SULFUR STATUS AND RESPONSES ON WISCONSIN ALFALFA1/

SULFUR STATUS AND RESPONSES ON WISCONSIN ALFALFA¹^/

K.A. Kelling and P.E. Speth²^/

For more than 30 years, agronomists, soil scientists, consultants and farmers have recognized the potential for significant responses to applied sulfur fertilizer in northern and western Wisconsin on lighter textured, low organic matter soils that have not received manure in recent years (Rand et al., 1969; Hoeft and Walsh, 1975; Schulte, 1976; Peters and Kelling, 1987). More recently, crop consultants and others have reported seeing sulfur responses on soils or in locations where they would not be expected. Kelling and Speth (1998) measured a sulfur response of alfalfa in the final 2 years of a 4-year experiment at Arlington, where S responses have traditionally not been observed.

Part of the reason S responses may be occurring more frequently is that precipitation S levels have been decreasing. From 1969 to 1987, precipitation S decreased an average of 42% across Wisconsin (Andraski and Bundy, 1989). Furthermore, a Wisconsin Department of Natural Resources estimate showed Wisconsin S emissions have declined another 40% from the mid-1980s to the mid-1990s.

In the early 1970s, Wisconsin started testing soils for SO₄-S based on the work of Hoeft et al. (1973). However, while this testing procedure determines the amount of sulfate-S in the plow layer of agricultural soils, it does not account for the several other sources of plant-available sulfur. That means that these tests are most useful in identifying crop production situations where the amount of plant available sulfur in the plow layer at the time of sampling is sufficient to supply crop sulfur needs, but they do not adequately determine if sulfur should be added if the test is low since adequate S may be coming from the other sources.

In 1991, based on survey data collected by Schulte (1976) and Schulte and Combs (1990), the Wisconsin soil test recommendation program switched its procedures in an attempt to account for available S from other sources. For the most part, however, this somewhat crude Aexpert system@ has not been thoroughly field tested.

This research has been conducted over the past several years to answer some specific questions about better identification of sulfur need and improved S management on Wisconsin alfalfa.

¹^/ Research support provided by the Wisconsin Fertilizer Research Council, Honeywell, The

Sulphur Institute and the UW College of Agricultural and Life Sciences is gratefully

acknowledged.

²^/ Professor/Extension Soil Scientist and Senior Research Specialist, respectively,

Department of Soil Science, University of Wisconsin-Madison.

Question #1 C Is a high rate of sulfur needed to improve alfalfa yield and quality?

We have conducted a trial at Spooner (a sulfur-responsive site) over the past 4 years that has examined the effects of topdressed sulfur rate and source on alfalfa yield and quality. Data from this experiment clearly show that topdressed elemental S is not available quickly enough the first year it is applied (Table 1); however, by the second season, enough of the elemental S had oxidized that it performed as well as the sulfate-S. These data also show little benefit to rates of S higher than 25 lb S/acre/year except when elemental S was used the first year.

Table 1. Effect of top dressed sulfur rate and source on alfalfa yield and average tissue S levels, Spooner, WI, 1997 to 2000.

S treatment________

Source Rate H 1997 I 1998 1999 2000

lb S/acre/year

Dry matter yield (ton/acre)

Check 0 0.74 4.08 4.33 3.30

Sulfate-S 25 0.97 4.27 5.09 3.74

75 0.93 4.22 5.04 3.67

225 1.03 4.28 5.25 3.85

Elemental S 25 0.66 4.40 4.96 3.43

75 0.85 4.04 5.12 3.92

225 0.95 4.37 4.96 3.76

Tissue S level (%)

Check 0 0.25 0.20 0.15 NA '

Sulfate-S 25 0.35 0.26 0.23 NA

75 0.35 0.27 0.25 NA

225 0.36 0.31 0.30 NA

Elemental S 25 0.23 0.26 0.23 NA

75 0.28 0.31 0.26 NA

225 0.29 0.31 0.29 NA

H Sulfur applied topdressed following first cutting each year.

I Only two cuttings taken in 1997 (seeding year).

' NA, data not available.

Evaluation of the tissue analysis data (Table 1) shows that when deficiency existed alfalfa tissue S levels were generally less than 0.23% S and usually below 0.20; when S was sufficient, tissue levels were generally above 0.25%. It is also interesting to note that even where large amounts of S were added, levels in the plant did not escalate dramatically.

Table 2 shows the influence of these treatments on harvested forage quality as measured by NIR scanning. There is no question that the addition of S at this S-deficient site increased the forage protein content and there is a slight tendency for the higher sulfur rates to increase protein slightly (0.3 to 1.0%) above the lowest S rate. However, since it took an extra 200 lb S/acre to achieve this increase, the cost to benefit ratio is very poor. Fiber analysis results as summarized by the relative feed value were apparently not affected by S treatment at any rate. At this point, we conclude that adding extra S in an attempt to bump yields more or to significantly increase crop quality is not a viable practice. Sufficient S should be used to optimize yield (about 25 lb S/acre/year), but adding excess is not cost effective.

