Frequently Asked Questions

This section provides brief answer to questions one might have on using zinc as a micronutrient. For more detailed information we advice to consult specialized literature one of which is “Zinc in soils and crop nutrition” by B. Alloway, funded by the International Zinc Association. The majority of the brief answers in this section are based on this publication which can also be downloaded from this site in the Publications section.

When was zinc defined as a vital micronutrient for crop and plant growth?

The essentiality of zinc for plants has only been scientifically established for around 70 years and in some parts of the world the existence of deficiencies has only been recognised during the last 20 or 30 years. The relatively recent discovery of widespread zinc deficiency problems in rice and wheat is linked to the intensification of farming in many developing countries. This has involved a change from traditional agriculture with locally - adapted crop genotypes and low inputs of nutrients to growing modern, high yielding plant varieties and the use of relatively large amounts of fertilisers. Many of the new crop varieties are much more susceptible to zinc deficiency than the traditional crops and the increased use of fertilisers, especially phosphorus, can render a deficiency of zinc more likely. A whole new type of farming, involving the sequential cropping of rice and wheat on the same land in some regions has been made possible by new crop varieties and agronomic expertise.

In plants, zinc plays a key role as a structural constituent or regulatory co-factor of a wide range of different enzymes in many important biochemical pathways and these are mainly concerned with:

1. carbohydrate metabolism, both in photosynthesis and in the conversion of sugars to starch
   
   
2. protein metabolism
   
   
3. auxin (growth regulator) metabolism
   
   
4. pollen formation
   
   
5. the maintenance of the integrity of biological membranes
   
   
6. the resistance to infection by certain pathogens

The main types of visible deficiency symptoms are:

1. Chlorosis – which is the change of leaf colour from the normal green chlorophyll colour to pale green and yellow, or even white, due to the reduced amount, or absence, of chlorophyll. In many cases in zinc deficient plants the chlorosis appears between the ribs in monocotyledons (grains and grasses) and between the veins of dicotyledon (broad leaf) plants and this is referred to as interveinal chlorosis.
   
   
2. Necrotic Spots on Leaves – this can occur in areas of chlorosis due to the death of the leaf tissue in small concentrated areas but the necrotic spots can grow in size as the plant ages if the deficiency is not treated.
   
   
3. Bronzing of Leaves – this symptom is also related to chlorosis and the yellow areas tend to turn bronze coloured.
   
   
4. Rosetting of Leaves – occurs when the internodes on the stems of dicotyledon crops fail to elongate normally and so the leaves form close together in a cluster instead of being spread out between nodes in a healthy plant. This is a very characteristic symptom of zinc deficiency in dicotyledon (broad leaf) crops, including bushes and trees.
   
   
5. Stunting of Plants – is a consequence of reduced dry matter production giving a smaller plant and/or reduced internode elongation of stems of developing crops.
   
   
6. Dwarf Leaves (also called ‘little leaf’) – are also fairly characteristic of zinc deficiency and these leaves may also show chlorosis, necrotic spots or bronzing.
   
   
7. Malformed Leaves – occur, often either narrower, or with wavy edges, instead of straight edges and the leaves may also be distinctly smaller (dwarfed).
   
   In comparison with other macro and micronutrients, the leaf symptoms of zinc deficiency are found on both old and new leaves, whereas symptoms of copper, iron, manganese and sulphur deficiency are found only on new leaves. In contrast, nitrogen, phosphorus, potassium, magnesium and molybdenum deficiency symptoms are found only on old leaves

Once identified, zinc deficient soils can be easily treated with zinc fertilisers to provide an adequate supply of zinc to crops. Several different zinc compounds are used as fertilisers but zinc sulphate is by far the most widely used material. Zinc sulphate is most frequently broadcast (or sprayed as a solution) evenly over the seedbed and incorporated into the topsoil by cultivation before sowing the seed. One application of between 20–30 kg ha–1 of zinc sulphate will often have an improving effect on the zinc status of the soil which will last for around five years before another application is required. However, this will vary in different areas; in some of the most deficient soils, such as those with a high content of calcium carbonate, zinc applications may have to be larger and more frequent.

Placement of the zinc fertiliser below and to one side of the seed at sowing is also frequently used. In this case, lower application rates are used because of the close proximity of the fertiliser band to the developing roots. Foliar sprays of zinc sulphate, zinc nitrate or chelated forms of zinc are mainly used on fruit trees and plantation crops but they can also be used to salvage annual crops and reduce yield loss. In all cases of treating zinc deficient soils, regular soil or plant testing is recommended to determine when additional applications of zinc fertilisers are required.

