Agronomic Effectiveness of Zinc Sources as Micronutrient Fertilizer

CHAPTER FIVE

Agronomic Effectiveness of Zinc Sources as Micronutrient Fertilizer D. Montalvo*,†,1, F. Degryse*, R.C. da Silva*, R. Baird*, M.J. McLaughlin*,** *

Fertilizer Technology Research Centre, University of Adelaide, Glen Osmond, SA, Australia CSIRO Land and Water Flagship, Glen Osmond, SA, Australia Division of Soil and Water Management, K.U. Leuven, Kasteelpark Arenberg, Heverlee, Belgium

** †

1

Corresponding author. E-mail address: [email protected]

Contents 1. Introduction 2. Importance of Zn for Plants and Humans 2.1 Zinc in Soils 2.2 Zinc Deficiency in Soils and its Correction 3. Sources of Zn for Fertilizer Production 4. Types of Zn Fertilizers 4.1 Zn Applied as a Single Nutrient 4.2 Zinc Applied as a Compound 5. Chemical Reactions of Zn Fertilizers 5.1 Chemical Reactions of Zn as Single Nutrient Fertilizer 5.2 Chemical Reactions of Zn-Enriched Fertilizers 6. Extraction Methods to Estimate Zn Fertilizer Effectiveness 7. Agronomic Effectiveness of Zn Fertilizers 7.1 Biofortification 7.2 Fertilizer Zn Effectiveness in Soil 7.3 Foliar Zn 7.4 Seed Treatment 8. New Technologies to Improve Zn Fertilizer Efficiency 8.1 Nanotechnology in Zn Fertilizers 8.2 Other Technologies 9. Conclusions and Future Needs Acknowledgments References

Advances in Agronomy, Volume 139 ISSN 0065-2113 http://dx.doi.org/10.1016/bs.agron.2016.05.004

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Abstract Zinc (Zn) is an essential micronutrient for plants and humans. Millions of hectares of agricultural land are affected by Zn deficiency and it has been estimated that about one-third of the world's population is Zn deficient. One of the strategies that has been successfully used to tackle Zn deficiency is the application of Zn fertilizers. A large array of Zn sources is available in the market, although the most commonly used fertilizers are ZnO and ZnSO4. The availability of Zn fertilizers is affected by the chemical reactions of Zn with the soil which are also affected by the chemical and physical properties of the fertilizer (eg, granule size or physical state solid vs fluid, water solubility of Zn) and the fertilizer application method. Application of fertilizers to the soil at sowing is an effective strategy to increase soil available Zn and crop yields, and the relative effectiveness of sulfate and oxide sources of Zn varies with placement. Cogranulating Zn with phosphorus fertilizers can reduce effectiveness due to formation of insoluble Zn phosphates, but various technologies are becoming available to circumvent this issue. The application of foliar Zn sprays should be considered when plants are grown in Zn-sufficient soils and the main goal is food biofortification.

1. INTRODUCTION Zinc (Zn) deficiency in plants and humans is a widespread problem in many regions of the world. The application of both soil and foliar Zn fertilizers has been used to correct Zn deficiency and to enhance plant Zn nutrition and yields. The effectiveness of Zn fertilizers can be diminished by chemical reactions occurring during fertilizer manufacture and with the soil components after fertilizer application. Sorption and precipitation are the main reactions that limit Zn availability in soils. A better understanding of these reactions is necessary to improve fertilizer formulations and to optimize fertilizer recommendations. The present review discusses fertilizer Zn sources and the chemical reactions that occur when they are applied alone or in combination with macronutrients. Other agronomic factors that affect the efficiency of Zn fertilizers are also discussed.

2. IMPORTANCE OF Zn FOR PLANTS AND HUMANS Zinc is essential for humans and animals. In plants, Zn plays a vital role as a catalytical, structural, and regulatory cofactor of many enzyme reactions. Zinc is necessary for the metabolism of carbohydrates, protein synthesis, the biosynthesis of growth hormones, in particular of indoleacetic acid, and the

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maintenance of the integrity of cell membranes (Broadley et al., 2012). Plants suffering from acute Zn deficiency exhibit stunted growth, chlorosis of leaves, shortened internodes and petioles, and clustering of small malformed leaves at the top of the plant (classic rosette symptom of dicotyledons) (Brennan et al., 1993). In marginally Zn-deficient soils, crop yields and quality can be reduced, without the appearance of Zn deficiency symptoms in plants (hidden deficiency) (Alloway, 2009). Zinc is an essential micronutrient for human health. Zinc is a component of a large number of enzymes (>300) and participates in various metabolic processes such as synthesis and degradation of carbohydrates, proteins, and nucleic acids. Zinc plays a vital role in the functioning of the nervous, reproductive, and immune systems and is important in the physical growth and cognitive development of children (Alloway, 2008b).

2.1 Zinc in Soils Zinc occurs naturally in the earth’s crust as a part of rocks and in ore minerals. The average concentration of Zn in the lithosphere is 80 mg kg
1 (Lindsay, 1979). The concentration of Zn in soil-forming rocks is very variable. Basaltic igneous rocks generally have a high concentration of Zn (48–240 mg kg
1); whereas silica-rich igneous rocks like granite and gneiss have much lower Zn content (5–140 mg kg
1). In the group of sedimentary rocks, black shales have the highest Zn content (34–1500 mg kg
1), followed by shales and clays (18–180 mg kg
1) and sandstone (2–41 mg kg
1) (Nagajyoti et al., 2010). To a large extent the concentration of Zn in natural (unfertilized and uncontaminated) soil is related to the chemical composition of the parent rock. Typical total Zn concentrations in uncontaminated soils vary widely and can range from 10 to 100 mg kg
1 (Mertens and Smolders, 2013). 2.1.1 Soil Solution Zn Soil solution Zn represents a small proportion of total Zn in the soil, but it is of critical importance as it is from this pool that plants absorb Zn. The concentration of Zn in soil solution is normally low, and can range from 4 × 10
10 to 4 × 10
6 M (Barber, 1995). Zinc concentration in soil solution has been found to be negatively correlated to soil pH, which can be explained by the stronger sorption on the solid phase at high pH values (Jeffery and Uren, 1983). In addition to the total concentration, the speciation of Zn in solution may affect its bioavailability. In the soil solution, Zn occurs in the form of free

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hydrated [Zn(6H2O)2+] and complexed species. Solution Zn speciation depends on pH as well as the abundance of ligands and competing ions in solution (Degryse et al., 2009). At high pH values (pH above 7.7), the free hydrated Zn ion can hydrolyze and form “hydroxy-complexes,” that is, ZnOH+ and ZnðOHÞ02 (Lindsay, 1979). Zinc in soil solution can form soluble complexes with inorganic (eg, sulfate, phosphate, nitrate, chloride) and organic (eg, fulvic and humic acids) ligands. Zinc complexes with inorganic ligands are usually of lesser importance. Complexation with dissolved organic matter (OM) (humic, fulvic acids) increases with increasing pH (Randhawa and Broadbent, 1965; Saeed and Fox, 1977; Weng et al., 2002). Chemical speciation models have been used to predict speciation of Zn in soil solutions, most commonly the Windermere Humic-Aqueous Model (WHAM) (Nolan et al., 2003) and the Nica-Donnan model (Cance`s et al., 2003). 2.1.2 Soil Solid Phase Zn and Reactions Controlling Zn Availability Zinc in the soil solid phase is present as adsorbed species bound to organic and inorganic particles and as Zn precipitates. Zinc precipitation includes the formation of a new solid phase compound, surface precipitation, and incorporation of Zn into an existent or recently precipitated mineral structure (Luxton et al., 2014). The term sorption has been used to refer to the transfer of Zn from the solution to the solid phase when the mechanism is unknown and includes adsorption and precipitation (Bradl, 2004). Soil pH is generally found to be the most important soil property determining the adsorption of Zn in soils, which can be attributed to the increasing negative charge on the soil surface as pH increases (Barrow, 1986b,c; McBride et al., 1997). It is generally found that the solid:liquid partitioning coefficient of Zn increases by about a factor of 5 per unit increase in pH in the pH range 4–7 (Sauve´ et al., 2000). Higher content of OM, oxides, and CaCO3 also favor Zn adsorption as these act as adsorbents (Degryse et al., 2009). The formation of Zn precipitates occurs where the concentration of Zn exceeds Zn adsorption maxima of the soil and the solubility product of Zn minerals is exceeded in soil solution (Bingham et al., 1964). Solubility calculations predict that under specific conditions of high pH (pH > 8) and very high Zn2+ concentration in solution (>10
4 M) Zn could theoretically precipitate as Zn(OH)2 (zinc hydroxide), ZnCO3 (smithsonite), ZnO (zincite), or Zn2SiO4 (willemite) (Lindsay, 1979). It is still being debated whether precipitation reactions control the concentration of Zn2+ in soil solution as many studies have shown that solubility of a single mineral phase

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cannot describe Zn solution concentrations (Bru¨mmer et al., 1983; Dang et al., 1996; Degryse et al., 2011). The existence of defined Zn minerals have been identified only in Zn fertilized soil bands (Kalbasi et al., 1978) or in heavily contaminated soils [eg, smithsonite in contaminated overbank sediments; Van Damme et al. (2010)]. Identification of Zn species in the soil solid phase is complex and advances have been possible with the use of synchrotron X-ray absorption spectroscopic techniques (XAS). These techniques have demonstrated the presence of more complicated Zn precipitates, such as Zn-layered double hydroxides (Zn-LDH), Zn-phyllosilicates, and Zn substituted in metal (hydr)oxide and phyllosilicate minerals (Luxton et al., 2014). Speciation of Zn in the soil solid phase is influenced by the source of Zn (geogenic, anthropogenic), the total concentration, and the soil chemical properties (pH and oxidation-reduction potential) (Luxton et al., 2014). Based on the combined use of synchrotron X-ray fluorescence (SXRF), diffraction (XRD), and absorption (EXAFS) Manceau et al. (2004) identified sphalerite (ZnS), zincochromite (ZnCr2O4), Zn-phyllosilicate, and lithiophorite, as the Zn species in a pristine horizon of a clayey acidic soil (pH 4.5–5.0). Jacquat et al. (2009) investigated the speciation of Zn in 49 field soils covering soil pH (CaCl2) values of 4.1–7.7 that were contaminated by runoff water from corroding galvanized power-line towers containing aqueous Zn2+ (total Zn ranged from 251 to 30090 mg kg
1, average of 4937 mg kg
1). Results from this study showed that in soils with pH values above 5.2 and high Zn loading relative to available sorption sites, the predominant Zn species in soil were Zn-LDH and Zn-phyllosilicate. In contrast, in acidic soils with low Zn loading the major species were Znhydroxy-interlayered minerals (Zn-HIM) reflecting the high affinity for Zn but limited sorption capacity of HIM. Hydrozincite [Zn5(CO3)2(OH)6] was the predominant Zn species in calcareous soils with extremely high Zn content. Retention of Zn in soils is characterized by initial fast reactions in which Zn is adsorbed to the solid particles, followed by slow reactions. These slow reactions are termed fixation or aging and are the cause for the gradual reduction (months or years) of Zn availability in soils (Barrow, 1986a, 1998; Ma and Uren, 2006). Fixation reactions increase the nonlabile fraction of Zn in the soil, which is the fraction that is not readily exchangeable with the Zn in the soil solution phase. The isotopic dilution technique provides a useful way to discriminate between labile (exchangeable) and nonlabile (nonexchangeable) Zn pools in soils. It has been suggested that the possible

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mechanisms for Zn fixation reactions are solid-state diffusion into mineral lattices and/or surface precipitation of Zn onto the oxide minerals and carbonates (especially in high pH soils) (Barrow, 1986b; Bruemmer et al., 1988; Nachtegaal and Sparks, 2004; Buekers et al., 2007). Studies of soils amended with Zn have shown that Zn fixation increases with increasing soil pH (Tye et al., 2003; Buekers et al., 2007; Donner et al., 2012) (Fig. 1). The decrease over time in the availability of applied Zn in soil to plants was demonstrated by the work of Brennan (1990). In this study, 54 soils with varying soil chemical properties (pH 4.8–8.6, clay % 1.5–59, organic carbon % 0.26–4.7) were incubated at 30°C for 30 days with Zn applied in solution as ZnSO4. Dry matter and Zn uptake of subterranean clover was determined in plants grown in incubated soils and in soils that received freshly applied Zn (control treatments). The relative effectiveness of the Zn fertilizer was defined by the ratio of the slopes of the linear relationships between added Zn and Zn uptake of the plants that received incubated and freshly applied Zn. In all soils, incubation decreased dry matter yield, Zn content, and Zn uptake by the plants. The relative effectiveness of incubated Zn ranged from 0.47 to 0.80. Lower relative effectiveness (more fixation) was observed in soils with high pH and high clay content (Fig. 2). It should be noted that fixation is not an irreversible process as “fixed Zn” can become available if the conditions that caused fixation are changed. For

Figure 1 Change in isotopic exchangeability of Zn initially added in solution to 23 soils categorized by pH range. The slow reaction is modeled as a reversible first-order kinetic process (solid lines). From Young (2013) with permission from Springer.

