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Production Paper

Early-Season Applications of Sulfur Fertilizers Increase Potato Yield and Reduce Tuber Defects

Univ. of Nebraska-PREC, 4502 Ave. I, Scottsbluff, NE 69361

^* Corresponding author (apavlista{at}unl.edu)

Received for publication June 22, 2004.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Sulfur recommendations are based on avoiding deficiency. Since S may have additional effects, the objective was to determine whether S fertilizers increase yield and reduce tuber defects. Potato (Solanum tuberosum L.) cultivar Atlantic was treated with wettable sulfur (WS), ammonium sulfate (AS), and ammonium thiosulfate (TS) at 0, 28, 56, 84, and 112 kg S ha^–1 applied at planting [in-furrow (IF)], 11 d later [pre-emergence (PE)], 11 d after emergence [postemergence (PT)], and at tuber initiation (TI). Separate trials were conducted for each fertilizer and application. Ammonium nitrate and urea were added to AS and TS to equalize N. Yield of U.S. "A" potato was increased 11 to 34% by fertilizers at S rates greater than 56 kg ha^–1 at IF, PT, and by AS and WS at TI. These fertilizers applied IF and at TI reduced the incidence of common scab (Streptomyces scabies) on tubers. When applied PT, AS and TS reduced common scab; WS did not. The incidence of black scurf (Rhizoctonia solani) on tubers was reduced when AS and TS were applied PT but not when applied IF and at TI. Pre-emergence application had no effect. Other tuber characteristics, e.g., specific gravity and vascular discoloration, were unaffected. Plant growth was inhibited by TS applied after emergence but recovered. Yield was increased by S at 56 kg ha^–1 applied IF, PT, and at TI except for TS applied at TI. Common scab and black scurf were reduced depending on application timing and fertilizer.

Abbreviations: AS, ammonium sulfate • DAE, days after emergence • IF, in-furrow • PE, pre-emergence • PT, postemergence • TI, tuber initiation • TS, ammonium thiosulfate • WS, wettable sulfur

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
COMMON SCAB caused by Streptomyces scabies (Loria et al., 1997) and black scurf caused by Rhizoctonia solani (Banville, 1989) are considered tuber blemishes that reduce market quality of harvested tubers. Common scab principally affects the fresh, process, and seed-potato markets while black scurf affects the fresh and seed-potato markets. Research on the inhibition of common scab through soil application of S has been reported as early as the end of the 19th century (Wheeler and Adams, 1897; Garman, 1898).

Common scab was found to be sensitive to soil pH and to have an optimal pH range of 5.4 to 7.4 for activity and infection of potato tubers as recently reviewed by Keinath and Loria (1989). Nearly all studies on using S for common scab control used large quantities of elemental S to lower pH on slightly acidic soils to below the optimum pH for S. scabies infection. Changing soil pH out of the common scab range works whether by acidifying with S (McCreary, 1967) or increasing the alkalinity with lime (Waterer, 2002). If the effect of S in controlling common scab was only to lower pH, then on slightly alkaline soils, the pH would be lowered into the optimum range for common scab infection and increase its incidence (Goss, 1934). However, as early as the 1920s, it was observed that lower quantities (340 kg ha^–1) of elemental S could also lower common scab and increase yield even without substantially lowering pH of slightly acid soil (Martin, 1920; Duff and Welch, 1927).

In North America, among the cheapest and easiest to obtain form of the sulfate ion is AS. In many situations, elemental S requires large amounts. Ammonium sulfate would be used in lower amounts, be a source of N, and fit into a fertilizer management scheme. Working with cultivar Irish Cobbler on slightly acid soils, Hooker and Kent (1950) reported that AS applied IF at rates resulting in 134 and 268 kg S ha^–1 increased total and marketable yields and reduced the incidence of common scab. Reporting preliminary data on the cultivar Atlantic on acidic soil, Hammerschmidt and Vitosh (1987) reported yield increases and reduction in common scab with AS applied at 128 kg S ha^–1 both IF plus early PE. They also reported a reduction in the soil population of S. scabies several weeks later when tubers were initiated.

