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Back to Basics Soil Fertility Part 4: Sulphur By Thom Weir, Senior Precision Agronomist For part four of Back to Basics Soil Fertility focus on the “secondary” nutrient: Sulphur. For much of the world and up until the 1980’s, Sulphur (S) or (Sulfur for those of you south of the 49th parallel) has been considered of secondary interest as compared to Nitrogen, Phosphate and Potassium. However, as organic matter depleted, and canola acres grew – a crop that is a heavy user of Sulphur– deficiencies became more common. That brings us to today where there are very few canola crops grown in western Canada without the application of some sort of Sulphur fertilizers. In fact, in the productive black soil zones and irrigated land, Sulphur is routinely applied to all crops.

What about Liebig’ Law of the Minimum in respect to Sulphur fertilization? In most cases Sulphur behaves like the other nutrients. Often most of the yield response occurs with 10 – 20 lbs of S. However, in some situations, we have seen what appears to be an interaction between Sulphur and nitrogen. As illustrated below, the application of either N or S without the other, did not produce a yield increase. Only when both were applied, did significant yield increases occur.

Nitrogen / Sulphur Interaction on S Deficient Soils Nyborg, M .et al 1974.

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5

10

15

20

25

30

35

No Fert. N-P-K P-K-S N-P-K-S

12.7

6.3

14.3

34.3

Canola Yield

(No S) (No N)

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A review of Sulphur fertilizer management, Malhi et al 2004

The Law of Diminishing Returns With Sulphur, yields level out quite quickly. Because of this, the point where economic returns are at or below 0°C often occur at rates under 20 lbs/acre, unless the soil is very deficient in S across the whole field. Due to the wide variation across a field and the fact that a Sulphur deficiency will result in vertically no yield, growers have learned that applying Sulphur across a field– especially when growing canola– is a good insurance application that often makes economic sense. The Right Source While more complicated than the 4R’s of Potassium management, the 4R’s of Sulphur management is still quite simple. Again, the first R we will look at is The Right Source. The first thing about Sulphur products is that there are only two products – Sulphate (S04) and Elemental (So). There is, however, a big difference in the way these two sources work, including:

• Immediately available to plants as Sulphate-Sulphur is the only form of S the plant can utilize.

• Sulphate Sulphur may be leached out of the rooting zone. • Elemental S is dependent upon time, temperature and moisture to be converted to

sulphate form that is available to the plant. • Elemental S is very acidifying whereas Sulphate products will vary in their ability to

acidify the soil.

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Ammonium Sulphate (20 or 21-0-0-24) (NH4)2SO4 AMS. This is often sourced as a bi-product from many industrial processes where ammonia is used as a reactant to strip hydrogen sulphide in flue gas desulphurization systems. This process prevents Sulphur from smoke and gas pollutants going into the air. It also is directly manufactured by reacting anhydrous ammonia with sulfuric acid. Whichever method is used, the result is either a crystalline or a granular product. The crystalline product is most commonly broadcast while the granular products are either broadcast or blended with other fertilizer products and applied as a seed-row or banded product. Ammonium Sulphate is very useful and flexible as a fertilizer product. It provides nitrogen and Sulphur in a single product. It is highly soluble and usually very flexible in application timing. It has been successfully broadcast or banded in fall or spring applications, as seedrow applied at seeding or as a top-dress material onto crops as late as the “bolting stage” of canola. A high percentage of the Sulphur is almost immediately available to the crops. Leaching losses of sulphates can occur in well drained soils especially when fall applied. Ammonium losses can occur in the form of volatilization soil contained >2% calcium carbonate (lime) which are likely to be found in the eastern prairies when pH levels are above 7.5. Amidas™ (40-0-0-5) is a homogenous granule containing urea and ammonium sulphate. Because of its N:S ratio matches most crop usage, Amidas is an ideal product for banding. It should not be used for broadcasting unless treated with a urease inhibitor product. In my previous post, I mentioned Sulphate of potash-magnesia (0-0-22-10.5Mg-22S) as a potassium product. It is mined from deposits in New Mexico and is commonly referred to as K-Mag or Sul-Po-Mag. Potassium-magnesium sulphate has a higher cost per unit of S than ammonium sulphate but as It also contains 10.5% magnesium and 22% Sulphur in water-soluble form it can be useful when all three nutrients are required. Sulphate of Potash (0-0-50-17S) K2SO4 is manufactured by combining Murate of Potash (0-0-60) with sodium sulphate. The resulting product contains both potassium and sulphate. It is used mainly on crops sensitive to chloride such as tobacco, potatoes and some vegetable crops. Ammonium Phosphate Sulphate (16-20-0-13) is a homogenous granule containing monoammonium phosphate (MAP) plus ammonium sulfate (AMS). This product has been used for years in high Sulphur demanding crops such as canola. While now mainly used in turf, it occasionally shows up in agricultural applications.