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0.9

(A)

RE uptake

0.8 0.7

y = –0.005x + 0.722 R 2 = 0.61

0.6 0.5 0.4 0 0.9

15

45 30 Clay content (%)

60

(B)

RE uptake

0.8 0.7 0.6 y = –0.075x + 1.152 R 2 = 0.74

0.5 0.4 4.5

5.5

6.5 Soil pH

7.5

8.5

Figure 2 The relationship of (A) clay content and (B) soil pH and the relative effectiveness of Zn determined by Zn uptake of clover shoots that received incubated (30 days) and fresh Zn fertilizer. Replotted from Brennan (1990) with permission from CSIRO Publishing.

example, Buekers et al. (2007) showed that the fraction of Zn fixed in soils with pH > 7 and high CaCO3 content decreased from 75% to 56% when the soil samples were acidified to remove 50% of the carbonates.

2.2 Zinc Deficiency in Soils and its Correction Zinc deficiency in agricultural soils is considered to be the most geographically widespread micronutrient deficiency constraint limiting crop production. In a global study carried out in 30 countries by the Food and Agriculture Organization of the United Nations (FAO) to assess the micronutrient status of soils, it was estimated that approximately 30% of the world’s agricultural soils are Zn deficient (Sillanpa¨a¨, 1982). Countries with extensive

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Zn-deficient areas include China, India, Iran, Pakistan, and Turkey where it has been estimated that between 50% and 70% of the cultivated land is affected by Zn deficiency (Fig. 3) (Alloway, 2009). Zinc deficiency has been also documented in Western and Southeastern Australia and Brazil. Generally regions with severe Zn-deficient soils are also regions where humans present Zn deficiencies, particularly in developing countries. Since total Zn concentration is a poor predictor of plant available Zn, soil extraction procedures have been developed to determine the readily available fraction of Zn in soils and to classify soils as either Zn deficient or Zn sufficient. The extracts used include chelating agents such as diethylenetriaminepenta-acetic acid (DTPA) and ethylenediaminetetra-acetic acid (EDTA), dilute acids (Mehlich-1, Mehlich-3, 0.1 M HCl), and neutral salts (0.1 M NaNO3, 0.01 M CaCl2). The use of dilute acids to extract available Zn is not advised in alkaline soils since nonlabile Zn may be mobilized (Lindsay and Cox, 1985). The extract that is still most widely used is DTPA, which was developed to measure available Zn in near-neutral and calcareous soils (Lindsay and Norvell, 1978). The calibration of soil test Zn against plant response allows the determination of the critical Zn concentration in the soil, below which a response in plant growth is expected from the application of Zn fertilizer. The critical values reported for DTPA range from 0.1 to 1 mg kg
1 and for 0.1 M HCl from 1 to 5 mg kg
1 (Brennan et al., 1993). Differences in critical values are likely related to differences in soil properties, crop species, and climatic conditions. Although there is no consensus in the literature about which extract better predicts plant available Zn, it appears that better correlations are often obtained with neutral salt extractions (eg, 0.01 M CaCl2), which

Widespread zinc deficiency Medium deficiency

Figure 3 Geographic distribution of severe [black areas (orange in the web version)] and moderate [gray areas (green in the web version)] Zn-deficient soils in the world. Adapted from Alloway (2008b) with permission from International Zinc Association.

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better mimic in situ soil solutions (Menzies et al., 2007). Given that fertilizer inputs are one of the major costs for farmers, better assessment of soil available Zn would help make more accurate and economically sound fertilizer recommendations. Zinc-deficient soils have typically low total Zn concentration, neutral to alkaline pH, high CaCO3 content, high OM content, high oxide content, high phosphorus (P) concentrations, or have been subjected to prolonged waterlogging (Alloway, 2009). Soils with low total Zn concentrations are commonly found in highly weathered soils of the tropics where the intense rainfall and warm temperatures have promoted weathering of the parent material and leaching of nutrients (eg, Zn). These highly weathered and oxide-rich soils are classified as Oxisols by the US soil taxonomy classification system and are present particularly in Africa and South America (Soil Survey Staff, 2003). In soils with low total Zn concentrations, low Zn availability can also be attributed to the presence of Fe oxide minerals that have strong affinity for Zn (Ghanem and Mikkelsen, 1988). The problem of Zn deficiency in Oxisols can be made worse by increase in soil pH due to lime applications to alleviate soil acidity (Fageria and Stone, 2008). Furthermore, due to the high P sorption capacity of the Oxisols, large P fertilizer applications are necessary for adequate plant growth. It has been shown that P sorbed on oxide surfaces increased the adsorption of Zn (Bolland et al., 1977; Barrow, 1987). Alkaline-calcareous soils represent an important type of agricultural soil in many countries with Mediterranean climates and are commonly utilized for the production of cereals. In the World Reference Base (WRB/FAO World Reference Base for Soil Resources, 2006) classification, these soils are classified as Calcisols. In Central Anatolia, the major wheat growing area of Turkey, 65% of the soils contain more than 20% of CaCO3 and the pH of the soils range from 7.5 to 8.1. These soils contain very low plant available Zn (0.23 mg kg
1 DTPA-Zn) although the total Zn content of the soils is relatively high (39.6–62.4 mg kg
1). The low Zn availability in these soils is due to Zn being strongly sorbed onto CaCO3 (Cakmak, 2008b) and, in common with other regions of Zn-deficient soils, has stimulated the introduction of measures to correct Zn deficiencies. Various strategies have been developed for the correction of Zn deficiency in crops. One approach is the selection of Zn-efficient plant species that are able to grow and yield under low soil Zn conditions (Graham and Rengel, 1993). It has been recognized that the tolerance of crop species to Zn deficiency varies widely, with beans, citrus, corn, and rice catalogued as

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Table 1 Relative susceptibility of various crop species to Zn deficiency. Susceptibility Low

Medium

High

Alfalfa Carrot Grapes Oat Rye Wheat

Barley Potato Soybean Sugar beet Spinach Tomato

Beans Citrus Corn Cotton Rice Sorghum

Source: Adapted from Alloway (2008a).

highly susceptible crops (Table 1). Differences in the response to Zn deficiency have also been observed between genotypes of the same crop species. Breeding Zn-efficient plants can be a cost-effective strategy; however, breeding programs require more time and resources. Hence, the use of Zn fertilizers is undoubtedly the most effective and quick solution for the correction of Zn deficiency in crops. In the long term, Zn export via harvested biomass needs to be compensated by proportional Zn inputs to avoid full depletion of the soil Zn reserves. Hence, breeding can never be a complete substitute for fertilization approaches, but must be seen as a complementary strategy.

3. SOURCES OF Zn FOR FERTILIZER PRODUCTION Zinc is mined from ore deposits in more than 50 countries in the world. According to the US Geological Survey data (USGS zinc statistics and information– http://minerals.usgs.gov/minerals/pubs/commodity/zinc/ index.html), in 2014 the Zn mine production was 13.3 million tons and the largest producers were China, Australia, Peru, USA, Mexico, and India. The most exploited Zn mineral is sphalerite (ZnS) which contains up to 67% Zn. Other Zn-containing ore minerals are smithsonite (ZnCO3), willemite (Zn2SiO4), and hemimorphite (Zn4Si2O7(OH)2·H2O). About 60% of Zn used worldwide comes from mined Zn, while the remaining 40% comes from recycling secondary Zn. Mined Zn is principally used in the galvanizing industry (50%), in the production of Zn-base alloys (17%), and in the production of brass and bronze (17%) (International Zinc Association— http:// www.zinc.org/sustainability-learning-annex/). All of these industrial processes generate by-products and residues that are used as feedstock for the production

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Table 2 Limits on metal contaminants in Zn fertilizers from hazardous material in USA. Maximum allowed concentration in Element fertilizer per 1% of Zn (mg kg
1)

As Cd Cr Pb Hg

0.3 1.4 0.6 2.8 0.3

Zinc fertilizers made from recycled hazardous secondary materials (USEPA, 2002).

of Zn fertilizers. Due to the increasing use of Zn fertilizers in agriculture, mined Zn is also starting to be used for the production of fertilizers. Potentially toxic metals like cadmium (Cd), lead (Pb), and nickel (Ni) may be present in by-products used to manufacture Zn fertilizers, depending on the industrial process from which it was obtained (USEPA, 2001). Highly variable concentrations of potentially toxic metals have been observed in Zn fertilizers made from industrial by-products. For example, the concentrations of Cd and Pb in two Zn-oxysulfate products prepared by partial acidulation of Zn flue dusts varied from several hundreds to thousands of milligram per kilogram of fertilizer (Westfall et al., 2005). The concern that high concentrations of potentially toxic metals could enter the human food chain through the application of the fertilizer to the soil, led the US Environmental Protection Agency (USEPA) to establish maximum limits for contaminants in fertilizers that are made from hazardous materials (Table 2) (USEPA, 2002).

4. TYPES OF Zn FERTILIZERS Various types of Zn fertilizers are used to correct Zn deficiency in crops. These fertilizers vary in their Zn content, chemical composition, price, and effectiveness to plants. The four main classes of Zn fertilizers are inorganic compounds, synthetic chelates, natural organic complexes, and inorganic complexes (Table 3) (Mortvedt and Gilkes, 1993). Zinc fertilizers can be applied as a single nutrient source or as compounds when incorporated or blended with macronutrient fertilizers (N–P–K). Zinc fertilizers are available in both liquid and solid forms and formulations have been developed for either soil or foliar application.

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Table 3 Commonly used Zn fertilizers. Zn source

Inorganic compounds Zinc sulfate monohydrate Zinc sulfate heptahydrate Zinc oxysulfate Zinc oxide Zinc carbonate Zinc chloride Zinc nitrate Basic zinc sulfate

Formula

Solubility in water

Zn content (%)

ZnSO4·H2O

Soluble

36

ZnSO4·7H2O

Soluble

22

×ZnSO4·yZnO ZnO ZnCO3 ZnCl2 Zn(NO3)2·3H2O ZnSO4·4Zn (OH)2

Variable Insoluble Insoluble Soluble Soluble Slightly soluble

20–55 50–80 50–56 50 23 55

Soluble Soluble Soluble

8–14 6–10 9–13

Soluble Soluble

50–100 50–80

Soluble (liquid)

10

Synthetic chelates Disodium zinc EDTA Na2ZnEDTA Sodium zinc HEDTA NaZnHEDTA NaZnNTA Sodium zinc NTA Natural organic complexes — Zinc polyflavonoid — Zinc lignosulfonate Inorganic complexes Zn(NH3)4SO4 Ammoniated zinc sulfate solution

Source: Adapted from Mortvedt and Gilkes (1993) with permission from Springer.