An alternative approach to the problem of zinc deficiency is to select and/or breed crops which are ‘zinc - efficient’ and able to tolerate low available concentrations of zinc in the soil. This approach is one of matching the plant to the soil, rather than modifying the soil to suit the plant. There are zinc - efficient cultivars of rice and wheat which are grown quite widely in areas of soils with a low zinc status.

The soil conditions most commonly giving rise to deficiencies of zinc can include one or more of the following:

• low total zinc content (such as sandy soils with low contents of organic matter)
  
  
• neutral or alkaline pH
  
  
• high salt concentrations (saline soils)
  
  
• high calcium carbonate content (calcareous soils)
  
  
• low pH, highly weathered parent materials (e.g. tropical soils)
  
  
• peat and muck (organic soils)
  
  
• high phosphate status
  
  
• prolonged waterlogging or flooding (paddy rice soils,
  
  
• high magnesium and/or bicarbonate concentrations (and in irrigation water)

Zinc in soils occurs in the following forms:

1. Free ions (Zn and ZnOH+) and organically complexed zinc in solution
   
   
2. Adsorbed and exchangeable zinc held on surfaces of the colloidal fraction in the soil, comprising: clay particles, humic compounds and iron and aluminium hydrated oxides
   
   
3. Secondary minerals and insoluble complexes in the solid phase of the soil

The zinc which is available to plants is that present in the soil solution or is adsorbed in a labile (easily desorbed) form. The soil factors affecting the availability of zinc to plants are those which control the amount of zinc in the soil solution and its sorption - desorption from/into the soil solution. These factors include: the total zinc content, pH, organic matter content, calcium carbonate content, redox conditions, microbial activity in the rhizosphere, soil moisture status, concentrations of other trace elements, concentrations of macro - nutrients, especially phosphorus and climate. Some of these factors are briefly summarised here in a practical crop production context:

• Sandy soils and acid highly leached soils with low total and plant -available zinc concentrations are highly prone to zinc deficiency.
  
  
• Availability of zinc decreases with increasing soil pH due to increased adsorptive capacity, the formation of hydrolysed forms of zinc, possible chemisorption on calcium carbonate and co-precipitation in iron oxides.
  
  
• Alkaline, calcareous and heavily limed soils tend to be more prone to zinc deficiency than neutral or slightly acid soils.
  
  
• When rapidly decomposable organic matter, such as manure, is added to soils, zinc may become more available due to the formation of soluble organic zinc complexes which are mobile and also probably capable of absorption into plant roots.
  
  
• Available zinc concentrations in soils with high organic matter contents (peat and muck soils) may be low due to either an inherently low total concentration in these organic materials and/or due to the formation of stable organic complexes with the solid - state organic matter.
  
  
• High levels of phosphorus may decrease the availability of zinc or the onset of zinc deficiency associated with phosphorus fertilisation may be due to plant physiological factors.
  
  
• Some forms of phosphatic fertilisers, such as superphosphate, contain significant amounts of zinc as impurities and also have an acidifying effect on soils. When these are replaced with "high analysis" forms of phosphatic fertilisers, such as mono-ammonium phosphate (MAP) and di - ammonium phosphate (DAP) the incidence of zinc deficiency has often been found to increase.
  
  
• Higher concentrations of copper in the soil solution, relative to zinc, can reduce the availability of zinc to a plant (and vice versa) due to competition for the same sites for absorption into the plant root. This could occur after the application of a copper fertiliser.
  
  
• In waterlogged soils, such as paddy rice soils, reducing conditions result in a rise in pH, high concentrations of bicarbonate ions, sometimes elevated concentrations of magnesium ions and the formation of insoluble zinc sulphide (ZnS) under strongly reducing conditions. The reducing conditions in periodically waterlogged soils also give rise to increased concentrations of divalent ferrous (Fe²⁺) and manganese (Mn²⁺) ions, from the dissolution of their hydrous oxide, and these could compete with zinc ions for uptake into roots.
  
  
• Nitrogen fertilisers, such as ammonium nitrate and sulphate of ammonia, can have a combined beneficial effect on the nutrition of crop plants by both supplying nitrogen, which is often the principal yield limiting nutrient, and also an increase in zinc availability through the acidification of the soil resulting in desorption of zinc, and through improved root growth (and hence an increased volume of soil explored by roots) in the more vigorously growing plant.
  