Table 2. Effect of topdressed sulfur rate and source on alfalfa crude protein content and forage relative feed value, Spooner, WI, 1998 to 1999. H

1998 1999______

S treatment Crude Relative Crude Relative

Source Rate H protein feed value protein feed value

lb S/acre/year % %

Check 0 21.2 145 18.9 139

Sulfate-S 25 22.2 136 20.5 140

75 22.4 142 20.6 145

225 22.5 139 21.1 146

Elemental S 25 21.7 136 20.1 139

75 22.2 139 20.7 145

225 22.7 147 21.0 149

H Average across three cuttings each year.

Question #2 C Can a preplant application of sulfur last for the entire life of the stand?

One of the components of the Spooner S work was to include several S sources (K₂SO₄, CaSO₄, and elemental S) as preplant treatments at a moderately high rate (75 lb S/acre). This was applied only once and the crop growth was monitored for the four following years (Table 3). Even on this quite sandy soil, it appears that all three sulfur sources were equally effective. Evaluation of the yield tissue data for 1999 suggests that the SO₄-S sources may have been tapering off compared to elemental S, but the yield data for 2000 show that the sulfate carriers were as strong as elemental S in this subsequent year. Unfortunately the tissue analyses have yet to be complete for this year.

Table 3. Lasting power of a moderately high rate of several S sources for alfalfa at Spooner, WI, 1997 to 2000.

S source H 1997 1998 1999 2000

Alfalfa yield (ton/acre)

None 0.74 4.08 4.33 3.30

K₂SO₄ 0.87 4.20 5.23 4.00

CaSO₄ 0.87 4.04 5.01 3.96

Elemental S 0.91 4.39 5.48 3.79

Tissue S (%)

None 0.25 0.20 0.15 NAI

K₂SO₄ 0.33 0.27 0.20 NA

CaSO₄ 0.34 0.28 0.21 NA

Elemental S 0.25 0.26 0.24 NA

H All S sources applied at 75 lb S/acre preplant spring 1997.

I NA, data not available.

Based on these data, we conclude that a moderate rate of S preplant is adequate to carry the crop for three or four seasons. Similar results were seen by Hoeft and Walsh (1975) in the early 1970s. On heavier soils, the lasting power of preplant S treatments would be even better.

Question #3 C Are older stands more likely to show S responses than younger stands?

In an actual farming situation, the answer to this question is likely AYes@ if the soils have a tendency toward being responsive and manure is usually not applied to the alfalfa. In this scenario, older stands would have a longer time, since the last manure application and, therefore, would be more likely to show a response to fertilizer S. We also speculated that since alfalfa is such a high S user that several years of alfalfa growth might out-strip the organic matter S mineralization and precipitation contributions.

To test these hypotheses, we selected alfalfa fields established in 1998 or 1999 that had not received manure in the last 3 to 5 years at the Arlington and Lancaster Agricultural Research Stations as well as a field in its third year of production at each location. Duplicate experiments were laid out using several treatments of topdressed S. In spite of the results we obtained at Arlington in the mid-1990s where we saw a sulfur response in the last 2 years of a 4-year trial (Kelling and Speth, 1998), the data from these trials (Table 4) show only slight yield responses to S and it is clearly not stand age related. The forage quality data are also quite mixed.

Table 4. Effect of stand age on alfalfa yield and forage quality, Arlington and Lancaster, WI, 1999 to 2000.

Treatment Newer stand Older stand

Source N rate S rate YieldH CP I RFVI Yield CP RFV

----- lb/acre ---- ton/acre % ton/acre %

Arlington

None 0 0 2.99 21.4 133 2.35 23.5 161

Gyp 0 48 3.08 20.9 127 2.37 24.1 174

AS 42 48 3.03 19.8 123 2.47 24.5 169

Gyp+AN 42 48 3.17 20.9 125 2.47 24.5 165

Lancaster

None 0 0 3.15 NA ' NA 3.22 NA NA

Gyp 0 48 3.31 NA NA 3.58 NA NA

AS 42 48 3.39 NA NA 2.97 NA NA

Gyp+AN 42 48 3.32 NA NA 2.95 NA NA

H Average from two cuts in 1999 and three cuts in 2000 at Arlington and three cuts in 2000

at Lancaster.

I CP, crude protein; RFV, relative feed value.

' NA, data not available.

Question #4C Are we more likely to need S fertilizer in southern and eastern Wisconsin

than we were a few years ago?

