Keywords

1 Introduction

‘Micronutrients’ are the trace elements required in small, but critical, quantities for the normal healthy growth of plants and animals. There are eight micronutrients required by higher plants and these are: boron (B), chlorine (Cl), Cu, iron (Fe), Mn, Mo, Ni and Zn. In addition, Co is essential for the bacterial fixation of nitrogen (N) in nodules on the roots of legumes [5, 8, 9]. With the exception of B and Cl, these elements are all classed as heavy metals. In higher animals and humans, the proven micronutrients are: chromium Cr, Cu, Fe, iodine (I), Mn, Mo, Se, V and Zn. Other elements which are being investigated for possible essential functions in animals and humans include: B, silicon (Si), As, fluorine (F), lithium (Li) and Sn. There is even some evidence that Cd, Pb and Sn may be essential at very low concentrations [12, 28, 31]. However, the micronutrients which have been conclusively proven to be essential in animal and/or human nutrition and whose concentrations in diets are critical are Co (ruminants only), Cr, Cu, Fe, I, Mn, Mo, Se and Zn.

An element can be considered ‘essential’ for plants if they cannot complete their life cycles without it, it cannot be replaced by other elements and it is directly involved in plant metabolism [8]. Essential trace elements are also known as ‘micronutrients’. For animals and humans, Reilly [28] quotes the definition of essential elements used by many nutritionists as: “The trace element must: (1) be present in healthy tissue, (2) its concentration must be relatively constant between different organisms, (3) deficiency induces specific biochemical changes, (4) the changes are accompanied by equivalent abnormalities in different species, and (5) supplementation with the element corrects the abnormalities.”

All micronutrient heavy metal(loid)s have homeostatic mechanisms in plants and/or animals which regulate the bioavailable concentrations of the elements. It is when these mechanisms break down and are unable to maintain the optimal range of supply of a particular micronutrient that either deficiencies or toxicities can occur.

2 Heavy Metal Micronutrients in Plant Nutrition

It is very important that the micronutrient requirements of crops are met as well as their macronutrient needs (e.g., N, P and K) if they are to yield satisfactorily and bear products, such as grains, fruits, vegetable leaves or storage roots, of acceptable quality. In comparison with macronutrients, which are often present in thousands of mg kg−1, micronutrients are only required in relatively small amounts (5–100 mg kg−1 DM). The dose response curves for all micronutrients show that yields can be affected by both deficiencies and toxicities. Typical dose-response graphs for micronutrients and non-essential elements are shown in Fig. 7.1. Although these are primarily based on generalised plant data, dose response graphs for animals show similar trends.

Fig. 7.1
figure 1

Typical dose-response curves for essential and non-essential trace elements (Reproduced from Alloway [3] (Copyright (2008) with permission from International Zinc Association and International Fertiliser Industry Association))

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Micronutrient deficiency problems in crops have only been widely recognised and treated in the field over the last 70 years. These deficiencies became apparent with the intensification of arable farming in many parts of the world and also with the cultivation of virgin and/or reclaimed land. Intensification involves the increased use of N, P and K fertilisers, growing new and higher yielding crop cultivars, increased use of pesticides and, where necessary, liming to optimise soil pH and/or increased use of irrigation. Prior to this intensification, much lower crop yields were usually accepted as the norm in many parts of the world, but the crop cultivars grown were generally well adapted to local soil and climatic conditions [2]. A similar situation has arisen with intensively managed livestock, but in the case of humans, trace element malnutrition is often related to poverty and restricted diets with either a shortage of foods containing adequate concentrations of micronutrients, or foods, such as grains containing relatively high levels of antinutrients such as phytates, which reduce the availability of some micronutrients [31, 33].

The functions of the plant micronutrients are briefly summarised in Table 7.1. All of these essential heavy metals have roles as constituents or activators of enzymes in physiological pathways. Although Fe is not normally considered to be a ‘heavy metal’ it is included in Tables 7.1 and 7.2 because of its very important functions and the fact that it interacts with several heavy metals such as Mn, Cu and Zn and macronutrients such as Ca.