4.1 Zn Applied as a Single Nutrient 4.1.1 Inorganic Compounds Oxides, carbonates, sulfates, chlorides, and nitrates of Zn are the inorganic forms of Zn fertilizers available in the market. Zincoxide (ZnO) is an inorganic compound nearly insoluble in water but soluble in acids. This is a cheap Zn source that is produced by two main processes—the indirect or French process and the direct or American process. In the indirect process developed by LeClaire in 1840, ZnO is obtained by burning metallic Zn. The physical properties and crystallography of the ZnO produced by this method can be manipulated by adjusting the combustion conditions (flame turbulence and air excess). The direct process involves the reduction-oxidation of Zn ore using carbon monoxide and air. Zinc sulfate (ZnSO4·xH2O) is produced by reacting ZnO with sulfuric acid (H2SO4). It is the most commonly used Zn fertilizer and it is available in

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the monohydrate and heptahydrate forms. Zinc sulfate is highly water soluble (96%) and dissolves quickly in the soil. Zincoxysulfate (xZnSO4·yZnO) is produced by partially acidulating ZnO with H2SO4. The Zn content and the water-soluble fraction of the final product are related to the degree of acidulation. 4.1.2 Synthetic Chelates Metal chelates have been used in agriculture for more than 50 years to enhance plant micronutrient uptake (Boawn et al., 1957; Wallace, 1963; Murphy and Walsh, 1972; Karak et al., 2005). The word chelate derives from the Greek word “chela” or “claw” and describes the complexation of a chelating agent with a metal cation through coordinate bonds forming a ring structure (Wallace, 1962; Mortvedt, 1991). The interest in the use of synthetic chelates has been related to their ability to retain metal cations in soluble forms in the soil solution, thus enhancing their diffusion to the plant roots (Wallace, 1963; Elgawhary et al., 1970). Common synthetic chelating agents used for Zn are the organic compounds EDTA and DTPA. Chelation of a metal cation (Zn2+) with an organic chelating agent (EDTA4
) causes the reversion of its charge (ZnEDTA2
) reducing its electrostatic attraction to the soil adsorption sites. Hence, when added in chelated form, Zn is less likely to be retained by the soil colloids and more likely to be transported through soil to the roots. Because of these characteristics, synthetic chelates are claimed to be very effective Zn fertilizers. Chelated Zn is not readily taken up by plant roots but the enhanced uptake of Zn observed in studies is probably due to enhanced diffusion of Zn to the roots (Zhao et al., 2015). An important characteristic of chelating agents is their capacity to form stable bonds with metals. Indeed for a chelate to be effective, the rate of substitution of the chelated cation with other cations present in the soil should be very low, maintaining the cation in a chelated form for sufficient time to be transported to the plant root (Mortvedt and Gilkes, 1993). 4.1.3 Natural Organic Complexes Natural organic complexes are produced by the reaction of Zn salts with organic complexing agents obtained as by-products of the wood pulp industry or related industries. Some of these complexing agents are lignosulfonates, phenols, and polyflavonoids and have been explored and promoted as cheaper and environmentally friendlier fertilizer alternatives than the conventional synthetic chelates (Martı´n-Ortiz et al., 2009a). Lignosulfonates are derived from lignin when wood pulp is produced by the sulfite method. These are the most common natural organic agents used to complex Zn by

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the fertilizer industry. Lignosulfonic acid is the only organic complexing agent authorized by the European Union to prepare complexed Zn, Cu, Fe, and Mn fertilizers (EuropeanCommision, 2012). Although lignosulfonates have been used as fertilizers for a long time, our understanding of the chemical reactions that affect their efficacy is still limited. For example, the exact chemical structure of lignosulfonates is unknown and it may well vary according to the source and/or production process used. It is generally accepted that lignosulfonates contain hydrophobic groups such as aromatic and aliphatic groups and a large number of hydrophilic groups such as sulfonic, carboxylic, and hydroxyphenolic, which can form coordinate bonds with metal cations (Carrasco et al., 2012). Martı´n-Ortiz et al. (2009b) showed that spruce lignosulfonate (softwood) had a higher Zn-complexing capacity than eucalyptus lignosulfonate (hardwood), which could be related to the higher phenolic content in the spruce lignosulfonate. Because of the complex structure of these polymers, stability constants have not been defined; however, it is presumed that they would be lower than those of synthetic chelates (Mortvedt, 1991). 4.1.4 Inorganic Complexes Inorganic complexes of Zn are also used as fertilizers. Ammonia forms coordinate bonds with Zn2+ to form a tetraamine complex, but it is assumed that the metal–ammonia complex decomposes after application to the soil (Mortvedt, 1991). The most common Zn inorganic complexes are ammoniated zinc sulfate solution Zn(NH3)4SO4 and ammoniated zinc chloride solution Zn(NH3)4Cl2. Commercial products usually contain 10–15% N, 10% Zn, and 5% S. The primary advantage of these sources is that they can easily mix with fluid fertilizers made from ammonium polyphosphate (Mortvedt, 1991).

4.2 Zinc Applied as a Compound Zinc application rates to the soil are normally low (<10 kg Zn ha
1) which can result in poor distribution of the nutrient in the crop-root zone if Zn is applied as a single-source high-analysis fertilizer (Mortvedt, 1991). Applying Zn in compound macronutrient fertilizers (as a coating or incorporated) results in a better field distribution, and also has economic benefits as no separate transport or application is required. The drawback is that Zn can react with the components of the macronutrient fertilizer reducing its availability. 4.2.1 Zinc–N–P–K Fertilizer Technology Zinc sources may be added to macronutrient fertilizers in three ways: (1) blending with granular fertilizers, (2) coating onto granular fertilizers, and (3)

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incorporated and cogranulated during the manufacturing of macronutrient fertilizers. Zinc can be also incorporated into fluid fertilizers. Bulk blending is a simple economical way of physically mixing different granular fertilizers to satisfy the nutrient requirements of individual fields and crops. If the particle sizes of granules are not matched in a bulk blend, segregation becomes a problem diminishing the uniform application of nutrients (Silverberg et al., 1972). Another problem of blending is a sparse distribution of Zn granules in the field due to the small amount of Zn required compared to the requirement of macronutrients. This problem is also made worse when using Zn sources of higher concentration as fewer granules will be required for a given application rate. The uniformity of Zn fertilizer application in the field can be improved by incorporating Zn during the granulation process or by coating Zn onto granular macronutrient fertilizers as in both cases Zn is an integral part of each granule. During incorporation of Zn into N–P–K fertilizers, Zn is in intimate contact with the other chemical components. Under the conditions of high temperature, varying pH, and moisture that exist during granulation, chemical reactions may occur (see Section 5.2.1) affecting the solubility of the added Zn (Mortvedt, 1991). The most commonly used coating procedure consists of mixing the granular macronutrient fertilizer with a finely (<0.15 mm) ground Zn source and spraying of a liquid binding agent that aids the adherence of the micronutrient to the surface of the granule. Acceptable binders should be cheap and should not cause undesirable physical problems (eg, caking) or chemical reactions. Typical binding agents that have been used are water, oil, waxes, ammonium polyphosphate, urea, ammonium nitrate, and lignosulfonates. Superphosphate fertilizers are applied to soils as a source of P, calcium (Ca), and sulfur (S); however, they may also contain varying amounts of trace metal impurities like Cd and Zn which are added simultaneously to the soil with the application of the fertilizer. These trace metals present in superphosphates are derived mainly from the phosphate rock from which they were produced. Williams (1974) reported that the concentration of Zn in superphosphates used in Australia ranged from 270 to 750 mg kg
1. Although superphosphates are an important source of Zn for crops (Ozanne et al., 1965); their usage has declined over the years in favor of high-purity phosphate sources (monoammonium phosphate and diammonium phosphate, <100 mg Zn kg
1). The shift from superphosphates to ammonium phosphate fertilizers was the main reason for the appearance of Zn deficiency symptoms in pastures and crops in calcareous soils of South Australia by the

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late 1980s. As a result, regular applications of Zn fertilizers have become a common practice since the last decade (Holloway et al., 2008b). 4.2.2 Manures and Biosolids Manures and biosolids (urban sewage wastes) contain high concentrations of OM, macro- and micronutrients which can be used beneficially when applied to agricultural land. In biosolids, the concentration of Zn and other trace elements vary depending on its origin (domestic or industrial sludge) and the treatment given at individual plants. Typical Zn concentrations reported in the literature are in the range 100–49000 mg kg
1 dry weight (Mortvedt, 1995). The high concentration of trace metals in biosolids has raised concerns regarding their potential as soil contaminants; hence its agricultural use has been regulated in many countries. Animal manures are applied to the soil to benefit principally from their nitrogen (N) and P contents. Regulation regarding their application is based on the total N and P loadings, giving less importance to the trace metal accumulation in the soil through manure application (Bolan et al., 2004). In intensive animal production systems, particularly of poultry and swine, the animals’ feeds are supplemented with Zn and other trace elements which are used as growth promoters. Since not all of the added metals can be assimilated by the animals, increasing amounts of these elements are found in the manures (Nicholson et al., 1999). Hence the continuous application of manures can lead to accumulation of Zn in the soil, but at a lower rate than use of sewage sludge because the concentration of Zn in manures is also lower. For example, assuming an application of 44 kg P ha
1, sewage sludge with 1.46% P would add 6.4 kg Zn ha
1; whereas poultry and swine manure would add 1.2 kg Zn ha
1, respectively (Bolan et al., 2004).

5. CHEMICAL REACTIONS OF Zn FERTILIZERS The soil chemical properties and the physical state of Zn applied (granular or fluid) determine the chemical reactions that occur when fertilizer is added to soil which ultimately determine the availability of Zn to the plants. For granular fertilizers the first process that occurs when a granular Zn fertilizer is applied to the soil is wetting of the granule. For surface-applied granules, wetting predominantly occurs by mass and capillary flow of water from the moist soil, and by water vapor transfer from the soil or the atmosphere to the granule (Lawton and Vomocil, 1954; Williams, 1969).

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This movement of water toward the fertilizer granule could restrict the outward diffusion of Zn to the surrounding soil. This physical process has been reported earlier as the mechanism that limited P diffusion in calcareous soils (Hettiarachchi et al., 2006). In contrast, no mass flow of moisture from the soil toward the fertilizer application site occurs with the injection of fluid fertilizers (Hettiarachchi et al., 2006). This different physical behavior around fertilizer granules and fluid bands can in part explain the differences in bioavailability of granular and fluid Zn fertilizers reported in the literature (Hettiarachchi et al., 2010). The movement of Zn into soil from surface-applied fertilizer granules can be assessed, for instance, by sampling soil sections at increasing distances from the fertilizer application point (Ghosh, 1990; Hettiarachchi et al., 2010). From such experiments, two zones have been identified with marked differences in the concentration of Zn: a Zn-saturated zone immediately adjacent to the fertilizer granule (within 1 cm), and an outer zone with low Zn concentrations (Ghosh, 1990). Different chemical reactions may occur in these two zones of soil. For example, Zn precipitates may form in the zone of high Zn concentration while in the zone of low concentration adsorption reactions most likely control the fate of Zn. Studies that investigated the solid-phase reaction products formed following the addition of Zn fertilizers in soils using direct identification methods (synchrotron techniques) are scarce in the literature. This could be in part due to the difficulty in the sample preparation as it can be challenging to isolate sufficient soil with high Zn concentrations necessary to obtain definitive qualitative data. Most information regarding the formation of Zn reaction products has been obtained from laboratory experiments that investigated the chemical reactions between Zn compounds and macronutrient carriers.

