophores, root ferric reductase activity, fertilization, grafting on Fe-tolerant rootstocks, crop management practices etc.). There are two mechanisms adopted by plants in order to take up Fe from soil: strategy I, and strategy II. Strategy I is a complex Fe uptake mechanism developed by all plants, with the exception of Poaceae plants, which belong to strategy II. Strategy I uptake mechanism is based on the reduction of external Fe³⁺ to Fe²⁺ through the induction of Fe³⁺ chelate reductase enzyme. Strategy II uptake mechanism is based on an increase in the synthesis and secretion of phytosiderophores (PS) to the environment of the rhizosphere. Then, the PS-Fe complexes are easily taken up by plants.

Many horticultural and agronomic crops (such as apple, grape, peach and Citrus), which belong to strategy I species, are sensitive to Fe deficiency. From strategy II species, rice and sorghum are among the most sensitive crops. Since Fe is involved in chlorophyll synthesis, chlorophyll content and photosynthetic rate, they are usually decreased under Fe deficiency; this is the reason why chlorosis is the most usual macroscopic symptom observed under conditions of Fe starvation. There are many mechanisms of tolerance adopted by plants in order to face Fe deficiency, like enhanced ability to induce H+ extrusion in strategy I plants, production of greater quantities of PS that are released by roots in order to mobilize Fe in strategy II plants, modification of the morphology of their root system in order to increase Fe uptake etc. Finally, there are two basic methods of supplying Fe in plants: through soil and foliar application; the foliar application is very advantageous under alkaline soil conditions. There are two basic categories of Fe fertilizers: the inorganic ones, based on inorganic Fe compounds, such as Fe salts (e.g., Fe(SO₄)7H₂O) and insoluble compounds, such as Fe oxides-hydroxides and the organic fertilizers, based on organic compounds, like Fe-EDTA and Fe-EDDHA.

All the above mentioned topics concerning soil and plant factors influencing Fe solubility and uptake, strategies of Fe uptake, mechanisms of tolerance adopted by plants in order to face Fe starvation, as well as methods of fertilizer application, and substances used to alleviate chlorosis and organic fertilizers, are fully analyzed in this chapter under the light of the most recent and important scientific papers.

Zinc deficiency is one of the most important micronutrient deficiencies and many crops exhibit symptoms. There are many, soil and non-soil, factors influencing Zn solubility in soils and uptake by plants. Some of the most important factors are: pH, CaCO₃, organic matter, cation exchange capacity (C.E.C.), soil humidity and temperature, soil texture, parent material, the quantity of Fe oxides and hydroxides in soils, the interaction with other nutrients, the genotypic ability to absorb Zn (such as the differential exudation capacity among genotypes), the formation of mycorrhiza, the different management and agronomical practices adopted by local farmers during crop production etc. Zinc may be taken up by plants either as Zn²⁺ or as Zn soluble organic chelates; Zn-deoxymugineic acid (DMA) complexes are the most referred ones in bibliography and the most prefered for uptake by some plant species (barley). Generally, Zn seems to be a mobile nutrient, easily transferred between vegetal tissues; under Zn deficient conditions many plant species are able to mobilize limited, but crucial for plant growth, quantities of Zn from older-mature- leaves to younger ones.

Zinc is very closely involved in N, carbohydrate and lipid metabolism of plants, as well as in protein and RNA synthesis. Of great importance is also the role of Zn on root membrane permeability. Generally, Zn levels below 20 p.p.m. are considered as deficient, so inorganic fertilization or organic amendment is needed. The non-typical Zn deficiency symptoms are usually those related to depressed plant growth, since Zn starvation negatively influences IAA concentration; from the typical symptoms, it should be distinguished the formation of clusters or rosettes of small stiff leaves at the ends of the young shoots in fruit trees etc. Some of the tolerance mechanisms adopted by plants in order to survive under Zn starvation include the enhanced exudation ability by tolerant genotypes, the antioxidant mechanisms in order to detoxify reactive oxygen species (ROS), the formation of mycorrhiza, the enhanced mobilization and translocation ability of Zn (usually from older to younger leaves) in tolerant genotypes etc. When leaf Zn concentrations are below the critical limits, Zn soil or foliar applications are needed. For that purpose, many substances, such as ZnSO₄, or ZnEDTA may be used for the alleviation of Zn deficiency. However, some industrial by-products, varying from flue dust, reacted with sulphuric acid, to organic compounds, derived from the paper industry, may be also used for the correction of Zn starvation.

All the above mentioned topics concerning soil and non-soil factors influencing Zn solubility and uptake, the deficiency symptoms, the mechanisms of tolerance adopted by plants in order to face Zn starvation, as well as the methods of fertilizers’ application and the substances used to alleviate Zn deficiency are within the purposes of this chapter and they are fully developed and discussed.

Despite the fact that Mn is one of the most abundant trace elements in the lithosphere, its’ concentration in soils greatly varies among soil types, depending on parent material and soil conditions. Apart from the parent material, other soil factors influencing Mn availability are pH, organic matter, soil moisture, CaCO₃ content, redox potential, soil microorganisms, soil texture, the availability of other nutrients, the kind of N fertilizers (NO₃^- or NH₄⁺) used by the farmers etc.