Table 7.1 Brief summary of the essential functions of metal micronutrients and beneficial elements in plants
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Table 7.2 Some common visible symptoms of micronutrient deficiencies in crops
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Critical deficiency concentrations of these micronutrient heavy metals in most crop plant species are generally in the range (in mg kg−1 DM): Cu 1.0–5.0, Fe 50–150, Mn 10–20, Mo 0.015–0.05, Ni 0.01–10 and Zn 15–20 [6, 7]. Critical concentrations in soils vary with the soil test procedure used, the crop and the type of soil. For Cu extracted with ethylenediamine tetraacetic acid (EDTA) deficiencies are highly likely <0.5 mg EDTA Cu kg−1 and still possible 0.5–08. Mg EDTA Cu kg−1 [1]. For Zn extracted with diethylenediamine pentaacetic acid (DTPA) typical threshold concentrations are 0.1–1.0 mg DTPA Zn kg−1 [21] (see Sect. 17.5.1), but around the world the critical range can extend up to 2.0 mg DTPA Zn kg−1 [3].

When the supply of a micronutrient to plants is either deficient or excessive, in addition to crop yields and quality being affected, visible symptoms of physiological stress are often observed, especially in cases of severe deficiency or toxicity [5]. Although plant species differ in the nature of the symptoms of micronutrient deficiencies and toxicities which they display, there are several generalizations which can be made. In most cases, severe deficiencies will cause stunted growth, discoloration and, in some cases, necrotic spots on leaves. The discolouration will usually commence as chlorosis when, instead of the normal green colour of chlorophyll, either all or part of the leaves turn yellow, or even white, but they can also turn brown. Deficiency symptoms can also include smaller or twisted leaves, and loss of turgour. Leaf symptoms are usually seen only on old leaves in the case of Mo, on new leaves with Fe, Mn and Cu, on both young and old leaves with Zn, and in terminal buds with B deficiency. Green veins (on chlorotic leaf laminas) are seen on new leaves with Fe and Mn deficiency and yellow veins with Cu deficiency [27]. A summary of the main types of deficiency symptoms associated with each of the plant micronutrients is given in Table 7.2. Less severe deficiencies may not manifest themselves until later stages in the development of the plant. Visible symptoms can provide a convenient and low-cost means of identifying micronutrient deficiency problems, but can sometimes be confused with deficiencies of other macro or micronutrients, toxicities of metal(loid)s or symptoms of disease, drought, heat stress, or damage by agrichemicals [5].

Symptoms of severe toxicity in plants vary with the metal(loid) involved (including non-essential and micronutrient elements), but certain symptoms tend to be commonly found. These include: chlorosis (Co, Cr, Cu, Hg, Mn, Mo, Ni, Se, Tl and Zn); stunted or deformed roots (As, Cd, Cr, Cu, Co, Hg, Mn, Mo, Ni, Pb, Ni and Zn); dark green leaves (Cu, Fe and Pb); grey green leaves (Ni); brown necrotic spots on leaves (As); brown leaf margins (Cd); white or necrotic leaf tips (Zn) and general stunting (Cd, Hg and Zn) [15]. Upper critical concentrations (in mg kg−1 DM) causing a 10% depression in yield (due to toxicity) are in the range: As 1–20, Cd 10–20, Cr 1–10, Cu 10–30, Hg 1–8, Ni 10–30 and Zn 100–500 [18].

In cases of marginal deficiency or toxicity, the manifestation of symptoms is often less distinct and more difficult to recognize in the field. The yields of many crops, especially cereals, can be significantly reduced (sometimes by 20% or more) and the quality of crop products impaired, without the manifestation of distinct visible symptoms due to marginal deficiencies of micronutrients such as Cu and Zn. These are usually referred to as hidden deficiencies, ‘hidden hunger’, latent, and/or sub-clinical deficiencies. In many parts of the world, this type of deficiency is more widespread and has a greater economic impact than acute micronutrient deficiencies. The apparent absence of deficiency or toxicity symptoms in a crop does not necessarily imply that the supply of micronutrients is optimal. More than one element may be deficient or causing phytotoxicity at a particular site (multi-micronutrient deficiencies or toxicities). In correcting a diagnosed deficiency of one element, there is a risk that the available concentration of another micronutrient may be reduced in some way, thereby inducing a deficiency of this element instead. This has been found with Cu and Mn, Cu and Zn, and Mn and Fe.