5.1 Chemical Reactions of Zn as Single Nutrient Fertilizer Using X-ray diffraction (XRD) analysis, Kalbasi et al. (1978) investigated the solid-phase reaction products that formed after the application of 0.1 g of Zn as ZnSO4, ZnS, and Zn-EDTA in four soils. The soils were incubated for 2–32 weeks and the soil used for the analyses was sampled from the zone immediately adjacent to the fertilizer placement site. Sphalerite was mostly identified in soils treated with ZnS, while no crystalline products were detected in soils treated with Zn-EDTA. In two calcareous soils, Zn carbonates were detected after application of ZnSO4. In an alkaline, noncalcareous

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soil, Zn(OH)2 was the reaction product detected shortly after the application of ZnSO4, but was not detected after 4 weeks of incubation. In a neutral (pH 7.2) noncalcareous soil, no crystalline solid-phase reaction products were detected in the soil treated with ZnSO4. This study illustrates the limitations of XRD for studying Zn speciation in soils, as only crystalline compounds in sufficiently high concentration can be detected. In most cases, Zn is likely to be mostly present as noncrystalline species, such as Zn adsorbed onto (or absorbed into) soil minerals. Several studies have indicated that the mobility of a soluble Zn salt added to soil is limited and dependent on soil pH and cation exchange capacity. Degryse et al. (2015) found that the extent of diffusion of spot-applied ZnSO4 (0.35 mg Zn) in three soils decreased with increasing soil pH and that even in an acid soil [pH(CaCl2) 4.3], added Zn only diffused about 1.5 cm from the point of application after 28 days. Similarly, Mortvedt and Giordano (1967b) found that mean Zn movement for spot-applied ZnSO4 (0.38 mg Zn) decreased from 2 cm in a very acid soil (pH 3.4) to 0.4 cm in alkaline soil (pH 7.7). Fewer studies have looked at mobility of Zn in soil when applied as ZnO. Available data in the literature suggest that ZnO dissolves within days (acid soils) to months (alkaline soils) when it is a fine powder mixed through soil (Smolders and Degryse, 2002; Voegelin et al., 2005). However, when ZnO is in granular form, the dissolution is expected to proceed much slower because of the decreased surface area in contact with the soil (Mortvedt, 1992).

5.2 Chemical Reactions of Zn-Enriched Fertilizers 5.2.1 Reactions of Zn With Macronutrient Compounds Zinc added to solid or liquid macronutrient fertilizers can react with other components in the fertilizer and precipitate as solid-phase compounds. Knowledge of the chemical interactions in fertilizer mixtures is important because the formation of reaction products can affect Zn solubility. Higher water-soluble Zn is desirable because it has been shown to strongly correlate with the agronomic effectiveness of Zn fertilizers (Mortvedt and Giordano, 1969). Lehr (1972) reviewed the chemical interactions between Zn sources and most commonly used macronutrient carriers and compiled a table with the Zn reaction products that have been identified or predicted from laboratory studies (Tables 4 and 5).

Zn3(PO4)2·4H2O

Low

Zn2KH(PO4)2·2H2O

Low

Zn2KH(PO4)2

Low

ZnNH4(PO4)2·H2O

Low

ZnNH4PO4

Low

ZnNH4PO4

Low

ZnKPO4

Low

ZnH2P2O7

Slowly soluble

Forms in dilute phosphate solutions (at high pH) containing dissolved Zn Forms rapidly when Zn reagents are added to solution of PK or NPK grades high in K Forms rapidly when Zn reagents added to solution of PK, or NPK grades high in K Reaction product of ZnSO4 with NH4H2PO4; forms in the pH range 3.5–4 Reaction product of ZnO or Zn salts added to NH4H2PO4 or mixtures of NH4H2PO4-(NH4)2HPO4 at pH’s below 5.5 Reaction product of ZnO or Zn salts added to more basic solution of (NH4)2HPO4 or APP of low polyphosphate content Reaction product of ZnO or Zn salts added to PK or NPK grades

A possible reaction product at placement site Metastable; slowly alters on standing to the anhydrous salt Forms during storage

Does not form if APP is present Hexagonal dimorph

Orthorhombic dimorph

Isotypic with hexagonal ZnNH4PO4; K+ ⇌ NH4+ Mg2+ and Fe2+ can substitute for Zn2+ 233

Forms on addition of Zn polyphosphoric acid prior to ammoniation

Remarks

Agronomic Effectiveness of Zinc Sources as Micronutrient Fertilizer

Table 4 Reaction products of Zn with P in macronutrient fertilizers. Reaction products Water solubility Occurrence in fertilizers

(Continued )

Zn(NH4)2H4(P2O7)2·2H2O

High

Forms on addition of Zn reagents to APP, where solution pH < 4

Zn3(NH4)2(P2O7)2·2H2O

Low

Zn(NH4)2P2O7·H2O

Low

Forms on addition of Zn reagents to APP where solution pH is 4–6 Forms on addition of Zn reagents to APP where solution pH is 6–8

(NH4)6Zn(P2O7)2·6H2O

High

Forms only in APP liquids of high polyphosphate content at pH > 6 at very high levels of added Zn

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Table 4 Reaction products of Zn with P in macronutrient fertilizers.—cont'd. Reaction products Water solubility Occurrence in fertilizers

Remarks

Isotypic with Mg salt; dissolves incongruently in H2O to form insoluble Zn(NH4)2P2O7·H2O Common postprecipitate in stored APP liquids Commonly forms on dilution of stable APP liquids Precipitation unlikely at the usual low concentration levels of added Zn

Source: Adapted from Lehr (1972) with permission from Soil Science Society of America.

D. Montalvo et al.

ZnNO3OH·H2O

Decomposes in water

Reaction product of surface-applied mixture of ZnO–Zn(NO3)2 with moisture

Zn(NO3)2·4NH3

High

Zn(NO3)2·NH4NO3·4NH3

High

3Zn(OH)2·NH4NO3

Low

Zn5NH4(NO3)2(OH)9·3H2O

Low

Zn(NO3)2·4Zn(OH)2·2H2O

Low

Zn(NO3)2·2CO (NH2)2·4H2O

High

ZnSO4·6CO(NH2)2

High

Reaction product of Zn reagents in NH3–NH4NO3 solutions Reaction product of Zn reagents in NH3–NH4NO3 solutions (25°C) Reaction product of surface-applied Zn°, ZnO, or Zn salts with solid NH4NO3 (pH>7) Reaction product of surface applied Zn°, ZnO, or Zn salts and solid NH4NO3 (pH 5–6) Reaction product of surface-applied ZnO or Zn salts and solid NH4NO3 (pH 3.5–5) Forms on addition of Zn reagents to ammoniating solutions containing NH4 NO3 and CO(NH2)2 Reaction product of ZnSO4 in ammoniating solutions containing CO(NH2)2

Remarks

Decomposed by H2O to form insoluble Zn(NO3)2·4Zn (OH)2·H2O Decomposed violently on heating Decomposes violently on heating Precipitates rapidly; forms insoluble coatings on residual Zn° and ZnO Precipitates rapidly; forms insoluble coatings on residual Zn° or ZnO Precipitates rapidly; water content varies from 0 to 2 moles Dissolves incongruently to form insoluble 3Zn(OH)2·NH4NO3

Agronomic Effectiveness of Zinc Sources as Micronutrient Fertilizer

Table 5 Reaction products of Zn with N in macronutrient fertilizers. Water Reaction products solubility Occurrence in fertilizers

Segregation of micronutrient in carrier solution

Source: Adapted from Lehr (1972) with permission from Soil Science Society of America.

235

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Zinc reacts particularly with the P component of macronutrient fertilizers. The Zn–P reactions are similar for both solid and fluid fertilizer formulations although they proceed at different rates (Lehr, 1972). In ammonium orthophosphate solutions, Zn precipitates as Zn-phosphates. For example, Zn added to monoammonium phosphate (MAP) or diammonium phosphate (DAP) was found to precipitate as insoluble ZnNH4H3(PO4)2·H2O at pH 3, as hexagonal dimorph of ZnNH4PO4 at pH 3.4–5.5 and as orthorhombic dimorph of ZnNH4PO4 at pH 6 (Frazier et al., 1966; Hossner and Blanchar, 1969). In ammonium polyphosphate solutions, the solubility of Zn sources is higher due to complex formation of Zn with polyphosphates (Silverberg et al., 1972), and hence polyphosphates are considered to be superior carriers for Zn (Mortvedt and Giordano, 1967a). Nevertheless, insoluble Zn-ammonium pyrophosphates were identified by Hossner and Blanchar (1969) in fertilizer mixtures containing 8% of Zn and with more than 20% of the total P in the solution as ammonium pyrophosphate. Holloway et al. (2008b) indicated that no more than 3% Zn (as Zn sulfate) can be added to fluid P fertilizers. Using X-ray diffraction analysis (Ghosh, 1990) identified ZnNH4PO4 as the major crystalline form of Zn in Zn-enriched MAP and DAP granules. However, no crystalline compounds were identified in superphosphate granules with high Zn content. Recently, Milani et al. (2015) used X-ray absorption near-edge structure (XANES) spectroscopy, to characterize the speciation of Zn added as nanoparticulate or bulk ZnO onto MAP and urea granules. The Zn species identified in the MAP granules after coating with either Zn source were scholzite (63%), Zn(NH4)PO4 (19
22%), and Zn(OH)2 (12
15%). Only a small amount of ZnO (0
6%) was detected in the coated MAP granules which suggested chemical reaction of Zn with P occurred during the manufacturing process. In contrast, zincite was the main (93
100%) Zn species present in the urea-coated granules. Jackson et al. (1962) reported that the water solubility of Zn added as zinc sulfate monohydrate (ZnSO4·H2O) or basic Zn sulfate (ZnSO4·4Zn(OH)2) to mixed N–P–K fertilizers decreased with increasing pH of the fertilizer. More than 80% of the total Zn was water soluble in the nonammoniated fertilizers with a pH range from 3.6 to 4.2. Ammoniation of the fertilizer mixture to a pH 4.6–6.1 significantly decreased water-soluble Zn in the sources to less than 20%. Similar results were found by Mortvedt and Giordano (1969) for orthophosphate fertilizers. Other researchers (Richards, 1969) also reported a negative relationship between % watersoluble Zn and the pH of the fertilizer (Fig. 4).

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237

Figure 4 The relationship between fertilizer pH and level of water-soluble Zn in granular mixed fertilizers for different methods of addition. From Richards (1969) reprinted with permission from Soil Science Society of America.

