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a Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, 1366 W. Altheimer Drive, Fayetteville, AR 72704
b Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, 115 Plant Science Building, Fayetteville, AR 72701
c Rice Research Extension Center, P.O. Box 351, Stuttgart, AR 72160
* Corresponding author (nslaton{at}uark.edu)
Received for publication May 1, 2004.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: DPRE, delayed preemergence • LIN, Lincoln County • LIN00, Lincoln County in 2000 • POST, postemergence • PPI, preplant incorporated • PTBS, Pine Tree Branch Station • PTBS00, Pine Tree Branch Station in 2000 • RREC, Rice Research Extension Center • RREC01, Rice Research Extension Center in 2001 • RREC02, Rice Research Extension Center in 2002 • ZnEDTA, zinc ethylenediaminetetraacetic acid • ZnOxS, zinc oxysulfate • ZnSO, zinc sulfate
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Zinc deficiency is the most common micronutrient deficiency of rice grown on neutral to alkaline soils in the USA (Norman et al., 2003), as well as other rice-producing regions of the world (Fageria et al., 2003). Slaton et al. (2002b) reported that 79% (>600000 ha) of the soils used for rice production in Arkansas have soil pH > 6.0 and may require Zn fertilization for normal crop growth and yield. In Arkansas, Zn fertilization is recommended for rice grown on silt and sandy loam soils that have soil water pH values 
6.0 and Mehlich-3 extractable Zn concentrations <3.5 mg kg–1 (Slaton, 2001; Slaton et al., 2002b). When recommended, Zn should be (i) applied to the soil before rice emergence as granular Zn at 11 kg Zn ha–1, (ii) applied to seedling rice foliage at 1 to 2.5 kg Zn ha–1 at least 5 to 7 d before flooding at the five-leaf stage, or (iii) applied directly to the rice seed at very low rates (Slaton, 2001; Slaton et al., 2001). While the general Zn fertilization recommendations are quite straightforward, a multitude of Zn fertilizers that vary in cost, elemental composition, solubility, and formulation are commercially available. Fertilizer recommendations cannot always account for differences among Zn sources and formulations, which often complicates the general recommendations and highlights the need to periodically evaluate the available fertilizer sources.
Pumphrey et al. (1963) showed that foliar application of ZnSO4 to corn (Zea mays L.) seedlings was generally less effective than preplant soil-applied Zn. Wells et al. (1973) reported the results of more than 20 Zn fertilization trials conducted in Arkansas, which included granular and liquid formulations of Zn applied across a wide range of Zn application rates and times. However, no Zn deficiency or yield benefit from Zn fertilization occurred in the trials that evaluated Zn application time. Giordano and Mortvedt (1973) showed, in greenhouse studies, that growth and Zn uptake of direct-seeded rice were similar for Zn placed in the water after flooding and Zn mixed with soil before seeding. However, our observations in commercial fields in Arkansas indicate that granular Zn fertilizer fails to prevent Zn deficiency when applied to the soil surface shortly before flooding seedling rice grown on Zn-deficient soils.
Giordano and Mortvedt (1972), in a greenhouse study, concluded that mixing Zn with soil was generally superior to deep or surface placement, although rice uptake of surface-placed Zn fertilizer was better under flooded soil conditions than moist soil (nonflooded) conditions. They also reported that Zn source influenced the downward movement of fertilizer Zn in soil. The movement of fertilizer Zn 4 wk after application was about 5 mm for ZnO, 20 mm for ZnSO4, and 60 mm for ZnEDTA (ethylenediaminetetraacetic acid). The mobility of fertilizer Zn in soil also decreases as soil pH increases (Mortvedt and Giordano, 1967). These data (Giordano and Mortvedt, 1972; Mortvedt and Giordano, 1967) suggest that when Zn is surface-applied to alkaline soils with no incorporation (i.e., conservation-tillage); some Zn fertilizer sources may be more appropriate than others due to their solubility, mobility, or both.