From Table 7.3 it can be seen that crop species vary considerably in their susceptibility to deficiencies of different micronutrients. Maize, rice, citrus and fruit trees are particularly susceptible to Zn deficiency, which is the most ubiquitous micronutrient deficiency disorder in crops. Small grain cereals, such as wheat, barley and oats as well as citrus and alfalfa are highly susceptible to Cu deficiency. However, although wheat is considered reasonably tolerant of Zn deficiency, in many countries, especially those with calcareous soils, Zn deficiency in wheat is a major problem (due to low availability) (see Chap. 17, Sect. 17.5) [21]. Intra-specific variations (between varieties or cultivars) can sometimes be even greater than differences in susceptibility between species, but all crops will be affected by a severe deficiency of any of the micronutrients. The main difference between genotypes is in the critical concentrations at which the supply of a particular micronutrient becomes inadequate. These will be significantly lower for the more tolerant genotypes (cultivars).

Table 7.3 The relative susceptibility of crops to deficiencies of micronutrients
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Cultivars which are able to grow normally in soils with marginally low available concentrations of a micronutrient are classed as being ‘efficient’ for that particular micronutrient and those which are unable to tolerate such low levels of this micronutrient are ‘inefficient’. Genotypic variations in efficiency have been reported for: B, Cu, Fe, Mn and Zn in crop plants [11]. Differences in micronutrient efficiency are probably due to genotypic variations in the volume and length of roots, root-induced changes in rhizosphere, increased absorption through vesicular mycorrhizae, release of root exudates to facilitate uptake, efficiency of utilization of the micronutrients once absorbed into plants, recycling of elements within the tissues of the growing plant, or tolerance of factors, which inhibit uptake, such as HCO −3 and Zn in rice [10, 13, 19].

In a large-scale programme of field experiments at 190 sites in 28 developing countries plus Finland, Silanpää [29] found that Zn deficiency occurred in 49% of the experimental sites, B deficiency in 31%, Mo deficiency in 15%, Cu deficiency in 14%, and Mn deficiency in 10%. However, for all elements except Zn, much higher percentages of these deficiencies were of the latent or hidden type (e.g., Cu 4% acute and 10% latent; Mo 3% acute and 12% latent), but in the case of Zn, there were almost equal percentages (25% acute and 24% latent) for the two types of deficiency.

3 Heavy Metal(loid) Micronutrients in Animal and Human Nutrition

Just as in plants, the heavy metal(loid)s essential for higher animals and humans are constituents and/or activators of enzymes. Their essential functions are given in Table 7.4.

Table 7.4 Functions of essential heavy metal(loid)s in higher animals and humans
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In grazing livestock, Mn deficiency is rare, but when it does occur it often results in lameness. Zinc concentrations in herbage need to be above 20 mg kg−1 in order to avoid Zn deficiency whose effects include, loss of appetite, poor growth and reduced fertility. With more severe deficiency (intakes <5 mg Zn kg−1) symptoms include loss of hair and wool, reduced immunity to disease and thickening and cracking of the skin. Copper deficiency can be associated with low Cu contents in herbage, but also to elevated Mo contents (see Sect. 18.4). Mild Cu deficiency symptoms, like those of Zn, tend to be non-specific and include poor growth and/or roughness and loss of pigmentation in coat. More severe Cu deficiency is accompanied by symptoms including diarrhoea, anaemia, and lameness. In sheep, Cu deficient lambs can develop a neurological condition called ‘swayback’ (or enzootic ataxia) [32].

The US Recommended Dietary Allowances (RDAs) [23] for Cu, Mn, Fe, Zn and Se for humans are shown in Table 7.5.

Table 7.5 Recommended dietary allowances (RDAs) in humans of the US National Academy of Sciences for copper, manganese, zinc, iron and selenium
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Apart from the proven essential trace elements, some other heavy metal(loid)s are being investigated to see whether they are also essential for animal and human nutrition. Several of these, such as As, Cd and Pb have only been considered as potentially toxic elements until recently, but there is some evidence that very small amounts of these elements may be essential. In most cases, the experimentation has involved using diets depleted of the elements of interest and some responses in growth rate and other parameters have been observed. This work is likely to be most relevant in medicine where patients may have to be maintained on total parenteral nutrition (TPN) [28]. However, it is highly unlikely that cases of deficiency of these elements will be found in humans consuming mixed diets or in livestock under normal agricultural conditions. The possible functions of the metal(loid)s currently under investigation are shown in Table 7.6.