Not only the carrier, but also the method of incorporation of Zn into the fertilizer may affect the solubility of Zn. Richards (1969) investigated the effect of method of incorporation of various Zn sources into a mixed N–P–K fertilizer on the level of water-soluble Zn. The Zn sources [Zn-chelates (Na2ZnEDTA, NaZnHEEDTA, ZnNTA), soluble Zn (ZnSO4·H2O, ZnSO4·(NH4)2SO4·6H2O, ZnCl2) and slightly soluble Zn (ZnO, ZnCO3)] were incorporated into the fertilizer prior to ammoniation or after ammoniation or were coated onto the granules after ammoniation. The level of watersoluble Zn from the soluble sources was significantly reduced (<10%) when the sources were incorporated before ammoniation, but when applied after ammoniation or coated, the water-soluble Zn increased up to 30 and 47%, respectively. The level of water-soluble Zn of the slightly soluble sources was higher when applied as coating but still less than 10% became water soluble. When incorporated into the fertilizer, the soluble and insoluble sources had similar water solubility because the extra alkalinity added through ZnO or ZnCO3 was offset by less ammoniation, resulting in a similar fertilizer pH. However, when they were coated postgranulation, the extra alkalinity of ZnO and ZnCO3 resulted in a higher fertilizer pH and hence lower water solubility of Zn in the fertilizer than when using soluble sources. The solubility of Zn chelates was in general not affected by the method of application as they remained largely water soluble (>70% water-soluble Zn). However, this is contrary to the work of Ellis et al. (1965) who reported that the level of watersoluble Zn of ZnEDTA was 10% when it was incorporated to ammoniated superphosphate fertilizer, but 100% when coated. The authors indicated that

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the acid reaction of superphosphate before ammoniation may have decomposed the chelate during the manufacturing process. Zinc can also react with the N component of fertilizers (Table 5), but in contrast with P, the products of the Zn–N reactions are generally watersoluble compounds (Lehr, 1972). In a recent study, Milani et al. (2012) evaluated the dissolution behavior and speciation of ZnO coated onto urea granules. Results from spectroscopic analysis showed that coating did not alter Zn speciation as ZnO was the dominant mineral on the surface of the granule, in contrast to the transformation of ZnO when coated onto MAP. Results from a column dissolution technique showed that the cumulative Zn release from the ZnO-coated urea granules was comparable to that of Zn release from ZnO alone. This suggested that the initial high pH (7.5) due to hydrolysis of urea was not inhibitory for the dissolution of ZnO. From solubility diagrams it was predicted that hydrozincite was most likely the mineral phase controlling the solubility of ZnO from the Zn-coated urea granules. 5.2.2 Chemical Reactions of Zn-Enriched Fertilizers Applied to the Soil In the previous section, we discussed the chemical reactions that occur between Zn and the macronutrient carriers and its effect on speciation and diffusion of Zn. However, as shown by Degryse et al. (2015) the fate of Zn fertilizers is also affected by the chemical properties of the soil. A visualization technique was used to evaluate the effect of P carrier on the diffusion of Zn from ZnSO4-coated MAP and ZnSO4-coated DAP granules applied to three soils of contrasting pH (soil pH in CaCl2 4.3, 6.1, and 7.7) (Degryse et al., 2015). In the most acidic soil, the extent of Zn diffusion after 28 days of incubation was much less for Zn-DAP granules than for Zn-MAP (Fig. 5), while the extent of diffusion was similar for Zn-MAP granules and ZnSO4 applied alone (control treatment). For the soil with pH 6.1, the diffusion of Zn from the fertilizers followed the order ZnSO4 > ZnMAP > Zn-DAP. For the soil with pH 7.7, Zn diffusion could only be visualized in the control treatment. The difference in diffusion of Zn from MAP and DAP granules in the noncalcareous soils can be related to the higher pH induced by DAP around the granule during dissolution which may have enhanced adsorption and promoted precipitation of Zn phosphates, likely hopeite [Zn3(PO4)2.4H2O]. Using X-ray diffraction analysis, Kalbasi et al. (1978) were able to identify crystalline Zn precipitates in the soil adjacent to fertilizer bands of calcareous

239

Agronomic Effectiveness of Zinc Sources as Micronutrient Fertilizer

ZnSO4

MAP+Zn

DAP+Zn

(A)

(B)

(C)

5.5 cm

Figure 5 Visualization of Zn diffusion at 28 days after addition of Zn fertilizer in soils with contrasting pH [soil pH in CaCl2 (A) 4.3, (B) 6.1, and (C) 7.7]. Zinc was added to the soil at a rate of 0.35 mg Zn, as a concentrated ZnSO4 solution, ZnSO4-coated MAP, and ZnSO4coated DAP. The line delineates the high-Zn zone as determined by image processing. From Degryse et al. (2015) with permission from Springer.

and noncalcareous soils that received ZnSO4 and NH4H2PO4. In the noncalcareous soil, hopeite was the reaction product detected, whereas in the calcareous soil ZnNH4PO4, ZnCO3 and hopeite were identified. Hettiarachchi et al. (2008) used a combination of bulk and focus synchrotron-based techniques and scanning electron microscopy to investigate the distribution and speciation of Zn in the soil-fertilizer reaction zones around granular fertilizers. Commercially available Zn-incorporated MAP granules were placed in the center of a Petri dish containing a calcareous soil (pH 8.5) and incubated for 5 weeks. Micro-X-ray fluorescence maps of the incubated granules showed that most of the fertilizer Zn was retained in the granule and its distribution was heterogeneous (Fig. 6). X-ray absorption fine structure spectra (EXAFS) of the incubated granule showed that the Zn

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Granule

P38

P37

P36 P39

Max

Mn Kα

5000 μm

Zn Kα

Min

Fe Kα

Ca Kα

Figure 6 Micro-X-ray fluorescence maps of Mn, Ca, Fe, and K or Zn for soil incubated with granular Zn. Area of single map is 8000/5000 μm for granular Zn added to the soil. Brighter colors (white-yellow, web version) for high fluorescence signal and darker colors (blue-black, web version) for low fluorescence signal. The markers noted as P36 to P39 represent locations for which μ-X-ray absorption near-edge structure (XANES) analyses were conducted. From Hettiarachchi et al. (2008) reprinted with permission from Soil Science Society of America.

species present in the residual granule were scholzite [CaZn2(PO4)2·2H2O)], willemite [Zn2(SiO4)], and zincite (ZnO). In the soil section adjacent to the fertilizer granule (0–4 mm) the Zn species detected were willemite, hopeite, and ferrihydrite-adsorbed Zn. When Zn was delivered with a fluid MAP formulation instead, willemite was the dominant Zn species in the 0–4 mm soil section, but as the distance from the application point increased ferrihydrite-adsorbed Zn and hopeite were the main Zn species in the soil.

6. EXTRACTION METHODS TO ESTIMATE Zn FERTILIZER EFFECTIVENESS The effectiveness of fertilizers is best assessed in plant yield response or in plant Zn uptake experiments. Extraction methods are a quick and cheap way to predict the agronomic performance of Zn fertilizers.

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241

Various chemical extraction methods have been tested to estimate the plant available Zn in the fertilizer. Mortvedt and Giordano (1969) reported significant positive correlations between the agronomic effectiveness of Zn cogranulated with macronutrient fertilizer and the concentration of Zn extractable in water, 0.01 and 0.001 N HCl, acidic K2SO4 solution or dilute solutions containing chelating agents. In their study, the extraction was undertaken using a fertilizer to solution ratio of 1:10 which was shaken for 1 h and filtered. All of the extractants gave strong relationships with plant response to fertilizer, with the water extraction performing the best. No significant correlation was obtained with Zn extractable in 6 N HCl (pseudo-total). Most other studies have used aqueous extraction to correlate solubility of fertilizer Zn to agronomic effectiveness, using various procedures. For example, Ghosh (1990) and Richards (1969) used a water to solid ratio of 1:100 to determine water extractable Zn, whereas Tanner and Grant (1973) determined effectiveness by leaching the fertilizer (25 g) with 1 L of water over a period of 3–4 h. Other researchers (Amrani et al., 1999; Liscano et al., 2000; Slaton et al., 2005) have used the 965.09 method described by the Official Methods of Analysis of AOAC International (1995) to determine the water-soluble Zn of various Zn sources (1:75 solid to liquid ratio). Based on the work by Vale and Alcarde (1999), the Brazilian fertilizer legislation now regulates that Zn fertilizers for soil application should contain a minimum of 60% soluble Zn determined by the 2% citric acid extraction method. However, recently the work by de Souza et al. (2013) has questioned the scientific validity of this directive. They indicated that the by-products galvanizing ash and brass ash would not be classified as Zn fertilizers according to the official method because the measured solubility was less than 60%, but that Zn uptake by plants that received these sources (as powder and mixed through the soil) was not significantly different from plants fertilized with ZnSO4. Furthermore, the uptake efficiency relative to ZnSO4 was higher with these sources than with a brass slag which contained 77% soluble Zn by 2% citric acid (Brass Slag II). Mineralogical composition of galvanizing ash and brass ash by X-ray diffraction analysis showed that Zn was predominantly present as ZnO and simonkolleite (Zn5(OH)8Cl2·H2O); whereas in Brass Slag II it was present as willemite. These results stress the importance of determining the chemical composition of by-products when used as fertilizers and the usefulness of extraction methods to predict the agronomic effectiveness of Zn sources. Apart from the chemical composition, the particle size of the Zn source is also a determinant factor for the agronomic effectiveness.

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While good correlations between water-soluble Zn and agronomic effectiveness have generally been found for granular Zn sources, as noted in the Brazilian work this relationship may not hold for powdered sources incorporated and mixed well through soil. For instance, ZnO has a low water solubility in a laboratory extraction test, but when mixed through soil as a powder, it is usually equally effective to ZnSO4 (see Section 7). Similarly, Mortvedt and Giordano (1967a) found that when liquid fertilizers with ZnO suspension were mixed through soil, they were all equally and highly effective, even though water solubility of Zn was very low for some of these fertilizers.

7. AGRONOMIC EFFECTIVENESS OF Zn FERTILIZERS The agronomic effectiveness of a micronutrient source is defined as the degree of crop response per unit of applied micronutrient (Mortvedt, 1991). From the perspective of a fertilizer technologist, an effective fertilizer is the one that gives the maximum plant response at the lowest application cost. The agronomic effectiveness of Zn fertilizers has been mainly related to the water solubility of the Zn source (Mortvedt, 1992; Amrani et al., 1999; Gangloff et al., 2002), though other management factors such as placement and source type can affect the efficiency of uptake of Zn from soil-applied fertilizers (Table 6). Whether increased Zn supply through fertilization translates into a yield increase or an increase in Zn concentrations in the plant depends on the Zn status of the soil. Under strongly deficient conditions, the effect will mostly be on yield, while under nondeficient conditions, the effect of fertilization will only be seen in the Zn tissue concentrations (Rengel et al., 1999). This latter effect is particularly important in situations where crops need to be enriched in Zn for human or animal nutrition—biofortification.