  
• Where topsoil has been removed, often as a result of levelling fields for irrigation, crops grown on the subsoil can be highly prone to zinc deficiency, especially in calcareous soils. The topsoil contains the most organic matter and when removed there are shortages of macronutrients as well as micronutrients. However, N,P,K fertilisers usually address the macronutrient requirements but the zinc status of these "cut" soils also needs to be considered.

Zinc appears to be absorbed by roots primarily as Zn²⁺ from the soil solution and its uptake is mediated by a protein with a strong affinity for zinc. Kochian ⁽⁵⁰⁾ proposed that the transport of zinc across the plasma membrane was towards a large negative electrical potential so that the process is thermodynamically passive. This negative electrical potential of the plasma membrane is the driving force for zinc by means of a divalent cation channel in dicotyledons and monocotyledons other than the Poaceae. In the Poacae, Kochian proposed that non-protein amino acids called ‘phytosiderophores’ or ‘phytometallophores’ form a complex with zinc and transport it to the outer face of the root-cell plasma membrane. These phytosidero-phores are released from the roots as a result of iron or zinc deficiency. This complex is then transported to the cell via a transport protein.

Nambiar ⁽⁵¹⁾ showed that plants could take up zinc from dry soil (matrix potential < –1.5 MPa) via excreted mucilage but this uptake was only 40% as effective as uptake from wet soil. Zinc is taken up as Zn²⁺ or as Zn(OH)₂ at high pH. As a result of low concentrations of zinc in the soil solution, uptake is mainly by direct root contact and is metabolically controlled. Extensive interactions take place between the uptake of zinc and other micronutrients, e.g. zinc - copper (Zn - Cu) which mutually inhibit each other indicating that both are absorbed through the same mechanism of carrier sites. Zinc - deficient rice shows increased uptake of cadmium but zinc is translocated to aerial parts to a greater extent than cadmium. Zinc addition to waterlogged soils has been found to increase DTPA extractable manganese but decrease the uptake and translocation of copper, iron and phosphorus ⁽⁵²⁾.

Zinc is transported in the plant either as Zn²⁺ or bound to organic acids. Zinc accumulates in root tissues but is translocated to the shoot when needed. Zinc is partially translocated from old leaves to developing organs. In rice seedlings, translocation of zinc from roots increases with manganese application.

Chaudry and Loneragan ⁽⁵³⁾ reported that alkaline earth cations inhibited Zn2+ absorption by plants non competitively, in the order: Mg²⁺ > Ba²⁺ > Sr²⁺ = Ca²⁺.

Although zinc deficiency is known to affect a wide range of crops in many parts of the world, genotypic differences between species render some crops more susceptible to deficiency than others. Apart from inter-specific differences, there are also important intra-specific differences which can in some cases be greater than differences between species. In the case of wheat, Durum wheat (Triticum durum) is more susceptible to zinc deficiency than bread wheat (Triticum aestivum). However, there are considerable varietal differences in both types of wheat.

Zinc is known to interact with the nutrients phosphorus and nitrogen; the macronutrients, calcium, magnesium, potassium and sodium; and the micronutrients copper, iron, manganese and boron.

There are 9 major factors affecting zinc availability (and hence likely to be related to deficiency) which included:

1. Soils of Low Zinc Content: Sandy soils, peat and muck soils with low total zinc contents (10 - 30 mg Zn kg–1) are highly likely to cause zinc deficiency in crops. Sillanpää (81) referred to deficiencies from this cause as ‘primary deficiencies’.
   
   
2. Soil with Restricted Root Zones: Restrictions to root penetration, such as those due to compaction by tractor wheels, plough pans and high water tables.
   
   
3. Calcareous Soils: Calcareous soils, generally with a pH > 7.4 have relatively low available zinc concentrations because the solubility of zinc decreases with increasing pH. Very often the total zinc content of calcareous soils is similar to those in soils of other types, or even higher, but the availability is low. Adsorption of zinc onto the CaCO3 is also a contributory factor. Sillanpää (81) uses the term ‘secondary deficiency’ for situations resulting in the low availability of zinc; these are sometimes also called ‘induced deficiencies’ by others.
   
   
4. Soils Low in Organic Matter: These soils are unable to retain very much zinc and hence tend to be more prone to deficiencies. In the USA one of the most frequent locations of zinc deficiency problems in crops is where surface soil has been removed as part of levelling of fields. The underlying soil has a lower organic matter content than the topsoil and, in many cases, the subsoil also has a higher pH. Several workers have shown a positive correlation between extractable zinc and organic matter content. Both the DTPA extractable zinc and organic matter content decrease with depth in the soil profile. In cases where soil has been disturbed and subsoil brought up to the surface, such as in drainage operations, zinc deficiency is also more likely to occur.
   