40 ROCK™ (12-40-0-6) is a homogenous granule containing Ammonium Phosphate Sulphate and Zinc Sulphate. This product has many of the advantages of products listed above.

Many metal-based micro-nutrients such as iron (Fe), copper (Cu) Manganese (Mn) and zinc (Zn) are commonly sold as sulphate products. These forms are plant available and will provide some sulphates but as they are used in very small amounts, the amount of sulphate applied is inconsequential.

The other group of Sulphur products available are classified as Elemental (So). So itself is an inert product. To become plant-available, So must undergo oxidation. Since the oxidation of Sº to SO4 is a biological process mainly by Thiobacillus bacteria, conditions must be favorable for

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growth of the organisms for oxidation to proceed at optimum rates. The following environmental conditions have been shown to have an influence on the rate of Sº oxidation:

• Particle size - The finer the particle size, the larger the surface area exposed to soil microorganisms and the more rapid the oxidation process. This has been shown to be the largest single factor on the rate of Sº oxidation.

• Temperature - optimum between 12.5 and 35 oC. • Soil moisture and aeration - Sº-oxidizing bacteria require oxygen: any condition that

reduces the oxygen supply in a soil will reduce the activity of Sº-oxidizing bacteria. Oxidation of So is most efficient at moisture levels close to field moisture capacity. Both waterlogging and excessively dry conditions greatly reduce the rate of So oxidation.

• Presence of Thiobacillus organisms – studies have shown more effective oxidation of So

in subsequent years following the initial application when the levels of Thiobacillus organisms have increased in the soil.

• Fertility status of the soil - Sulfur-oxidizing bacteria require the nutrient package that plants require. Studies have shown that oxidation of So proceeds more rapidly in fertile soils. However, there may be competition between the bacteria and plant roots for nutrients and this has been found to cause temporary nitrogen deficiencies in plants under high Sº oxidation rates.

There are a variety of So products available. Most contain 85 – 95% finely ground So combined with bentonite clay (5-15%). This clay swells when wetted, forcing the fine So particles to disperse, making them available for oxidation by the bacteria.

Because of the requirement for So to be oxidized, and the factors involved, the amount of plant-available sulphate tends to be quite variable. As a result, much higher rates are often applied compared to sulphate rates to ensure there is product available for the plants in season.

The third group of Sulphur products are combinations of the two groups above – in various formulations and combinations. I wrote about these in this post.

The first product is Ammonium Thiosulphate (12-0-0-26) or ATS. H8N2O3S2. This product is the standard fluid or liquid Sulphur source. It contains both sulphate and elemental portions of S in approximately a 1:1 ratio. This can offer both advantages and disadvantages. The oxidation rate of elemental Sulphur from a liquid application of ATS would depend on the same factors affecting the oxidation of powder elemental sulfur. In sandier (lighter) soils, this means that only the sulphate portion of S is susceptible to leaching in a heavy rain event.

However, it also means that only half the Sulphur rate applied is immediately available to the crop. ATS has also been shown to be a weak urease inhibitor when used with UAN (28-0-0). It can slow the rate of urea hydrolysis, slow the conversion of urea to ammonium (NH4

+), and reduce volatilization losses of ammonia (NH3) to the atmosphere. It should be noted that this effect is small compared to a product such as AgroTain.

MES - Micro Essentials® S15 (13-33-0-15S); SZ 1 (2-40-0 10S 1Zn): S10 12-40-0-10S product comprised of monoammonium phosphate (MAP) plus ammonium sulfate (AMS) and elemental S in equal proportions] as an S and P fertilizer source. This product is promoted as an all in one alternative to a MAP/Ammonium Sulphate (AmSul) blend. While safer in the seedrow

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versus such a blend, it is not as seedrow-safe as MAP alone. Again, the advantages and disadvantages discussed above exist.