Table 7.6 Possible functions of other heavy metal(loid)s in animals and humans
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4 Soil Types Commonly Associated with Micronutrient Deficiencies

The effects of soil type and soil conditions on the bioavailable concentrations of heavy metalloids are covered in detail in Chaps. 3, 6 and 9–18. However, Table 7.7 summarises the most typical soil conditions leading to either deficiencies in crops, or low concentrations of micronutrients required by animals in herbage and crop products. Boron and Fe are also included because of the agronomic importance of them being deficient.

Table 7.7 Soil factors associated with micronutrient deficiencies
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Low total concentrations of micronutrients (and most non-essential trace elements) are often found on sandy textured soils (e.g., Podzols) and/or on heavily weathered tropical soils (e.g., Ferralsols) and can give rise single and/or multi-element deficiencies. High soil pH values (>7.0) such as are found on calcareous, heavily limed or saline soils are likely to cause deficiencies of B, Cu, Fe, Mn, Ni and Zn. Multi-element deficiencies are also highly likely on these soils. In contrast, Mo is likely to be highly available in high pH soils, but can be deficient in acid soils. Organic matter-rich soils, such as peats (Histosols), muck soils and heavily manured mineral soils are likely to have low available concentrations of B, Cu, Mn, Se and Zn [2]. However, as shown in Table 7.7, low organic matter contents can also predispose to deficiencies of B, Cu and Zn.

Clay-rich soils which usually have relatively high adsorptive capacities and tend to be imperfectly to poorly drained (gleyed) can be deficient in Cu, Mn and Zn. Paddy soils used for wetland rice production are prone to Zn deficiency, but can have toxic Fe concentrations due to the reduction of insoluble Fe oxides. In general, micronutrient deficiencies in crops and herbage, tend to be reflected in low intakes in animal and human diets. However, in the case of grazing livestock the soil-plant-animal pathway may be bypassed by direct (usually accidental) ingestion of soil and so the amount of metal(loid)s ingested by the animals may not be affected by soil-plant barriers. However, the presence of adsorbent minerals and antagonistic elements in the animal GI tract can modify the availability of the micronutrient to the animal.

5 Plant Factors Associated with Micronutrient Deficiencies

The plant factors associated with the onset of deficiencies in crop plants are covered in detail in Chap. 6, but one or more of the following factors are often found to be involved:

  • Plant genotype (i.e., micronutrient efficient/inefficient cultivars),

  • Nitrogen supply (effects on growth rate, dilution, elements locked up in proteins in foliage),

  • Phosphate supply (effects on growth rate – dilution e.g., Cu, and metabolism e.g., Zn)

  • Moisture stress (uptake reduced in drought conditions),

  • Temperature stress (high and low temperatures),

  • High/low light intensity,

  • Rooting conditions (restrictions in rooting zone will reduce the volume of soil explored by roots),

  • Mycorrhizal infection (increases the effective volume of roots),

  • Secretion of root exudates (e.g., phytosiderophores),

  • Pathological disease,

  • Agrochemicals (e.g., glyphosate-induced deficiencies of Mn & Zn),

  • Antagonistic effects of other micronutrients (e.g., Cu–Zn, Fe–Cu, Fe–Mn, Cu–Mn etc.),

  • Previous crop species – there is some evidence that the mineralization products of some plant species can render certain micronutrients less available in the soil. An example of this is Cu deficiency in wheat following oil seed rape (canola).

Based on: [2, 7, 9, 12, 13, 19, 22]

6 Concluding Comments

This short chapter has been included in this latest edition of ‘Heavy Metals in Soils’ to draw attention to the importance of many heavy metals such as Cu and Zn which are micronutrients in both plants and animals and the metalloid Se, a vitally important micronutrient in animals and humans (see Chap. 16, Sect. 16.4). More details are given in Chaps. 9–17 and 21 dealing with the respective heavy metal(loid)s, but it is useful to see them dealt with as a group, to allow comparisons to be made between them. Ongoing and future research may increase the number of heavy metal(loid)s classed as micronutrients.