Manganese uptake by plants is metabolically controlled, i.e., it is absorbed as Mn²⁺, with energy consumption. In some cases Mn may be also taken up as chelate. Manganese is highly important for plant metabolism and growth, since it is contained in three enzyme complexes (those of photosystem II, MnSOD and acid phosphatases), while it is also an activator for a large number of enzymes (about 35). Manganese plays a very important role in photosystem II and photosynthesis, while it is also associated with the structure of chloroplasts, with N metabolism, with CO₂ assimilation in C4 plants, with the cycle of tricarboxylic acids etc. Critical Mn deficiency concentrations for plant growth vary from 15 to 20 mg/kg d.w. (depending on plant species). The most characteristic symptoms of Mn deficiency are the appearance of small yellow spots on the leaves and the interveinal chlorosis. In cases of Mn deficiency symptoms, soil or foliar application of Mn (either as MnSO₄ or MnEDTA) should be preferred. Particularly, foliar application of Mn is advantageous in calcareous/alkaline soils, where Mn is quickly immobilized after soil application.

Under conditions of Mn deficiency plants adopt tolerance mechanisms in order to survive. Some of the most important mechanisms include the acidification of their rhizosphere, the greater ability for remobilization and redistribution of Mn from the more to less tolerant tissues (such as the young leaves), the formation of arbuscular mycorrhiza to root system environment, the adjustment of root morphology in order to take up more Mn²⁺ etc. From an agronomical point of view, under conditions of limited Mn availability particular emphasis should be given to the choice and cultivation of genotypes with increased ability of Mn uptake and internal use efficiency (e.g., enhanced transport from the root system to shoot).

All these topics concerning Mn content and availability in soils, Mn uptake by plants, the role of Mn in biochemical and physiological reactions, the symptoms of Mn deficiency, as well as the strategies adopted by plants in order to survive under Mn starvation and the adaptation mechanisms of the Mn-use efficient genotypes are fully discussed in this chapter.

Copper is needed in very low concentrations by plants. The critical Cu deficiency level for adequate plant growth is around 2-4 p.p.m., depending on plants species and genotypes. Copper is involved in many physiological and biochemical functions, such as photosynthesis, electron transport in photosystem II (PSII), chloroplast ultrastructure, carbohydrate and nitrogen metabolism, water permeability of xylem vessels, as well as in the production of DNA and RNA; it is also related with mechanisms of disease resistance. The critical Cu concentration, according to the DTPA solution, is 0.2 p.p.m. Soil available concentrations of Cu depend on parent material, pH and CaCO₃ content, organic matter, phosphoric ion content, cation exchange capacity (C.E.C.), soil type, structure and moisture, the availability of other nutrients etc. Some of the most characteristic symptoms of Cu deficiency include the formation of narrow and twisted leaves, as well as leaf curling, while their petioles bend downward. Enhanced remobilization and retranslocation in order to overcome Cu starvation and survive are within the most important mechanisms of tolerance adopted by plants. Increased antioxidant defence mechanisms under Cu stress conditions include enzymatic and non-enzymatic antioxidant responses. When crops suffer from Cu deficiency, both soil or foliar applications may be a good solution; CuSO₄5H₂O is usually the substance that is preferred. Apart from Cu sulphate (25% Cu), other Cu sources that could be used as Cu fertilizers are: cupric oxychloride (50% Cu), cuprous oxide (32.8% Cu) and chelated Cu (5% Cu).

All these topics concerning Cu availability in soils and uptake by plants, the roles of Cu in plant metabolism, the critical concentrations and symptoms of Cu deficiency, as well as the tolerance mechanisms and strategies adopted by plants in order to survive under Cu starvation, and the Cu-fertilizers that can be used to overcome Cu deficiency in crops, are fully discussed and analyzed.

Boron is a very important micronutrient, playing a crucial role in many physiological and biochemical functions, as well as in plant metabolism. More specifically, B is involved in cell elongation and division, cell wall biosynthesis and structure, N, carbohydrate and IAA metabolism, photosynthesis, as well as in membrane integrity, seed production, sugar metabolism, regulation of lignin biosynthesis etc. Flowering and fruit setting are two of the mostly known functions that are negatively influenced by B deficiency.

Boron deficiency is most probably found in calcareous and alkaline soils, as well as in soils formed on parent materials inherently low in B, such as sandstones. Boron is absorbed from soil solution mainly as undissociated boric acid. Boron uptake is not yet clear as to the extent to which this process is either passive or active; however, the metabolically controlled process seems to be relatively minor. The most possible explanation is that when B supply is high, B uptake by roots is believed to occur by passive diffusion. In contrast, under low B supply, a significant portion of B may be taken up via active pathways. Under low B conditions (leaf concentrations of less than 10 mg/kg dry weight in young sampled expanding leaves), flower sterility and floral abnormalities are within the first symptoms of B starvation. In addition, the formation of incomplete or damaged embryos and malformed fruits are also within the most common and important symptoms of B starvation for plants. Other symptoms of B deficiency include rapid cessation of root elongation, inhibited growth and thickening of young leaves, loss of apical dominance in trees etc.