Ammonium thiosulfate is another form of S, and the thiosulfate ion is readily bioconverted to sulfate. Ammonium thiosulfate is used by some growers as thiosul and applied through sprinkler irrigation systems at midseason to promote vine regrowth after hail and for disease protection. Thiosul applied for this purpose amounts to 5 to 12 kg S ha^–1. The use of TS as a fertilizer or for tuber disease control has not previously been explored.

The objective of this study was to measure the S rate response on potato yield and tuber defects using different forms of S applied at various stages in potato development.

	MATERIALS AND METHODS

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Trials were conducted on potato cultivar Atlantic in alternating years (1992, 1994, and 1996) at Scottsbluff (Panhandle Research and Extension Center) and in successive years (1993–1995) at Alliance (Western Potatoes, Inc.), NE (Table 1). At Scottsbluff and Alliance, the soils were fine sandy loams characterized as Tripp (plustolls) and Alliance (griustolls), respectively, with an alkaline pH from 7.6 to 8.1 and organic matter from 1.1 to 1.7%. General fertilization was 140 or 200 kg N ha^–1, 70 or 100 kg P ha^–1, and no K at Alliance and Scottsbluff, respectively. Preplant soil SO₄–S was 11 to 17 kg ha^–1 in the top 30 cm as determined using calcium phosphate as an extractant. In Scottsbluff, fertilizer (N and P) was applied in three equal doses before emergence, and in Alliance, a third of the total N and all the P were applied before emergence, and the rest of the N was applied during the growing season through the sprinkler irrigation system. Irrigation at Scottsbluff was by an irrigation gun or a rolling sprinkler line while at Alliance, it was through a center-pivot system. Weather conditions were cool and wet in 1992 and 1993, whereas near-normal temperature and rainfall conditions existed in 1994 and 1995 and wet with near normal temperatures in 1996.

Planting was done in early to mid-May, and plants emerged 20 to 24 d later. Vines were desiccated mechanically and chemically at the end of August or early September, and tubers were harvested about 2 wk later. Yields and tuber characteristics were measured within 2 wk after harvest. Sulfur fertilizer applications (treatments) were made IF, PE, PT, and at TI (Pavlista, 1995). Granular applications (AS in 1992–1994) were broadcast with a granular spreader designed for fertilizing individual rows. Liquid treatments (AS in 1995–1996, TS and WS) were applied IF in a band of 30 cm over the open furrow using 4002E TeeJet nozzles (Spraying Systems Co., Wheaton, IL), PE and PT at 61 cm over the rows after hilling using a 7004E nozzles, and at TI at 91 cm over the canopy using a 9506E nozzles, respectively. All liquid treatments were applied in water at 935 L ha^–1.

Each trial consisted of each S fertilizer applied at a single time with the treatment variable within the trial being rate. Four replicated plots were used for each treatment, and the trials were designed as Latin squares. Plots were four rows, each 91.4 cm wide and 11 m long of which 9.1 m of the center two rows was harvested. For liquid foliar treatments, all four rows were treated in the plot, and for granular treatments, the center two rows were treated. The trial areas were also bordered by at least four untreated rows. Seed pieces were spaced 24 cm within the rows. Trials were not done in the same field twice.

Wettable S was applied as a liquid in a 80DF formulation (Microthiol Special, supplied by Cerexagri then Elf Atochem) at rates to achieve 0, 28, 56, and 112 kg S ha^–1. Additional N fertilization was not applied, so the final N rate was 140 and 200 kg ha^–1 for Alliance and Scottsbluff, respectively. Ammonium sulfate was applied as 21–0–0–24S granules in 1992, 1993, and 1994. The rate of AS was such that 0, 56, and 112 kg S ha^–1 was applied. The added N in AS was compensated by adding granular ammonium nitrate to AS at application. The added N in AS plus ammonium nitrate amounted to 98 kg ha^–1 in addition to the general fertilization N in all AS granular plots. In 1994 at TI and in 1995 and 1996, AS was applied as a liquid to achieve the S rates of 0, 28, 56, and 84 kg ha^–1, and the extra N was compensated with liquid urea (70%, 32–0–0), reaching 74 kg N ha^–1 in AS liquid plots. Ammonium sulfate at 112 kg S ha^–1 could not be achieved as a liquid in 935 L ha^–1. Ammonium thiosulfate was obtained as a 60% liquid formulation called thiosul (12–0–0–26S). As with AS and WS, the amount of TS used was to achieve 0, 28, 56, and 112 kg S ha^–1. As with liquid AS, the additional N from TS was compensated with liquid urea to maintain an additional 52 kg N ha^–1 to the general N fertilization in TS plots.