Low rates of Zn fertilizer solutions are commonly tank-mixed with some herbicides or liquid fertilizers and applied to seedling rice to reduce application costs. However, little or no field research has compared the use of commercially available Zn sources applied at low Zn rates sprayed alone before or after rice emergence with granular Zn fertilizers applied before or after rice emergence. The primary objective of this research was to evaluate the Zn nutritional status and grain yield of rice grown on Zn-deficient soils as affected by various liquid and granular Zn fertilizer sources and application rates when applied: (i) before seeding and incorporated; (ii) to the soil surface after seeding, but before rice emergence; or (iii) 5 to 7 d before flooding at the three- to four-leaf stage. Our hypotheses were that: (i) granular Zn sources applied at recommended rates (i.e., 11 kg Zn ha–1) would perform well when applied before rice emergence, but not when applied shortly before flooding; (ii) liquid sources sprayed uniformly at low rates (1–2 kg Zn ha–1) would perform well applied to seedling rice (preflood) but not when applied preemergence; and (iii) in general, rice receiving Zn fertilizer before rice emergence would produce greater yields than rice fertilized with Zn shortly before flooding.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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For each site-year, composite soil samples (0–10 cm) were collected from each unfertilized control for routine soil analyses. The soils were oven-dried, crushed, and passed through a 2-mm sieve before analysis. Soil pH was determined on a 1:2 soil/water suspension with a glass electrode. Extractable soil nutrients were determined by the Mehlich-3 procedure (Mehlich, 1984) and analyzed by inductively coupled atomic plasma spectroscopy (ICAP, Soltanpour et al., 1996). Selected soil properties are listed in Table 1. Based on the University of Arkansas interpretations, Zn fertilizer was recommended for each site. Each site received 75 kg K ha–1 as KCl and 60 kg P ha–1 as triple superphosphate to ensure these elements were not yield-limiting factors.
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The remaining two Zn treatments included a finely granulated, dry formulation of ZnSO4 (ZnSO35, 355 g Zn kg–1) with high water-soluble Zn content and a liquid Zn chelated with EDTA (ZnEDTA, 90 g Zn kg–1). Both products were mixed with water and applied with a CO2 backpack sprayer calibrated to deliver 94 L ha–1. Application rates were 1.1 kg Zn ha–1 for ZnEDTA and 2.2 kg Zn ha–1 for ZnSO35.
Unopened bags of each dry, granulated Zn fertilizer source were obtained from local fertilizer dealers or their manufacturers and subsamples from each of the four Zn sources were tested for water-soluble Zn and total Zn using AOAC methods 965.09 and 957.02, respectively (AOAC, 1990). The mean water-soluble percentage of the total Zn in each product was 100% for ZnSO31 and ZnSO35, 50% for ZnOxS20, and 14% for ZnOxS36.
All tests were drill seeded (100 kg seed ha–1) following incorporation of preplant fertilizers, including the PPI Zn treatments. Specific seeding and fertilization dates are listed in Table 2. Each plot contained nine, 5 m long rows of rice with 18-cm row spacings. A 0.5 m wide unfertilized border surrounded each plot. After the POST Zn treatments were applied at the four-leaf stage, 135 kg N ha–1 as urea was broadcast to a dry soil surface and a 10- to 15-cm deep permanent flood was established within 5 d. Management with respect to irrigation and pest control followed guidelines recommended by the University of Arkansas Cooperative Extension Service for the dry-seeded, delayed-flood rice cultural system (Slaton, 2001).