To answer this question, in addition to the stand age trials we are conducting, we are getting help from three county faculty to conduct on-farm trials in Manitowoc in 1999 and 2000 and in Dodge and Fond du Lac counties in 2000. Table 5 shows that yield responses were observed at Manitowoc in both 1999 and 2000 and Dodge County in 2000, but not at Fond du Lac. It is interesting to note that when field responses were seen, the increases were mostly during first cut, sometimes second, but rarely third and fourth cuts. Magnitude of the responses were similar to those frequently seen at other responsive locations (0.2 to 0.7 ton/acre/year). These responses are noteworthy because these counties are in the eastern and southeastern part of the state where S responses have been less frequent.

Table 5. Effect of sulfur on alfalfa yields at several on-farm locations, 1999 and 2000.

Yield

S rate Cut 1 Cut 2 Cut 3 Cut 4 Total

lb S/acre ------------------------------------ ton/acre ------------------------------------

Manitowoc 1999

0 -- 1.38 1.34 0.56 3.28

25 -- 1.48 1.20 0.55 3.23

50 -- 1.80 1.34 0.70 3.83

Pr > F 0.15 0.65 0.17 0.08

LSD_0.05 0.49 NSH 0.19 0.58

Manitowoc 2000

0 1.44 1.09 0.72 0.82 4.08

25 1.92 1.21 0.66 0.70 4.48

50 2.61 1.02 0.58 0.70 4.91

Pr > F 0.02 0.07 0.46 0.22 0.06

LSD_0.05 0.70 0.15 NS NS 0.66

Dodge 2000

0 2 .00 1.78 1.33 -- 5.11

50 2.12 1.78 1.37 -- 5.27

Pr > F <0.01 0.93 0.47 -- 0.15

LSD_0.05 0.07 NS NS -- 0.24

Fond du Lac 2000

0 1.24 1.94 1.31 -- 4.99

25 1.27 1.94 1.32 -- 5.03

Pr > F 0.84 0.92 0.89 -- 0.86

LSD_0.05 NS NS NS -- NS

H NS, not significant.

The ICP tissue analysis for Manitowoc 1999, Manitowoc 2000, and Dodge 2000 show that S concentrations below 0.21 to 0.23% are deficient and responses are likely, whereas values above 0.23% were sufficient and responses did not occur. Sulfur treatment had few other consistent or meaningful impacts on tissue levels of other nutrients. The forage quality analysis for Manitowoc in 1999 illustrates that while S addition, when deficiency existed, can increase protein content, it had little effect on other quality parameters (data not shown).

In addition, we asked crop consultants, county faculty, and industry agronomists to collect alfalfa tissue and soil samples from fields that had not received sulfur fertilizer or manure for the past 2 to 3 years. Fifty three sites were included in the survey in 2000. Seventeen of the samples collected contained less than 0.23%, which was arbitrarily chosen to separate sites designated AS deficient@ from AS sufficient@ sites. Unfortunately, we did not receive any samples from many counties in southern and western Wisconsin we want to include in this survey.

ADeficient@ samples were received from Sheboygan (4), Iron (1), Fond du Lac (1), Pierce (3), Manitowoc (1), Dunn (8), and Waupaca (1) counties. Average sulfur availability index (SAI) for the Adeficient@ sites was 32.9 (when two apparent anomalies were excluded) compared to 40.3 from the other sites. In addition, all but five of the sufficient sites had SAI greater than 35 and the other five were between 30 and 35. This shows that the SAI seems to work quite well although there is a slight indication that the estimate of precipitation S may be somewhat too high for some of the eastern Wisconsin counties.

Collectively these data do suggest that the potential for S responses is higher in southern and eastern Wisconsin than it was a few years ago. On soils where manure or S fertilizer have not been applied for 2 or 3 years, where soil organic matter is less than 3%, where a high S-demanding crop is being grown, and where there is a tendency toward sandiness, there appears to be some potential for sulfur responses. In general, the SAI appears to work very well in that values below 30 to 32 indicate a clear S need and high potential for response and values above 40 are very likely unresponsive. As has been our recommendation in the past, any uncertainty about the need for sulfur can be addressed by doing plant tissue analysis. It is an excellent confirmation tool.

Acknowledgments C

Significant parts of this work were dependent upon cooperation in conducting the on-farm trials and collecting samples for the sulfur survey. Our sincere appreciation is expressed to the following cooperators for their help:

Mike Ballweg Scott Hendrickson

Shawn Eisch Greg Kern

Kevin Erb Mike Rankin

Jim Fanta Joe Stellato

Jim Faust Tom Syverud

Freedom Agricultural Education Department

References C

Andraski, T.W., and L.G. Bundy. 1989. Sulfur, nitrogen and pH levels in Wisconsin

precipitation. J. Environ. Qual. 19:60-64.

Hoeft, R.G., L.M. Walsh, and D.R. Keeney. 1973. Evaluation of various extractants for

available soil sulfur. Soil Sci. Soc. Am. Proc. 37:401-404.

Hoeft, R.G., and L.M. Walsh. 1975. Effect of carrier, rate, and time of application of S

on the yield, and S and N content of alfalfa. Agron. J. 7:427-430.