Small tubers, <4.8 cm diameter, were eliminated using a chain grader set for 4.8 cm. The remaining tubers, U.S. Grade A, were weighed and comprised yield. From these, samples of 50 tubers were randomly removed from each plot and scored for external and internal defects. External defects included diseases such as common scab, black scurf, tuber rots (Fusarium spp. and Erwinia carotovora), and misshaping; internal defects were hollow heart and vascular discoloration. Also from the U.S. Grade A tubers, 3.6-kg samples were randomly removed from each plot, and specific gravity using a Potato Chip Association/Snack Food Association hydrometer was measured (Gould and Plimpton, 1985). A second set of 2.3-kg samples were also removed and fried to make potato chips whose color were visually estimated using the Potato Chip Association/Snack Food Association color chart (Pavlista, 1997). Data within the rate trials and across years for each S source and application timing were analyzed using PROC ANOVA (SAS Inst., 1996). Quadratic equations compared yield to S rate with r² based on yearly means. Treatment means for each material at each timing were separated using Duncan multiple range and, for canopy height comparisons, using least significant differences. Interactions of year x S rate were calculated separately for each S source and application timing.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The rate of application at various timings of the S-containing fertilizers had a pronounced effect on yield and, to a lesser extent, on the incidence of common scab and black scurf. In-furrow application at planting of AS granules and WS and TS at 56 and 112 kg ha^–1 increased yields of U.S. Grade A (>4.8-cm diam.) tubers, cultivar Atlantic, by 16 to 23% (Table 2). Sulfur applied at 28 kg ha^–1 was intermediate. The quadratic equations representing yield in t ha^–1 (Y) versus S rate in kg ha^–1 (S) for each material applied IF were (WS) Y = 31 + 0.18S – 0.0010S², r² = 0.70; (AS) Y = 32 + 0.17S – 0.0011S², r² = 0.54; and (TS) Y = 34 + 0.14S – 0.0007S², r² = 0.87. There also was a decrease in the number of tubers with common scab. For AS, there was a rate x year interaction because in 1992 the incidence of common scab was about 10%, too low to detect a significant decrease, whereas in 1993 and 1994, the incidence of common scab was greater than 20% and a significant decrease by AS was observed. The incidence of black scurf was decreased by WS in 1994 but not in 1992 and 1993. There was no significant decrease by AS and TS.

Early postemergence applications were made 11 d after emergence (DAE) and when applied at this timing and at S rates greater than 56 kg ha^–1 and when WS was applied as a liquid, AS as either a granule or liquid, and TS as a liquid, yields were increased 14 to 26% (Table 3). The lower rate, as with IF applications, was intermediate. The quadratic equations representing yield in t ha^–1 (Y) versus S rate in kg ha^–1 (S) for each material applied PT were (WS) Y = 34 + 0.12S – 0.0005S², r² = 0.85; (AS/granular) Y = 28 + 0.10S – 0.0005S², r² = 0.63; (AS/liquid) Y = 33 + 0.03S – 0.0007S², r² = 0.73; and (TS) Y = 27 + 0.18S – 0.0013S², r² = 0.41. Ammonium sulfate (granular and liquid) and thiosulfate applied at 112 kg S ha^–1 also decreased the incidence of common scab; WS did not. There was no rate x year interaction observed. In addition, AS and TS decreased the incidence of black scurf. In 1995, in the liquid AS trial, the incidence of black scurf on checks was 100%, and no significant decrease was observed. Ammonium thiosulfate in addition to increasing yield also inhibited vine growth as depicted by plant height (Fig. 1a). From 1 wk after treatment (18 DAE) to 5 wk after treatment (46 DAE), vines showed a significant height reduction. For the first 2 wk after treatment, height was reduced by >25% and then gradually recovered by the sixth week after treatment. Plant width was measured the first year and showed the same trend as height (data not presented).