For the RREC01 and RREC02 trials, aboveground whole-plant samples were harvested from a 1-m section of the second inside row for determination of total dry matter, tissue Zn concentration, and seedling Zn uptake about 14 d after flooding. Tissues were oven-dried at 60°C until a constant moisture was reached, weighed, ground to pass a 1-mm sieve, and digested with concentrated HNO3 and 30% H2O2 (Jones and Case, 1990). Before drying, tissue samples were washed immediately and sequentially in deionized water and 0.1 M HCl, and rinsed in deionized water to remove possible sources of Zn contamination. The digests were analyzed for Zn by ICAP (Soltanpour et al., 1996). At maturity, grain yields were measured by harvesting 2.8 m2 of the four center rows of each plot with a small-plot combine. Grain yields were adjusted to a uniform moisture content of 120 g kg–1.
Each site-year was arranged as a randomized complete block with a split-plot treatment arrangement and four replications of each treatment. The main plot was Zn application time and the subplot was Zn source. Locations were analyzed separately. Analysis of variance procedures were conducted with the PROC GLM procedure in SAS (SAS Inst., Cary, NC). Mean separations were performed by Fisher's Protected Least Significant Difference method at a significance level of 0.05.
LIN00
The LIN00 trial established in 2000 contained three additional Zn fertilizer sources plus four Zn sources described previously and only two application times. The ZnOxS20 source was not included as a treatment at this site. Zinc fertilizers were applied DPRE and POST to plots with the same dimensions as described previously for PTBS00, RREC01, and RREC02 (Table 2). Two of the additional Zn treatments were dry-granular fertilizers (ZnOxS10, 100 g Zn kg–1; and ZnOxS30, 300 g Zn kg–1) that were, at the time, being newly marketed in Arkansas. Water-soluble Zn and total Zn content were determined as described previously. The water-soluble Zn contents were 53% for ZnOxS10 and 41% for ZnOxS30. The remaining Zn fertilizer was a liquid formulation derived from ZnSO4 and according to the label contained some EDTA (ZnSO10, 100 g Zn kg–1). Field management with respect to N, P, and K fertilization; pest control; and irrigation were performed by the grower and closely followed production guidelines recommended by the University of Arkansas Cooperative Extension Service for the dry-seeded, delayed-flood rice cultural system (Slaton, 2001). Zinc treated ‘Cypress’ rice seed was planted in all areas of the field, except where the experiment was located, which was seeded with untreated Cypress seed. At maturity, grain yields were measured by harvesting 2.8 m2 of the four center rows of each plot with a small-plot combine. Grain yields were adjusted to a uniform moisture content of 120 g kg–1.
The experiment was arranged as a randomized complete block design with a 2 (application time) x 7 (Zn source) factorial arrangement of treatments and compared with an unfertilized control. Each treatment was replicated four times. Analysis of variance procedures were conducted with the PROC GLM procedure in SAS (SAS Inst., Cary, NC). Mean separations were performed by Fisher's Protected Least Significant Difference method at a significance level of 0.05.
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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At all locations, Zn treatments applied PPI and DPRE generally prevented Zn deficiency symptoms. Occasionally a few random rice plants within plots receiving granular Zn from some sources showed deficiency symptoms suggesting that: (i) Zn granules were distributed nonuniformly; (ii) assuming uniform distribution, Zn granules were not close enough to provide adequate Zn nutrition to all seedlings when applied at 11.2 kg Zn ha–1; and/or (iii) that granular fertilizer Zn was not immediately soluble or available. When Zn was applied POST, Zn deficiency was regularly observed after flooding in rice receiving granular formulations of Zn at 11.2 kg Zn ha–1, suggesting that Zn applied to the soil surface was not positioned for immediate root uptake. However, the Zn fertilizers that were sprayed POST, with the Zn solutions contacting both seedling leaves and soil, were generally very effective at preventing Zn deficiency symptoms.
