by Joel Bagg, Forage Specialist, & Bonnie Ball, Soil Fertility Specialist, OMAFRA
Sulphur (S) received from atmospheric sulphur dioxide emissions (acid rain) in Ontario has steadily declined by over 50% during the last 25 years. We are beginning to see yield responses in more situations when applying S to alfalfa. Sometimes the response is dramatic, while in other situations there is no response. Tissue sampling of alfalfa is a useful diagnostic tool in predicting whether there will be an economic response to applying S.
S availability varies from site-to-site and from year-to-year according to temperature and rainfall. Soil organic matter plays an important role in providing available S to plants. Sulphate is very mobile in soils, similar to nitrate, and can be leached into the subsoil and become unavailable to plants (but not as easily as nitrate). S deficiencies have also increased due to some reductions in organic matter, and higher crop and protein yields. There is considerable S in manure. S deficiencies are more likely to occur on low organic matter soils, and soils that have not had a manure application within a couple of years. Within fields, sulphur deficiency symptoms may show up first on eroded knolls and other low organic matter areas.
What Does S Deficiency In Alfalfa Look Like?
Alfalfa has the highest S requirements of any of the field crops. A 4 ton/acre crop of alfalfa removes about 20 lbs/ac of S. S deficient alfalfa plants will be spindly and uniformly light green or yellowish (as opposed to a yellow top and green bottom, etc), with weak growth. (Figure 1)
How Do I Know If Have A Deficiency?
There currently is not a reliable soil test for S in Ontario. Sulphate levels are quite variable, and may be leached from the soil between soil sampling and plant growth.
Tissue testing of alfalfa (at mid-bud to early-flower stage) is considered a suitable diagnostic approach for determining S deficiencies. Sample the top 6 inches of 35 stems and send them to a laboratory for tissue analysis. The critical level below which alfalfa is considered S deficient and may benefit from applying sulphur is 0.25%. If a check is desired, take a similar sample from an area with no visual S-deficiency symptons.
A 2012 field survey of Ontario alfalfa stands indicated that 21% of fields had S- tissue analysis below this level. Put another way, 79% of these fields would have been unlikely to have an economic response to applying sulphur. It is also noteworthy that 37% of these fields tested below the critical K value of 1.7%, almost twice as many than were S deficient. Neglecting K fertility, while attempting to improve S fertility is not an effective strategy.
What Form of S?
What is the most economical source of S to use with alfalfa? The sulphur must be in the sulphate form to be taken up by the plant. Sulphate fertilizers include:
- ammonium sulphate 21 – 0 – 0 – 24)
- potassium sulphate (0 – 0 – 50 – 18)
- sulphate of potash magnesia (Sul-Po-Mag or K-Mag) (0 – 0 – 22 – 20)
- calcium sulphate (gypsum) (0 – 0 – 0 – 17)
All are equally effective as sources of sulphate. Depending on what assumptions you make, current prices make S in the sulphate form worth about $0.90 or more per lb S. To determine the most economical source of sulphate, get some local price quotes and do the math.
Ammonium sulphate provides nitrogen which should not be needed by the alfalfa. K-mag and potassium sulphate also provide potassium which is usually also required in alfalfa, but potassium sulphate is difficult to source and more expensive in some areas. Gypsum can be a good source of sulphate, but has no advantage in improving soil pH. Thiosulphate liquid forms, ammonium thiosulphate (12-0-0-26) and potassium thiosulphate (0-0-25-17), are readily available, but liquids are less convenient for fertilizing alfalfa and generally more costly per unit of S than dry forms.
Elemental sulphur (0-0-0-90) consists of finely ground sulphur that has been pelletized, and must be converted by oxidation to sulphate by soil bacteria before plants can utilize it. The rate of availability depends on particle size, method of application and moisture. Incorporating it into the soil before establishment makes it more readily available. In some circumstances, 50% of the sulphur may be available in the year of application, while the remainder is more slowly available. Elemental sulphur is currently worth about $0.35 per lb S. Applying a single application of elemental sulphur rather than sulphate, supplies a cheaper S source over a longer period of time and reduces the need for annual applications. An application of 50 lbs/ac of S should last the life of a productive 3 year alfalfa stand.
When Should I Apply It?
Sulphate-S should ideally be applied in the spring at green-up to improve plant utilization, minimize losses due to leaching, and receive a first-cut yield boost (Figure 2). Elemental sulphur can be applied by:
- incorporating it into the soil with other fertilizer at establishment (Figure 3) , or
- blending it with P and K (and possibly boron) and broadcasting it after a cut.
How Much S Should I Apply?
A general thumb rule for S application on alfalfa is 5 lb/ac per ton of dry matter yield. Some S is still available in reduced amounts from atmospheric deposition and organic matter. The University of Wisconsin recommends 15 – 25 lbs/ac of S in the sulphate form broadcast on established stands annually, or 25 – 50 lbs/ac of elemental S incorporated at seeding. Research is required to verify these numbers in Ontario.
Sulphur deficiencies in alfalfa have been more common in the mid-western US and north-western Ontario, because they are located upwind of much of the sulphur producing industrial pollution that has been cleaned up. Ontario research on sulphur rates, source, and timing for alfalfa has been more limited. Results from recent research trials applying sulphate to alfalfa have been mixed. Some sites have shown no response to applying sulphur. However, the most responsive site showed a dramatic yield increase in an alfalfa-grass mix of 1.55 ton/ac, a crude protein increase of 4 percentage points, and a percentage of alfalfa in the harvested forage improved from 33 to 56%.
To confirm that an actual yield response has occurred, farmers may want to leave a test strip where no S is applied. (Figure 4)
Tissue test alfalfa fields showing potential deficiency symptoms to determine if S should be applied, especially fields with low organic matter soils and those that do not receive manure. Applying elemental-S bulk blended with other fertilizer is the most cost effective method of providing S. Spring applications of sulphate can provide a more immediate yield response.