7.1 Biofortification Biofortification is the process that aims to increase the concentration of nutrients in edible portions of crop plants either through fertilization (agronomic biofortification) or plant breeding (genetic biofortification) (White and Broadley, 2005). Foliar Zn application alone can be a very efficient way of biofortification. However, there are large differences between species

Table 6 Comparative performance of different Zn sources in greenhouse and field studies. Parameters Application Zn sources Findings studied Crop method

ZnEDTAmixed > Znlignosulfonatemixed = ZnSO4mixed > Znlignosulfonategranules > ZnSO4granules ZnDTPA > Zn fulvate ZnDTPA, Zn fulvate, > ZnEDTA > Zn ZnEDTA, Zn citrate, citrate > ZnSO4 ZnSO4 ZnSO4 = ZnEDTA = Powder and granular Zn-polyflavonoid ZnSO4, ZnEDTA, Zn-polyflavonoid and (mixed in the soil) ZnNH4PO4 ZnEDTA > Znpolyflavonoid > ZnSO4 (spot placed)

ZnSO4, Znlignosulfonate (powder, granules), ZnEDTA (liquid)

ZnEDTA ZnSO4, Zn- ZnEDTA > ZnSO4 = oxysulfate (26 and 55% Zn-lignosulfonate > water-soluble Zn), Zn Zn-oxysulfate 55% > sucrate, Zn Zn-oxysulfate 26% = lignosulfonate (all Zn sucrate sources granules except ZnEDTA)

Dry matter, Zn uptake

Maize

Dry matter, Zn uptake

Corn

Dry matter

Corn

Zn uptake

Corn

Type of study

Soil pH

Mixed Greenhouse 8.1 (powder and liquid) spot placed (granules) Greenhouse 8.4

Mixed and spot Greenhouse 8.1 placed All powder materials were applied as a solution, except ZnNH4PO4 Granules spot Greenhouse Limed to placed pH 7.2 2.5 cm below the seed

References

Goos et al. (2000)

Prasad and Sinha (1981) Brown and Krantz (1966)

Gangloff et al. (2002)

(Continued )

Table 6 Comparative performance of different Zn sources in greenhouse and field studies.—cont'd. Parameters Application Zn sources Findings studied Crop method

ZnO, ZnSO4, ZnEDTA (all sources finely ground) ZnSO4, ZnEDTA, metallic Zn, fritted Zn

Zn uptake: ZnO > ZnSO4 > ZnEDTA All sources equally effective for dry matter production ZnSO4 > ZnEDTA > metallic Zn > fritted Zn

ZnSO4, ZnO, Zn frits

ZnSO4 > ZnO = Zn frits

ZnSO4 and ZnO-coated urea ZnSO4, Znlignosulfonate, Zn-oxysulfate (41 and 14% water-soluble Zn)

ZnSO4-coated urea > ZnO-coated urea ZnSO4 = Znlignosulfonate > Znoxysulfate (41%) > Zn-oxysulfate (14%) No source effect for grain yield

Dry matter, Zn uptake

Zn uptake, dry matter yield

Flooded Mixed rice

Flooded Mixed rice (metallic Zn as powder, other sources not specified) Grain yield, Zn Flooded Broadcast and uptake rice disked (the state of the source not specified) Grain yield, Zn Flooded Band applied uptake rice Zn concentration, Flooded Broadcast and grain yield rice incorporated (all sources granules)

Type of study

Soil pH

References

Greenhouse Limed to pH 7.5

Giordano and Mortvedt (1973)

Greenhouse 6.7

Kang and Okoro (1976)

Field

10.4

Nayyar and Takkar (1980)

Field

8.2

Field

7.4

Shivay et al. (2008) Slaton et al. (2005)

ZnSO4, ZnO, ZnEDTA, Zn–NH3 complex (all sources mixed in ortho and polyphosphate carriers) ZnEDTA, Zn– NH3 complex, ZnO, ZnSO4, Zn(NO3)2 (all sources mixed in polyphosphate carrier)

No significant differences among sources

Grain yield

Corn

Band applied

Field

7.2, 7.8, 6.3

Rehm et al. (1980)

No significant differences (at 1 and 5% significant level)

Grain yield

Corn

Band applied

Field

6.4, 6.3, 7.6, 5.4

Hergert et al. (1984)

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in terms of effectiveness of foliar-applied Zn. Increasing the concentration of Zn in seeds is a desirable quality factor. From the agronomic perspective, sowing seeds of high Zn content can enhance crop growth and yield through better germination, seedling vigor, and stress tolerance particularly in Zn-deficient soils (Rengel and Graham, 1995; Yilmaz et al., 1998). Additionally, biofortification of grains with Zn can bring beneficial effects for human health by helping to overcome malnutrition in populations with cereal-based diets (Cakmak, 2008a). Rengel and Graham (1995) evaluated the effects of seed Zn content on early wheat seedling growth in a glasshouse experiment. They reported that for Zn efficient and deficient genotypes, increasing the seed Zn content from 0.3 to 0.7 μg seed
1 resulted in greater shoot and root growth. Furthermore, the plants derived from seeds with high Zn content required less Zn applied to the soil to achieve 90% optimal yield compared with plants with low Zn content in the seed. The authors concluded that a high content of Zn in the seed acted similarly to a starter fertilizer by improving early vegetative growth and dissipating differences between genotypes of different Zn efficiency. Similar results were observed in field experiments under rainfed conditions. Yilmaz et al. (1998) found a significantly higher grain yield for wheat plants that emerged from seeds with high Zn content (1.5 μg seed
1) than for plants derived from low Zn seed (0.4 μg seed
1). Nevertheless, the high content of Zn in seed could not overcome the effects of Zn deficiency without Zn application to the soil. Results from field studies with wheat and rice conducted in various countries under different soil and environmental conditions have demonstrated that foliar Zn application or a combination of soil and foliar Zn are highly effective ways to increase grain Zn content (Phattarakul et al., 2012; Zou et al., 2012). In wheat grown in a Zn-deficient soil, (Cakmak et al., 2010) found that a combined application of soil and foliar Zn increased the Zn concentration in grain up to 2.4-fold. The increase in Zn concentration in the grain and the endosperm fraction was most pronounced with foliar sprays at late growth stages (milk and dough). While foliar fertilization is often very effective in increasing grain Zn concentrations of wheat, it is usually found to be less effective for corn (Cakmak, 2012). In the case of rice, foliar application of Zn before flowering had no effect on the concentration of Zn in the grain, but foliar spraying 1 and 2 weeks after flowering (two applications) increased the concentration of Zn in the unhusked seed from about 17 mg kg
1 (no foliar Zn) to 60 mg kg
1 (Boonchuay et al., 2013).

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These results suggest that timing of foliar Zn is an important issue that should be considered in biofortification of wheat and rice grain with Zn.

7.2 Fertilizer Zn Effectiveness in Soil 7.2.1 Placement Effect Zinc applied to the soil is commonly broadcasted and incorporated or banded to the side and below the seed. For selection of placement method it should be considered whether Zn availability is controlled by precipitation or adsorption reactions. Broadcast application of Zn fertilizers is considered an effective placement method as it allows better distribution of the fertilizer in the soil and hence increases possibilities for fertilizer-root interception (Murphy and Walsh, 1972). In calcareous soils where Zn availability is controlled by Zn-P precipitation, broadcasting soluble sources may be beneficial. The superiority of broadcast application over banding has been shown in field (Pumphrey et al., 1963; Carsky and Reid, 1990) and in greenhouse experiments (Brown and Krantz, 1966; Soper et al., 1989). For example, in a greenhouse study the uptake of Zn by corn from ZnSO4 was 0.34 mg pot
1 (rate 6.4 mg Zn pot
1) when the fertilizer was mixed thoroughly in a calcareous alkaline soil (pH 8.1), whereas when the fertilizer was spot placed under the seed, the uptake of Zn was only 0.07 mg pot
1 (Brown and Krantz, 1966). Similar conclusions were drawn by Soper et al. (1989) who reported that increasing the volume of soil treated with ZnSO4 increased the concentration of Zn in beans grown in a calcareous soil. It is generally recommended that sources of slow solubility (eg, ZnO) should be broadcast and incorporated, as this mixing is necessary to enhance dissolution and plant nutrient uptake (Boawn, 1973; Martens and Westermann, 1991). The effect of placement on plant response can be influenced by the source of Zn applied. Moraghan (1996) showed that dry matter yield and Zn uptake by navy bean (grown to maturity) were the same when ZnSO4, ZnEDTA, and Zn-lignosulfonate were mixed in a calcareous soil. However, when the sources were banded only the plants that received ZnEDTA performed well while Zn uptake was significantly lower than in the mixed treatments for ZnSO4 and Zn-lignosulfonate. Banding did not reduce the effectiveness of ZnEDTA probably due to its higher mobility in the soil (Brown and Krantz, 1966). Similar findings were reported by Kang and Okoro (1976) for flooded rice, who found that fritted Zn was much less

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effective when banded than when mixed, while there was no effect of placement method for ZnEDTA. 7.2.2 Source Effect 7.2.2.1 Physical Form

Few studies have directly compared the effectiveness of Zn fertilizers applied in different physical states (fluid vs granular). An early pot experiment by Mortvedt and Giordano (1967a) compared the effectiveness of Zn as ZnO applied in fluid or granular carriers and mixed or banded in a noncalcareous soil (pH 7.3). The results showed that the application of fluid Zn significantly increased corn dry matter yield and Zn uptake compared to Zn applied in granular form. No differences in effectiveness were observed among the fluid sources when they were mixed in the soil and were similar to powder ZnO thoroughly mixed with the soil. However, when the fluid sources were banded their effectiveness was not consistently superior to the granular sources. Mortvedt and Giordano (1967a) argued that the superiority of the fluid fertilizers over the granular was due to a better distribution of Zn in the soil with the fluid sources which promoted solubilization, while banding resulted in a reduced Zn solubility. Conversely, for the granular sources, the effectiveness was similar whether banded or mixed through the soil. Field experiments in Australian calcareous soils have shown an increase in wheat grain yield of 11 and 17% with fluid Zn compared with granular products (Holloway et al., 2006). The enhanced distribution of the fertilizer cannot solely account for the better performance of fluid Zn because the application volume of fluid fertilizers is normally very low 30–150 L ha
1 (Holloway et al., 2008a). The greater crop response to fluids is explained by the different chemical behavior of fluid and granular Zn fertilizers. Hettiarachchi et al. (2010) used a 65Zn isotopic dilution technique to determine the potential available Zn (labile Zn) from either granular or fluid Zn fertilizers applied to calcareous and noncalcareous soils at different distances from the point of application. More than 90% of fertilizer Zn was recovered in the soil closest to the granule (0–7.5 mm) in both soils irrespective of the Zn form. This result revealed the limited mobility of fertilizer Zn and agrees with results from other studies. Interestingly when Zn was applied in fluid form, more fertilizer Zn remained in a labile form (Hettiarachchi et al., 2010). Hence the superior agronomic effectiveness of fluid Zn in field experiments in Australian calcareous soils reported by Holloway et al. (2006) may be due to less

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sorption of applied Zn and to the formation of different reaction products when Zn is applied in fluid or granular form, as discussed earlier. 7.2.2.2 Water Solubility

Almost all of the published studies that evaluate the plant response to varying levels of water-soluble Zn, agree that at least 30–50% of total Zn in a granular fertilizer should be water soluble to effectively supply Zn to the immediate crop (Mortvedt, 1992; Amrani et al., 1999; Liscano et al., 2000; Gangloff et al., 2002). In a greenhouse study on corn, Amrani et al. (1999) evaluated the effectiveness of commercial granular Zn fertilizers (ZnSO4·H2O and Znoxysulfate) applied to loamy sand Zn-deficient soils limed to pH 6.3 and 7.4. The Zn fertilizers were produced by acidifying Zn-rich feedstocks with H2SO4. The water-soluble Zn of the fertilizers ranged from 0.7% to 100% and was related to the degree of acid added during the manufacturing process. The highest dry matter yields and measured uptake of Zn by plants were from the fertilizers that contained between 66% and 100% watersoluble Zn. Similar results were obtained by Mortvedt (1992) who found that corn dry matter production was seriously compromised with granular fertilizers containing less than 40% water-soluble Zn. However, when the fertilizers were applied as a powder and mixed into the soil no relationship was observed between plant growth and the level of water-soluble Zn in the fertilizer. The water solubility of the fertilizer appears to be of importance mainly in the short-term and hence is critical for the crop grown immediately after fertilizer application. Slaton et al. (2005) showed that for the first rice crop, water solubility of the fertilizers influenced early tissue Zn concentration, but no differences among Zn sources were observed in grain yield. Furthermore, in the second rice crop Zn concentration and grain yield were also not affected by Zn source. In greenhouse and field experiments, when applied at similar rates, often high dry matter and Zn uptake have been measured in plants fertilized with Zn chelates than with inorganic Zn fertilizers (Boawn, 1973; Prasad and Sinha, 1981; Goos et al., 2000; Gangloff et al., 2002; Naik and Das, 2008). In contrast, other studies have shown that Zn chelates are equally or less effective than the inorganic sources (Giordano, 1977; Rehm et al., 1980; Hergert et al., 1984). The better performance of Zn chelate enhancing plant growth has been attributed to less sorption of Zn by the soil. Obrador et al. (2003) reported that when Zn chelate (Zn-DPTA-HEDTA-EDTA) was applied to a calcareous soil and incubated for 60 days the concentration of Zn in the