Seedling Zinc Accumulation
Zinc fertilizer source, averaged across application times, was the only factor that significantly affected Zn uptake of rice at the RREC01 (P = 0.001) and RREC02 (P = 0.001, Table 3) locations. For RREC01, rice receiving ZnSO31, ZnSO35, and ZnEDTA had greater Zn uptakes than the unfertilized control, but only ZnSO31 resulted in significantly greater Zn uptake by rice than most other Zn sources. For RREC02, all Zn sources, except ZnOxS36, resulted in greater Zn uptake by rice than the unfertilized control. For RREC02, the ZnOxS36 resulted in low seedling Zn uptake (Table 3) and grain yield (Table 4), suggesting that this Zn fertilizer, low in water-soluble Zn, cannot supply enough Zn to maximize early season rice growth on some Zn-deficient soils, even when applied at 11 kg Zn ha–1. In contrast, ZnEDTA and ZnSO35 were applied at low rates and produced seedlings with intermediate to high Zn uptakes for both site-years (Table 3).
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Grain Yield
PTBS00, RREC01, and RREC02
Rice grain yield response to Zn fertilization varied among the site-years and was likely due to the differences in the severity of Zn deficiency. The interaction between Zn application time and source did not significantly affect (P = 0.225 for PTBS00, 0.186 for RREC01, and 0.755 for RREC02) rice grain yield for any site-year. However, Zn fertilizer source, averaged across application times, was significant for all three site-years (P = 0.001 for PTBS00, RREC01, and RREC02). All Zn sources, averaged across application times, increased grain yields compared with the unfertilized control (Table 4). The range of grain yield increases from Zn fertilization was 12 to 25% for PTBS00, 40 to 72% for RREC01, and 19 to 26% for RREC02. For PTBS00, where the lowest yield response to Zn fertilization occurred, the numerically greatest yield was produced with ZnEDTA, which produced yields similar to that of rice receiving ZnSO31 and ZnOxS20 and was significantly greater than yields produced by application of ZnSO35 and ZnOxS36. For RREC01, ZnEDTA, ZnOxS20, and ZnSO35 all produced equal yields, which were greater than the yield from rice fertilized with ZnOxS36. Although, grain yields among the five Zn fertilizer sources were not different for RREC02, ZnEDTA produced the second greatest numerical yield. Across the three site-years, consistent responses to specific Zn sources were observed. The granular sources ZnSO31 and ZnOxS20, which were applied at 11.2 kg Zn ha–1, as well as ZnEDTA and ZnSO35, which were applied at 1.1 and 2.2 kg Zn ha–1, respectively, tended to produce near-maximum to maximum rice grain yields for each site-year. In contrast, ZnOxS36 produced only intermediate rice grain yields at PTBS00 and RREC01 suggesting that this granular Zn source may be inferior to the other tested Zn sources when applied at the recommended rate and Zn deficiency is severe.
Zinc application time, averaged across Zn sources, significantly affected grain yields only for RREC01 (P = 0.001), which had severe Zn deficiency. Rice grain yields were similar [LSD(0.05) = 758 kg ha–1] for Zn applied PPI (6915 kg ha–1) and DPRE (7456 kg ha–1), but significantly greater than when Zn was applied POST (5526 kg ha–1). Zinc application time was not significant for grain yield at RREC02 (P = 0.566) or PTBS00 (P = 0.057).
LIN00
At the LIN00 site, rice grain yield was not significantly affected by the interaction between Zn application time and source (P = 0.523), but was affected by the main effects of Zn application time (P = 0.015) and Zn source (P = 0.003, Table 4). Zinc fertilizer applied DPRE (7682 kg ha–1) produced numerically greater, but statistically similar (LSD = 1006 kg ha–1), rice yields as Zn applied POST (7073 kg ha–1). Both DPRE and POST Zn applications significantly increased yields compared with the unfertilized control (2967 kg ha–1).
All Zn sources, averaged across Zn application times, increased rice grain yields compared with the unfertilized control (Table 4) with yield increases ranging from 104 to 180%. Among Zn sources, the ZnOxS30 fertilizer produced significantly lower yields than all other Zn sources, which were not different.