   
5. Microbially Inactivated Zinc: It has been found that zinc - sensitive crops, such as maize can show increased deficiency problems when following certain crops, such as sugar beet (Beta vulgaris). This appears to be due to the incorporation of sugar beet leaves into the soil bringing about a reduction in the available zinc concentration. However, this has not been found by all workers and alternative explanations, such as the high phosphate applications normally used with sugar beet could also apply.
   
   
6. Cool Soil Temperatures: Zinc deficiencies are often worse during the early growing season due to low temperatures. In Colorado, zinc deficiency problems are often severe during cool wet springs and disappear by mid - July. The explanation for the effects of low temperatures are that they are due to poorly developed root systems and reduced microbial decomposition of organic matter which would release zinc to the new crop. There have been reports of phosphorus - induced zinc deficiency being more severe at low temperatures than at high temperatures. Growth can often be normal in young plants until a cool and wet period when new growth appears chlorotic and sometimes shows an abrupt colour contrast between green and yellow colouration of the leaves.
   
   
7. Plant Species and Varieties: Plants differ markedly in their sensitivity/tolerance to zinc deficiency. Intra - specific variations are sometimes as great as inter - specific variations. Several workers have demonstrated that wheat varieties can display a wide range of efficiency of zinc utilization. The most ‘zinc - efficient’ cultivars were able to produce more dry matter and grain under conditions of low available zinc supply than zinc - inefficient varieties.
   
   
8. High Levels of Available Phosphorus: The mechanism responsible for this antagonism is not fully understood. Phosphorus could affect either the uptake of zinc through the roots and/or the translocation of zinc within the plant. An excess of phosphorus can interfere with the metabolic functions of zinc. High levels of phosphorus can also lead to a reduction in vesicular arbuscular mycorrhizal infection and this could reduce the absorbing area of the roots.
   
   
9. Effect of Nitrogen: Nitrogen can affect zinc availability in two possible ways. Firstly, increased protein formation following nitrogen fertiliser additions can lead to zinc being retained in the root as a zinc - protein complex and not translocated around the plant. Secondly, acidifying nitrogen fertilisers, such as ammonium nitrate and ammonium sulphate can lead to a decrease in soil pH and an increase in zinc availability.

Zinc, in common with the other plant micronutrients, can limit growth when it is present both in inadequately low concentrations and also in excessive concentrations, due to deficiency and toxicity, respectively. So, while excess zinc can be toxic to plants, just as in the case of zinc deficiency, plants vary widely in their tolerance to zinc toxicity.

Although zinc toxicities are possible, they are not likely to be important for most agricultural land anywhere in the world. Apart from areas polluted by industry and some fields receiving excess zinc - rich sludges or manures, most soils will mainly have either normal (sufficient) or deficient levels of zinc.

For a more detailed discussion on zinc toxicity in plants, please refer to Section 2.5 of Professor Brian Alloway's book in our publications section titled: Zinc in Soils and Crop Nutrition. To view a Table of Contents of this publication, and to download individual chapters, click here.

Zinc is an essential trace element for animals and humans as well as plants. There are more than 300 enzymes involved in key metabolic processes in humans which contain zinc and therefore an adequate zinc intake is essential for normal healthy growth and reproduction. The International Zinc Nutrition Consultative Group (IZiNCG) has estimated that as much as one third of the world’s population is at risk from inadequate zinc intake. Many food products are derived directly from plants, including staples such as rice, wheat, maize and sorghum, but the zinc content of animal products is also affected by the soil - plant relationships of zinc since ruminants consume herbage and other animals such as pigs and poultry consume cereals. Zinc deficient plants generally have low tissue zinc concentrations and therefore, in addition to reduced crop yields, deficiencies of zinc in the field also reduce the nutritional value of the crop with regard to its contribution to the zinc content of the diet. This can be vitally important in subsistence rural economies where there is often insufficient diversity in the diet to enable low concentrations of zinc in one component of the diet to be compensated for by a higher zinc content in another. For example, lean meat, whole grain cereals and pulses generally contain the highest concentrations of zinc (20 - 50 mg Zn kg–1 fresh weight). However, taking meat as an example, many people in developing countries either do not have access to a sufficient amount of meat or their social and religious customs do not permit them to eat it so this significant source of zinc in the diet is often not available to many people.