Under B deficiency conditions some tolerance and adaptation mechanisms, such as enhanced translocation from root system to leaves and lower shoot/root dry weight ratio in resistant genotypes, usually take place. In order to alleviate B deficiency, the most commonly used B fertilizer is borax; nevertheless, its’ basic drawback is that it leaches easily from sandy soils. Other fertilizers that can be used, either as soil, or foliar application, are those of boric acid and solubor 20-21% (leafy sprays are particularly preferred when soil is potentially capable of fixing high amounts of B).

All these topics concerning B availability in soils and the factors influencing it, the uptake and transport of B, the roles of B in plant metabolism and growth, the critical concentrations of B starvation in plant tissues, the symptoms of B deficiency and the methods of its’ detection, as well as the tolerance mechanisms adopted by plants in order to face B deprivation and the B-fertilizers used to alleviate B stress in crops are fully analyzed and discussed in this chapter.

The average concentration of Mo in the lithosphere is 2.4 mg/kg and in soils varies greatly, depending on parent material. Molybdenum solubility, thus its’ availability for plants, is affected by many factors, such as soil pH, organic matter, the content of Fe oxides/hydroxides in soils, P concentration, liming, crop management, soil humidity and interactions with other nutrients. Molybdenum is mainly absorbed as MoO₄^2- and it is readily and highly mobile in xylem and phloem. The form by which Mo is translocated is probably that of MoO₄^-. From the physiological functions of Mo, the most important is that in atmospheric N capturing and uptake. Particularly, nitrogenase and nitrate reductase, which are involved in N fixation and NO^3- reduction respectively, are Mo-containing enzymes and their activity is depressed under conditions of Mo deficiency. Apart from the involvment of Mo in nitrogen metabolism and fixation, Mo also participates in N transport in plants, as well as it occurs in more than 60 enzymes and catalyzes diverse oxidation and reduction reactions. Furthermore, Mo is also involved in processess concerning the synthesis of the phytohormones abscisic acid and indole-3 butyric acid.

Generally, Mo foliar level 0.5 p.p.m. is considered for most plant species as critical deficient, while for some others may be lower, for example 0.1 or 0.3 p.p.m. Molybdenum deficiency symptoms include deep chlorosis of old leaflets, spreading to young growth and intensification of chlorosis leading to bleaching. The leaf chlorosis of Mo deficiency somewhat resembles to that of N deficiency, due in part to Mo role in N utilization. In order to detect Mo deficiency before the appearance of symptoms, induciable nitrate reductase activity can be used as an indicator of the Mo nutritional status of plants, since Mo is an essential component of two major enzymes (nitrate reductase and nitrogenase). Finally, in order to correct Mo starvation, Mo can be supplied either in mixtures as fertilizers, or as seed coating, or foliar sprays of watersoluble Mo salts, mostly ammonium and sodium molybdates.

All the points referring to Mo availability in soils, its’ uptake, the physiological roles of Mo in plant metabolism, the critical Mo concentrations, the symptoms of deficiency and the methods used for its’ detection, as well as the Mo-fertilizers used to alleviate Mo stress in crops, are fully analyzed and discussed in this chapter.

Chlorine is an important micronutrient and despite the fact that plant tissues usually contain substantial amounts of Cl^-, often in the range from 2 to 20 mg/g d.w., the demand for Cl^- for optimum growth is for most species considerably lower (deficiency symptoms usually occur in the range 70-700 μg/g d.w.). Chlorine is taken up by plants as Cl^- and it is highly mobile, so after absorption it can be easily transported inside plants. The negative charge of Cl in soil makes it prone to leaching in regions with high rainfalls. In contrast to that, in regions with high evapotranspiration (arid and semi-arid regions) Cl^- may be highly accumulated in surface soil horizons. Apart from the climatic conditions determining the accumulation or leaching of Cl^- in soils, the distance from the sea is another important factor influencing Cl concentrations in soils; so, Cl in soils exhibits a clear trend of decreasing concentration with increasing distance from the sea.

Chlorine is implicated in several physiological functions, such as in osmotic and stomatal regulation, in oxygen evolution in photosynthesis, in disease resistance and tolerance, as well as in fruit quality and crop yields. In recent publications it is referred that the critical Cl deficiency concentration is 2 g/kg d.w. (i.e., 2000 mg/kg d.w.). Below that concentration Cl deficiency symptoms, such as chlorotic leaves, leaf spots, brown edges, restricted and highly branched root system, as well as wilting of leaves at margins and leaf mottling, may occur. In order to alleviate Cl starvation symptoms, some Cl-containing fertilizers that may be supplied to plants are those of KCl (47% Cl), MgCl₂ and CaCl₂ (64% Cl). Other (anthropogenic) sources of Cl supply to plants are the irrigation water, the use of de-icing salt to frozen roadways during winter months and the atmospheric pollution.