View larger version (30K): [in this window] [in a new window]   Fig. 1. Canopy height of potato cultivar Atlantic after application of ammonium thiosulfate at S rates of 56 and 112 kg ha–1 applied at (a) 10 to 11 and (b) 20 to 21 d after emergence. Vertical bars show the LSD at P < 0.05 for each evaluation timing.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
These trials were conducted on the potato cultivar Atlantic (Webb et al., 1978) because of its susceptibility to common scab and because it is a principle potato chip cultivar. The location of the trials were chosen on ground with a history of common scab infection of harvested susceptible tubers. Although trial emphasis was on common scab, primarily a soil-borne bacterium (Pavlista, 1996), other tuber defects were evaluated, and of them, only black scurf showed a response to S application. Black scurf results from a soil infection of tubers by the same fungus causing stem canker (Stevenson et al., 2001). Besides observation on the incidence (percentage infected tubers), measurements were also taken on yield of U.S. Grade A tubers, tubers greater than 4.8 cm disregarding defects.

Sulfur application of 28 kg ha^–1 gave a yield-increase response intermediate between no added S and 56 kg S ha^–1, and above this up to 112 kg ha^–1, yield was level (plateau) when WS, AS, or TS was applied IF, PT, and at TI. There was no S effect when these materials were applied PE. Since these trials were conducted on alkaline soils (pH 7.8–8.1) and these low rates were effective except at PE, the data suggest that the S effect is not due to a lowering of the pH (Hooker and Kent, 1950). However, the mode of action of S in reducing common scab and black scurf may not be altogether clear. The timing of application may be important. While application IF, PT, and at TI effectively increased yield and in many cases reduced disease, PE application did not. Preplant soil-surface applications of AS have also been reported as having no effect on yield, common scab, or black scurf (Gray et al., 1961). However, reduction of soil pathogen may not explain activities of WS, AS, or TS applied PT and at TI. A reduction of common scab and yield promotion by AS applied PT was earlier reported as possible using Atlantic by Hammerschmidt and Vitosh (1987). It is also important to note that liquid applications were made in a large volume, 935 L ha^–1, chosen for maximum coverage since this is near runoff for the potato canopy (Lipe and Thomas, 1980). No explanations as to the foliar action of S are forthcoming.

Another interesting observation is the change in activity of TS when applied PT and at TI. Ammonium thiosulfate at 56 and 112 kg S ha^–1 applied IF and PT increased yield and reduced common scab and black scurf. But at PT, vine growth, as measured by canopy height, was also dramatically inhibited, albeit temporarily. Application at TI had a similar effect on the canopy that was less pronounced since vine size was greater than at PT. However at TI, there was a dramatic reduction of yield as well. Common scab at both application times was still reduced by TS. In a single experiment conducted at Alliance, TS at 112 kg S ha^–1 was applied PT through a center-pivot irrigation system applying 0.25 cm of water. Although a yield increase was observed, there was no effect on canopy size, nor on common scab and black scurf (data not shown).

The reduction of common scab, at least with IF application, is due to the S in the sulfate ion and the thiosulfate ion as the ammonium ion is not effective (Hooker and Kent, 1950). Additional evidence suggesting that the sulfate ion is effective in promoting potato yield and reducing tuber disease incidence is suggested from reports on the application of heavy metal ions such as Mn (Mortvedt et al., 1963). The sulfate form of Mn was the only one active compared with other forms such as carbonate or oxide. When the rates are recalculated to S rate (107 kg S ha^–1 in Mortvedt et al., 1963) and the data compared based on S rate, then the yield promotion and common scab reduction fit well with the S rates reported here. Potassium reduced the incidence of stem canker caused by R. solani (Panique et al., 1997) when applied in the sulfate form but not the chloride form when applied shortly after emergence. The use of an organic sulfate form, ammonium lignosulfonate (a soluble derivative of lignin produced through the use of sulfuric acid), has been reported to reduce common scab and verticillium wilt (Verticillium dahliae) on potato when incorporated in the soil (Soltani et al., 2002).

	ACKNOWLEDGMENTS

 
Author thanks Carl Gall for his technical support and financial support from the Nebraska Potato Promotion Board, Colorado Potato Administrative Committee (Area III), Cerexagri, and Agrium Corp.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Univ. of Nebraska Agric. Res. Div., Journal Ser. 14160.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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