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Compared with rice receiving adequate Zn before emergence, results from the RREC01 indicate that rice seedlings that are stressed by severe Zn-deficiency have lower yield potential, even when Zn is applied shortly before flooding (i.e., POST) in the direct-seeded, delayed-flood cultural system. When Zn fertilizer is not applied PPI or DPRE and Zn deficiency of rice seedlings occurs, some of the potential yield loss attributed to Zn deficiency can be restored by POST applications of Zn fertilizer, which supports University of Arkansas recommendations to apply Zn to Zn-deficient rice (Slaton, 2001). Snyder and Jones (1991) also reported that postemergence applications of Fe solutions helped recover some lost yield potential of Fe-deficient rice, but were not as effective as Fe fertilization at seeding. Apparently, postemergence Zn applications are an acceptable alternative to preemergence Zn fertilization only when rice is grown on marginally Zn-deficient soils (i.e., PTBS00 and RREC02) or when Zn is applied several weeks before flooding as shown for LIN00. For LIN00, the POST Zn treatments were applied 21 d before flooding, which differed from the PTBS00, RREC01, and RREC02 trials where the POST application of Zn was applied only 5 d before the permanent flood was established.
Despite being more expensive, chelated Zn products are used commonly and often preferentially to inorganic Zn sources for postemergence Zn fertilization of rice in Arkansas. Data from these four studies suggest that chelated or inorganic ZnSO4 fertilizer sources are equally effective for Zn solutions applied both pre- and postemergence. Although Zn solutions containing ZnSO4 were applied at higher Zn rates than chelated Zn, the lower price of ZnSO4 reduces fertilizer costs, even when higher Zn rates are used. Haslett et al. (2001) reported that Zn from ZnSO4 and ZnEDTA solutions applied to wheat (Triticum aestivum L.) foliage at the same rates was absorbed and translocated with equal efficiency. One possible advantage that chelated Zn might have over ZnSO4 would be its increased mobility in soil, which may lead to increased root uptake (Gangloff et al., 2002) when Zn solutions are applied to seedling crops with limited leaf area. The portion of foliar-applied Zn that contacts the soil when ZnSO4 is sprayed would probably remain very near the soil surface, whereas the chelated Zn would be more mobile and likely to reach seedling roots (Giordano and Mortvedt, 1972).
For growers, selecting the appropriate Zn-fertilization strategy, which includes Zn fertilizer source, application rate, and application time, depends on a variety of economic and agronomic factors, as well as the chemical and physical properties of the selected fertilizer. Rice response to Zn fertilization depends on soil pH and soil-test Zn (Slaton et al., 2002b). Because alkaline soils very low in Zn (i.e., <2.5 mg Mehlich-3 Zn kg–1) are likely to respond positively to Zn fertilization, Arkansas growers with such soils are encouraged to apply granular Zn fertilizers with high levels of water-soluble Zn at 11 kg Zn ha–1 before rice emergence. This approach should provide sufficient Zn for the current rice crop, increase soil-test Zn, and provide residual Zn nutritional benefits for several years (Brown et al., 1962, 1964; Carsky and Reid, 1990).
Rice response to Zn fertilization also differed among granular Zn sources, especially when Zn deficiency was severe (Tables 3 and 4). Amrani et al. (1999), Liscano et al. (2000), and Mortvedt (1992) all reported that the immediate availability and, consequently, the effectiveness of a particular granular fertilizer was directly related to the level of water-soluble Zn in granular Zn fertilizers. In the current study, the granular Zn fertilizers with the greatest levels of water-soluble Zn were consistently among the most effective sources. In contrast, sources like ZnOxS36 and ZnOxS30, which have low to intermediate levels of water-soluble Zn, did not always maximize rice growth, Zn uptake, and grain yield. Zinc fertilization recommendations provided to growers should, therefore, account for differences in Zn availability in granular Zn fertilizers.