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most available forms (water soluble plus exchangeable Zn) was larger than with a Zn-amino acid fertilizer. Furthermore, Zn-DPTA-HEDTA-EDTA was more mobile as 49% of added Zn was collected in leachates from the soil amended with this source. The effectiveness of Zn chelates can be reduced by leaching of the source under high rainfall conditions or by degradation of the chelate in the soil. The fact that some studies found Zn chelates to be superior while others did not may also be related to differences in placement or in soil properties. Mortvedt and Giordano (1969) conducted a greenhouse experiment with corn to evaluate the agronomic effectiveness of ZnSO4 and ZnO granulated with various macronutrient fertilizers (Table 7). Relative to ZnSO4 and ZnO mixed alone to the soil, the Zn sources granulated with concentrated superphosphate, triammonium polyphosphate, and ammonium polyphosphate were the most effective fertilizers. The good performance is explained by the higher content of soluble Zn. In contrast other ammoniated phosphate sources were not effective carries for Zn due to the low water-soluble Zn (0.1–7%). The lower water solubility can be explained by the higher pH values of these fertilizers (5.0–7.9). At these pH values, Zn

Table 7 Fertilizer solution pH, water-soluble Zn, and agronomic effectiveness of Zn when ZnSO4 and ZnO were granulated with various macronutrient fertilizers. WaterAgronomic soluble Zn effectiveness (%) (% of total) Solution pH Macronutrient

Phosphate fertilizers Superphosphate NH4H2PO4 Ammonium phosphate nitrate Nitric phosphate Ammoniated superphosphate Urea ammonium phosphate (NH4)2HPO4 Polyphosphate fertilizers Ammonium polyphosphate Triammonium pyrophosphate Nitrogen fertilizers NH4NO3 Urea

ZnSO4

ZnO

ZnSO4

ZnO

ZnSO4

ZnO

2.8 3.6 5.0 5.2 5.4 7.3 7.5

2.8 4.1 5.6 6.7 5.9 7.9 8.2

90 70 7 0.1 1 2 9

100 6 2 0.1 1 1 0.1

72 63 9 16 12 9 25

54 28 4 4 10 11 32

5.5 6.1

5.7 6.4

90 100

96 100

49 93

55 60

4.8 6.3

6.9 7.6

100 80

6 2

58 64

4 15

Source: Adapted from Mortvedt and Giordano (1969) with permission from American Chemical Society.

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may have precipitated as ZnNH4PO4 or Zn3(PO4)2 in the granule hence decreasing its bioavailability. An inverse relationship was found between the pH of the fertilizer solutions and the agronomic effectiveness of Zn in these fertilizers. The effectiveness of the fertilizer decreased drastically when the pH of the fertilizer solution was near 4. Exceptions were the polyphosphate and pyrophosphate sources that were good carriers even though their solution pH ranged from 5.5 to 6.4. A study by Tanner and Grant (1973) showed that yield and Zn content of maize were similar to granular ZnO and ZnSO4 mixed through an acidic sand (pH 5.6). In this study, banding ZnO did not significantly decrease Zn uptake compared with the mixed application. Brennan and Bolland (2006) compared the response of wheat to powdered ZnSO4 and ZnO mixed– applied to an acid sand (pH 5.5) or an alkaline sandy clay (pH 7.5). In agreement with findings by previous researchers, both Zn sources were equally effective in the acid soil. In contrast, ZnO was only half as effective as ZnSO4 in the alkaline soil (Brennan and Bolland, 2006). These results would suggest that for alkaline soils (pH > 7.4) highly water-soluble sources may be more effective Zn fertilizers even when the fertilizer is powdered and mixed through soil. However, given that most studies have indicated that ZnO solubilizes relatively fast when mixed through soil as a powder, it is unlikely that this difference would persist for more than one crop. Recently, McBeath and McLaughlin (2014) investigated in a laboratory and greenhouse experiments Zn solubility and plant availability of seven ZnO fertilizers. When the fertilizers were uniformly mixed through the soil, little difference in Zn availability to plants was observed compared to ZnSO4, but when the fertilizer was banded, ZnO was much less effective than ZnSO4. The authors indicated that the homogeneous distribution of ZnO powder mixed through soil allowed for soil minerals and OM to provide pH buffering as well as a sink for Zn2+ ions necessary to drive the dissolution of ZnO until completion. In contrast, when the fertilizer is banded, the reduced contact with soil results in much slower dissolution of ZnO. 7.2.2.3 Total Zn Content

The concentration of Zn in the granules may also affect the effectiveness of the fertilizer. A lower concentration of Zn in the granule results in more granules being applied for a given Zn rate. This increases the opportunities for root interception (more homogeneous distribution) and is also expected to increase the rate of solubilization in the case of sparingly soluble Zn (eg,

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ZnO or Zn phosphates). Giordano and Mortvedt (1966) compared concentrated superphosphate (CSP) granules containing 0.5, 2, and 8% Zn as ZnO in a greenhouse experiment. At the same rate, Zn uptake by corn and dry matter yield were inversely related to the concentration of Zn in the carrier. Mortvedt (1968) found no difference between ammoniated concentrated superphosphate (ACSP) with two different Zn rates (0.45 or 1.8% Zn) for fine granules, but crop response increased with a decrease in Zn concentration for the medium and coarse granules. 7.2.2.4 Granule Size

In sparingly soluble Zn fertilizers, the effect of granule size on agronomic effectiveness can be explained in this way: larger granule size results in less granules for a given amount of soil and lower surface area of contact between soil and fertilizer, resulting in lower dissolution rate. The effect of granule size has been overlooked by researchers as in most studies the granule size is not even mentioned, but has been demonstrated in a few studies (Mortvedt, 1968, 1992; Liscano et al., 2000). For instance, Mortvedt (1968) evaluated plant response to ACSP cogranulated with ZnO or ZnSO4 and screened to provide coarse, medium, and fine granules (<2.38, <1.68, and < 0.5 mm). Dry matter yield and Zn uptake decreased with increasing the size of the granule although there was little difference between the medium and coarse granules. For the second crop, maximum yields were much lower and the size effect of the granules was not as evident. Liscano et al. (2000) evaluated the effect of particle size (powder, 1.5, 2, 2.5 mm) on the availability of Zn for rice from Zn oxysulfate fertilizer with a low water solubility (4% water soluble Zn). Dry matter yield and plant uptake of Zn increased as the granule size decreased. The granules larger than 1.5 mm had no effect on yield compared to the control treatment (no Zn applied) (Fig. 7). Mortvedt (1992) assessed the response of corn to powdered and granular Zn fertilizers (sulfate, oxide, or oxysulfates) and found no difference between the various sources when finely ground, while the response increased with the level of water-soluble Zn when the fertilizer was in granular form. 7.2.3 Application Rate Fertilization rates of 2.5–34 kg Zn ha
1 as ZnSO4 or 0.3–6.0 kg Zn ha
1 as Zn-chelate are reported in the literature for application of broadcast or banded Zn to correct deficiencies in soils (Martens and Westermann, 1991; Takkar and Walker, 1993). The large variation in the range of the fertilizer rate is related to the application method, soil type and deficiency

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Figure 7 Effect of granule size of a Zn oxysulfate fertilizer with low water solubility on (A) dry matter yield (DMY) and (B) Zn uptake by flooded rice. Different letters denote significant difference among granule size at P ≤ 0.05 level. Data from Liscano et al. (2000) replotted with permission.

level and the sensitivity of the crop (Takkar and Walker, 1993). In addition, the initial fertilizer rate can also affect the decline in Zn availability. 7.2.4 Residual Effectiveness The application of Zn fertilizers to the soil has been shown to have residual value for subsequent crops. The residual value of fertilizer Zn depends on the fertilizer rate, the type of soil, and the crop system (Takkar and Walker, 1993). Longer residual effects have been reported in soils that received relatively large application rates of broadcast Zn. In a 5-year field study, Boawn (1974) reported that a single application of ZnSO4 (11.2 kg Zn ha
1) in a noncalcareous alkaline soil was sufficient to sustain adequate levels of soil Zn for corn for at least 4 years (DTPA extractable-Zn > 0.5 mg kg
1). The corn that was grown annually in this soil did not show a decrease over time in the concentration of Zn in the leaves and in Zn taken up by the plants. However, when the same rate was applied to a calcareous soil, the DTPA-extractable Zn in the fourth year after application was at the lower limit for deficiency (0.5 mg kg
1) and hence Zn utilization by the plant was lower compared to the previous years. Although large single Zn fertilizer rates have been recommended historically, questions have been raised regarding the efficient use of the fertilizer by this practice. It is accepted that the longer the time Zn is in contact with the soil, the effectiveness of added Zn decreases due to slow sorption reactions (aging). Ma and Uren (2006) found that the relative loss of bioavailable Zn

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after 1 and 2 years of incubation was higher when a large Zn fertilization rate was used. This suggests that regular small additions of Zn may be more efficient than occasional high dose applications. For example, Singh and Abrol (1985) suggested that a yearly small application was more effective than a single large application in a rice–wheat rotation system. They compared single applications (2.25–27 kg Zn ha
1) to continuous applications before each crop over a period of 4 years. They found that the application of 18 kg Zn ha
1 once to the first crop provided sufficient residual Zn to increase the grain yield of the third rice crop and fourth wheat crop to the maximal level. However, the Zn uptake in the crop was higher with a continuous application of 2.25 kg Zn ha
1 than with an equivalent amount of Zn added in a single application at the start of the experiment. Though, it should be noted that a comparison in the last year of the study puts the single application at the start of the study in an unfavorable light. Over the long term, continuous application compared to more irregular application (eg, once every 5 years) may not make a difference if the average availability over all crops is considered. To our knowledge, no such long-term studies have been carried out. Since the amounts of Zn removed by cropping systems are typically low compared to the amounts of Zn applied to the soil, knowledge on the residual value of fertilizer Zn is necessary to determine the rate and frequency of fertilizer application. For example, Bender et al. (2013) showed that in high-yield hybrid corn (12 t ha
1) the amount of Zn removed in the grain was 300 g ha
1 while Zn applied was 4.2 kg Zn ha
1. This positive mass balance leads to accumulation of Zn in the soil which will have varying residual availability depending on soil pH.