Tiger 50 (12-0-0-50) is a physical blend of So in the form of 0-0-0-90 and Ammonium Sulphate (21-0-0-24) in a So: Sulphate S ratio of approximately 3:1. One of the selling points of this product is because such a high percentage of S is in the So form, seedrow safety is greatly improved. However, the banding or seedrow application of So products greatly reduces conversions of the S to plant useable forms.

The Right Rate Like P and K, the Right Rate is going to depend on 3 factors. The first factor is the crop that is being grown. Of course, the first crop we think of when discussing Sulphur is canola. However, while canola is a heavy user of S and while it responds aggressively to Sulphur applications, there are other crops that require S, albeit at rates lower that canola. The exception is Alfalfa in that its Sulphur requirements are quite high. The Right Rate is going to depend on the soil test. The problem with Sulphur however, is that Sulphur is quite variable in many glacial till soils in western Canada. This is caused by a few factors. Till soils may have sand or gravel areas below the surface which, over years, may have reduced the amount of Sulphur in the soil by leaching. Sulphates are also found in many salts in the soil including the two dominant salts in our soils – sodium sulphate and magnesium sulphate. These two salts account for the dominate causes of salinity in our soils. When sampling, if a couple of samples are taken from moderately saline areas, their relatively very high levels of sulphates can result in a high Sulphur result even when much of the field is deficient. Always be suspect of your S results if your E.C. (salinity rating) is above 4 dS/m, I start to suspect sulphate salts may be influencing the results. Gypsum is a soft sulfate mineral composed of calcium sulfate, with the chemical formula CaSO4·2H2O. It often is observed to participate out as crystals in our arid soils. A couple of these crystals in a sample can impact the composite sample. Because of the variability, I usually use the following decision tree when making Sulphur recommendations.

1. If the soil test recommendation is in the deficient level for Sulphur, I follow the high end of the recommendation – especially with canola.

2. If the soil test recommendation is in the marginal level for Sulphur, I recommend as if it was deficient.

3. If the soil test recommendations show enough levels of Sulphur, I look for reasons such as E.C. to show why the test is showing high. I also talk to the grower to discuss the possible past applications of manure or So product. I also compare this field to others to see if the levels are consistently high. If I can’t find a reason for the high levels or if I see E.C. levels above 4 dS/m, I usually recommend Sulphur applications.

This usually means I recommend 15 – 30 lbs Sulphate S/acre when I have a 60-bushel canola crop targeted. I usually routinely make a recommendation of 8-10 lbs Sulphate S / acre for other annual crops.

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The other crop I mentioned is Alfalfa. It is a heavy user and may result in the early demise of a strong stand. My rule of thumb is to apply 50 lbs sulphate S or 100 lbs So the fall before seeding. I then recommend applying 50 lbs either product every second year. The Right Place The Right Place for Sulphur fertilizers will depend on the product being selected. Sulphate products are very flexible. Broadcast and banded products work equally as well and within safe rate restrictions, seedrow placed products also work well. With Elemental Sulphur (So), surface broadcast is the correct application. Banding these products, especially in reduced or no-till fields may severely reduce the conversion to plant-available sulphate (SO4) forms. The Right Time And finally, the Right Time for Sulphur fertilizers. Sulphate products are very forgiving. Except for sandy soils where leaching may occur, fall broadcasting or banding work well as do spring operations. At seeding applications as side or midrow banding work well as does seedrow applications within recommended limits. Finally, top dressing up to late vegetative stages works surprisingly well if rainfall or irrigation occurs shortly after applications. Elemental Sulphur (So) is completely different. To be most effective, these products should be applied by broadcasting in the fall without incorporation and leaving them exposed to the elements. Any other timing will compromise the product’s performance (except on established forage). I have encouraged many farmers to move to an “Every Field – Every Year” approach to work well with Sulphur. Cereal crops like wheat and oats benefit from Sulphur as do pulse crops which have a high demand. One of the key functions of S in a plant is in the synthesis of oils. Because of its importance with this process, any oilseed crop including flax and soybeans benefit from a Sulphur application. I personally like to see an application of 8-10 lbs of S with every crop and possibly doubling this rate with canola.

Sulphur Deficiency in an Oat Field near Saltcoats, Sk.

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Sulphur Deficiency in an Oat Field near Saltcoats, Sk.