Although ZnSO4 and ZnEDTA solutions applied PPI and DPRE at 1.1 to 2.2 kg Zn ha–1 allow for uniform distribution of Zn and generally performed equal to the best granular Zn treatments in our studies, preplant application of at least 9 kg Zn ha–1 as granular fertilizer is generally needed to consistently produce maximum rice yields on Zn-deficient soils (Mikkelsen and Brandon, 1975; Slaton et al., 2002a; Wells et al., 1973). Uniform application of micronutrient granular fertilizers applied at low rates may not provide sufficient distribution of Zn granules to maximize crop growth and yield. Giordano and Mortvedt (1966) showed that when Zn application rate was held constant, Zn uptake by corn decreased as the number of Zn fertilizer granules decreased below some critical granule density (i.e., curvilinear response). They also showed that dry matter yield was limited at low Zn application rates, despite uniform distribution of small granules. Liscano et al. (2000) showed that Zn uptake by rice increased as granule size decreased for a Zn source with low-water soluble Zn content. Therefore, fertilizer solutions containing low rates (1–2 kg Zn ha–1) of an inorganic (ZnSO4) or chelated Zn source applied alone or tank-mixed with other crop inputs (i.e., crop protectants or other liquid fertilizers) applied before or shortly after rice emergence could be used quite effectively in Zn fertilizer programs for alkaline soils with intermediate soil Zn levels (2.5–3.5 mg Mehlich-3 Zn kg–1). This conclusion is supported by Amer et al. (1980), who reported that foliar-applied ZnSO4 and higher ZnSO4 rates mixed with the soil produced similar rice yields on a moderately Zn-deficient soil.
One risk of planning to apply Zn fertilizer solutions to rice at the three- to five-leaf stage is that seedlings may become Zn deficient before Zn fertilizer is applied, especially when cool, wet environmental conditions follow seedling emergence. Zinc is a cofactor for alcohol dehydrogenase, which is required for the reduction of acetaldehyde to ethanol during anaerobic metabolism under flooded soil conditions. Zinc-deficiency symptoms of rice seedlings are not always easily distinguishable before flooding (i.e., from irrigation or excessive precipitation), but usually appear within 2 to 7 d after flooding (Norman et al., 2003). Moore and Patrick (1988) suggested that flooding inhibits the ability of Zn-deficient rice seedlings to survive flooded soil conditions due to the decline in alcohol dehydrogenase activity, which partially explains why seedlings express dramatic Zn-deficiency symptoms after flooding. Cool, wet environmental conditions that occur before the permanent flood is established may also exacerbate Zn deficiency of seedling rice, especially when rice is seeded very early (Norman et al., 2003). Although the flooded soil conditions used to grow rice play an important role in the occurrence and diagnosis of Zn deficiency, delaying Zn application until after crop emergence has also been shown to reduce yield potential of corn (Pumphrey et al., 1963). Once seedlings become Zn deficient, some minimum amount of time is likely needed for the uptake and translocation of foliar-applied Zn and subsequent restoration of normal plant growth and metabolism.
Data from these field studies show that a wide range of Zn sources can be used in a variety of ways to prevent growth and yield limitations of rice attributed to Zn deficiency. Planned foliar Zn applications have performed quite effectively in commercial fields and research trials. However, environment-induced Zn deficiency could occur before Zn application, resulting in increased production expenses, stand loss, and decreased yield potential. Therefore, Zn fertilization recommendations should reflect the advantages of a preventative Zn fertilization program implemented before crop emergence. Growers can confidently apply Zn fertilizer solutions or granular Zn fertilizers with high levels of water-soluble Zn to the soil surface without incorporation before seedling emergence with recommended rates of granular Zn preferred for alkaline soils with very low soil-test Zn. When postemergence Zn applications are preferred by the grower, Zn solutions containing either chelated or other soluble sources of inorganic Zn should be applied soon after rice emergence to allow for maximum foliar and root uptake by rice before flooding.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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