7.3 Foliar Zn Foliar sprays with Zn have been used as an agronomic practice to supplement soil Zn fertilizer applications during plant growth stages of high Zn demand, particularly when soil and climatic conditions may limit the availability of soil-applied Zn (Ferna´ndez and Brown, 2013). Foliar Zn applications are considered more effective than soil Zn applications to alleviate Zn deficiency symptoms when they appear during the crop cycle. However, the disadvantage of foliar Zn is that multiple applications may be required to correct Zn deficiency due to the limited mobility of foliar-absorbed Zn to nonsprayed new leaves and roots (Swietlik, 2002). Also a yield penalty may be incurred by the time the symptoms of Zn deficiency are diagnosed and a foliar spray

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program initiated. Brennan (1991) showed that in a Zn-deficient soil banding ZnSO4 at sowing resulted in higher wheat grain yield compared to a foliar Zn spray. The limited retranslocation of foliar-applied Zn to other organs of the plant has been related to poor penetration of the nutrient into the leaf or to the high binding capacity of Zn to leaf tissues, rather than limited phloem mobility (Ferna´ndez et al., 2013). Using the radioactive isotope 65Zn, Haslett et al. (2001) demonstrated that Zn applied to wheat leaves translocated above and below the treated leaf and also to the root tips. These results are in agreement with the study by Erenoglu et al. (2002) where up to 40% of foliar-absorbed 65Zn translocated out of the treated area to the roots and shoots of wheat plants. The nutritional status of the plants did not affect the amount of foliar Zn taken up by the wheat, but it determined the proportion of Zn that translocated to other parts of the plant. In contrast, experiments with 65Zn applied to leaves of avocado seedlings showed that less than 1% of Zn applied as ZnSO4 or Zn metalosate (6.8% Zn) was taken up by the leaf tissue and there was little translocation of Zn into the parenchyma tissue adjacent to the application spots or into the leaves above or below the treated leaves (Crowley et al., 1996). In a recent study, Du et al. (2015) used synchrotron-based X-ray fluorescence microscopy for in situ examination of the absorption and relocation of foliar-applied Zn to either the adaxial or abaxial surface of leaves from Znsufficient and Zn-deficient tomato and citrus plants. Results from this study showed that the concentration of Zn in the underlying tissue increased by up to 600-fold in tomato but only up to 5-fold in citrus. In tomato leaves the concentration of Zn in the underlying tissues was also 2 times higher when Zn was applied to the abaxial leaf surface (with substantial higher density of stomata and trichomes) than to the adaxial surface. In general Zn redistribution in the leaf was very limited with concentrations of Zn decreasing to background levels within 10 mm, but in Zn-deficient plants the mobility of Zn was restricted to an even greater extent as the concentration of Zn reached background levels within 2-mm distance. The major factors that affect the absorption and translocation of foliarapplied nutrients are the timing of application, the physicochemical properties of the formulation, the environmental conditions during application, and the particular characteristics of the plants (Ferna´ndez and Brown, 2013). The most common Zn sources used for foliar fertilization are soluble salts and chelates. Surfactants and adjuvants are also included in the formulation to aid on penetration of the nutrient through the cuticle. Crowley et al. (1996)

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studied the effects of three surfactants used in combination with ZnSO4, ZnO, and Zn metalosate. The surfactants included a methylated seed oil adjuvant, a nonionic polyoxyethylenesorbitan detergent, and a nonionic silicone oil-based surfactant. The application of surfactants did not significantly affect the foliar Zn content compared to the treatments where Zn was applied in water without surfactant. The only treatment where a positive effect of a surfactant was observed was the mixture of seed oil adjuvant with ZnO. The authors indicated, the possible reason might be that insoluble ZnO was partially solubilized to form a water-soluble complex with components of the surfactant. In this study ZnSO4 was a better source of Zn when applied to the leaves than ZnO and Zn metalosate. The phytoavailability of Zn from 11 commercially available Zn products of different chemical compositions was evaluated when applied to the foliage of apple trees at postbloom stage (Peryea, 2006). The Zn sprays had no effect on fruit number, bitter pit incidence, or on leaf green color, but in general all products increased the concentration of Zn in the leaves to values higher than the desirable concentration of 15 mg kg
1. Ranking of the products to increase the Zn leaf concentration was in the order: Zn phosphate < Zn oxide = Zn oxysulfate < chelated/organically complexed Zn ≤ Zn nitrate. In wheat, foliar-applied Zn as Zn-EDTA was up to 1.7 times more effective than ZnSO4 when applied at GS 14 growth stage; however, both products were equally effective when applied at the GS 22–24 growth stage (Brennan, 1991).

7.4 Seed Treatment Seed priming is a low-cost technique of soaking seeds in a solution containing Zn (or other micronutrient) for a specified time after which the seeds are redried and sown (Rehman et al., 2012). Due to the small amounts of Zn needed for seed priming and the ease of the process, this is a practical way to increase the content of Zn in seeds to enhance seedling vigor, plant growth, and yields if compared with soil Zn applications. Harris et al. (2007) reported that priming maize seeds in a solution with 1% Zn for 16 h increased the content of Zn from 15 to 560 mg kg
1. Furthermore, average results from seven field studies showed significant increases in grain yield from primed seeds (3.8 t ha
1) relative to the nonprimed seeds (3.0 t ha
1). In rice, seed priming was also a more cost-effective solution to soil-applied Zn as similar grain yields were obtained with the application of 11 kg Zn ha
1 incorporated to the soil and with 2.8 g Zn kg seed
1 (Slaton et al., 2001).

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Seed coating is another way of seed treatment that differently to priming seeds are not soaked in a nutrient solution but just sprayed to form a continuous layer on the seed (Rehman et al., 2012). Tunes et al. (2012) found that coating wheat seeds with a layer of ZnSO4, a fungicide and a polymer did not affect the viability of the seeds stored for 6 months and further increased the number of grains per spikelet and grain weight per plant.

8. NEW TECHNOLOGIES TO IMPROVE Zn FERTILIZER EFFICIENCY 8.1 Nanotechnology in Zn Fertilizers Nanotechnology comprises the use of material with at least one dimension smaller than 100 nm. The small size and hence high surface area to volume ratio of nanoparticles may result in different physicochemical properties compared to their bulk counterparts. It has been suggested that the implementation of nanotechnology in fertilizer research can enhance nutrient uptake and fertilizer efficiency and thus lead to economic and environmental benefits. Nanofertilizers could be designed to release nutrients in a controlled way synchronized with plant demand, or be designed to prevent the immobilization of nutrients in the soil, or could be directly taken up by the plant and thereby improve the nutrient uptake (DeRosa et al., 2010). Zinc oxide nanoparticles are among the most widely used manufactured nanoparticles in industrial, commercial, and medicinal products. In a few recent studies, it was suggested that ZnO nanoparticles may have potential as fertilizers of improved effectiveness for soil and foliage application. Prasad et al. (2012) demonstrated that treatment of peanut seeds with ZnO nanoparticles (25 nm) resulted in greater seed germination, seedling vigor, stem and root growth, pod yield, and chlorophyll content than treatment with chelated bulk ZnSO4. In a field experiment, foliar application of ZnO nanoparticles significantly increase pod yield and shelling percent compared to foliar Zn applied as chelated bulk ZnSO4, even when the nanoparticle treatment was applied at a 15 times lower dose than the ZnSO4. Another study investigated the effects of bulk ZnO and ZnO nanoparticles on germination, growth, and biochemical parameters of cabbage, cauliflower, and tomato (Singh et al., 2013). The authors reported that ZnO nanoparticles increased seed germination, seedling growth, and the content of pigments, sugar, and activities of nitrate reductase enzyme while

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bulk ZnO had phytotoxic effects. The enhanced germination and seedling growth in common chickpea exposed to ZnO nanoparticles (20–30 nm) was related to the high levels of indole acetic acid measured in the sprouts (Pandey et al., 2010). Similarly, Adhikari et al. (2015) reported that plant growth parameters such as plant height, root length and volume, and biomass of maize grown in Hodgson nutrient solution (pH 6.5) were all improved in plants that received Zn as ZnO nanoparticles compared to ZnSO4. Zinc nanoparticles prepared from tire rubber waste by milling process were also a more effective Zn source than ZnSO4 for cucumber plants grown in nutrient solutions that were buffered to pH 6.0 (Moghaddasi et al., 2013). However, it is more likely that at that pH all the ZnO (1 mg L
1) would have been completely dissolved as Zn2+ and any effect that was observed by the rubber waste compared to ZnSO4 may have been due to other components present in the rubber. For a proper interpretation of experiments that use nanoparticles, it is critical to adequately characterize the chemical speciation of the fertilizer Zn in the growth media. In contrast to the previously mentioned studies, other researchers have observed null effects from the exposure of ZnO nanoparticles to plants. For example, in the study by Watts-Williams et al. (2014) no significant differences in plant biomass and shoot Zn concentration were observed in plants fertilized with either bulk ZnO (300 nm) or ZnO nanoparticles (40 nm). As discussed earlier, even bulk ZnO dissolves relatively fast when it is mixed through soil in powdered form and hence no benefit is to be expected from nanosized ZnO mixed through soil compared to bulk ZnO or soluble fertilizers. Similarly, no differences in plant growth, Zn accumulation, and Zn speciation were observed in cowpea plants grown in soils amended with soluble ZnCl2 or ZnO nanoparticles (Wang et al., 2013).

8.2 Other Technologies Given that the most cost-effective way to apply Zn fertilizers is incorporated into granular or fluid macronutrient fertilizers, or coated on granular products, the goal is to maintain a high water solubility of Zn in the final product. For Zn-enriched phosphatic fertilizers this is problematic due to precipitation of Zn in the products as various Zn phosphates. Hence there is need to develop chemical or physical methods to prevent this interaction, which may involve the use of chelates, polymers, or other techniques (Peacock et al., 2011). With chelates, it will be important to select products that have high plant availability (Stacey et al., 2008).

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9. CONCLUSIONS AND FUTURE NEEDS Zinc fertilizers will continue to be used in agriculture to sustain crop yields to meet the demand for food in a growing population. Since Zn deficiency in humans has become a problem of major concern, current fertilizer research programs seek to improve not only yields but also grain Zn concentrations to address both food security and quality. It has been experimentally shown that under Zn-deficient conditions the application of Zn fertilizers to the soil is an effective strategy to increase crop yields, whereas foliar Zn application is highly effective when the goal is Zn biofortification. Timing foliar sprays is a critical factor that determines the effectiveness of foliar-applied fertilizer in increasing grain Zn concentrations. Although various sources of Zn are available in the market, the inorganic compounds ZnSO4 and ZnO are the most commonly used Zn fertilizers and for cost reasons are unlikely to be replaced by other compounds. Zinc sources can be applied alone into the soil, but the incorporation of Zn in macronutrient formulations has become popular as it allows a more uniform distribution of Zn into the soil and eliminates the need of additional field operations. In Zn-enriched fertilizers, the availability of Zn can be affected by the chemical reactions of Zn and the P component of the macronutrient carrier which reduce the water solubility of Zn. The crop recovery of Zn applied in fertilizers to the soil is generally low (<1%). The effectiveness of Zn fertilizers has been related to the water solubility of Zn in the fertilizer. However, studies have shown that the fate of fertilizer Zn depends not only on the fertilizer composition but also on the interaction of Zn with the soil and the fertilizer application method. A better understanding of these interactions may lead to the selection of appropriate fertilizer management practices. While much effort has been put in understanding the reaction of Zn in soils there is a need of long-term trials to fully understand the residual effectiveness of Zn fertilizers. This information is of importance to adjust Zn fertilizer rates and to determine when reapplication of fertilizer Zn is needed. Furthermore, commonly used soil extracts to test for Zn deficiency in soils do not always correctly predict fertilizer response. This is the key research needed for improving fertilizer recommendations. Much effort is needed for the development of more effective and cheap soil-applied fertilizers. For example, new technologies may reduce or prevent the precipitation of Zn during fertilizer manufacture of Zn-enriched

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phosphatic fertilizers and enhance Zn solubility by reducing Zn sorption to the soil. The use of nanotechnology and the role for new polymers and chelates should be further explored as strategies to improve the effectiveness of Zn fertilizers.

ACKNOWLEDGMENTS The authors acknowledge the support of the International Zinc Association and The Mosaic Company.

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