Sulfur can hold the limit on a crop’s potential

This week, the “nutrient back to the basics” series will discuss the secondary nutrient — sulfur.

For much of the world, sulfur has been considered of secondary interest compared to nitrogen, phosphate and potassium.

However, as organic matter was depleted and canola, which is a very heavy user of sulfur, grew in popularity, sulfur deficiencies became more common on the Plains and Prairies.

Very few canola crops grown in Western Canada don’t have an application of some sort of sulfur fertilizer. In fact, in the productive black soil zones and irrigated land, sulfur is routinely applied to all crops.

So what about Liebig’s law of the minimum in respect to sulfur fertilization?

In most cases, sulfur behaves like the other nutrients. Often, the majority of the yield response occurs with the first 10 to 20 pounds of sulfur that is applied. However, in some situations, we have seen what appears to be an interaction between sulfur and nitrogen. Research has established that in some cases the application of either nitrogen or sulfur without the other did not produce a yield increase. Only when both were applied did significant yield increases occur.

When it comes to sulfur, the law of diminishing returns usually shows a levelling out of yields quite quickly. Because of this, the point where economic returns are at or below zero often occurs at rates of less than 20 lb. per acre, unless the soil is very deficient across the whole field.

However, due to the wide variation that one finds across a field, and the fact that a sulfur deficiency will result in virtually no yield, growers have learned that applying sulfur across a field, especially when growing canola, is good insurance and often makes economic sense.

While more complicated than the 4Rs of potassium management, the 4Rs of sulfur management are still quite simple.

The first one is the right source

The first thing about sulfur products is that there are really only two products: sulfate (S04) and elemental (SO). There is a big difference in the way these two sources work:

  • Sulfate sulfur is the only form of sulfur the plant can use immediately.
  • Sulfate sulfur may be leached out of the rooting zone.
  • Elemental sulfur is dependent upon time, temperature and moisture to be converted to sulfate.
  • Elemental sulfur is very acidifying, while sulfate products will vary in their ability to acidify the soil.

Ammonium sulfate (20 or 21-0-0- 24) – (NH4)2SO4 AMS

This is often sourced as a byproduct from industrial processes where ammonia is used as a reactant to strip hydrogen sulfide in flue gas desulfurization systems.

The process prevents sulfur from smoke and gas pollutants from going into the air. It also is manufactured by reacting anhydrous ammonia with sulfuric acid.

Whichever method is used, the end 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 in the seed row or banded down.

Ammonium sulfate is very useful and flexible as a fertilizer. It provides nitrogen and sulfur 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 seed row applied at seeding or as a top-dress material onto crops as late as the bolting-stage of canola.

A high percentage of the sulfur is almost immediately available to the crops. Leaching losses of sulfates can occur in well-drained soils, especially when fall applied. Ammonium losses can occur in the form of volatilization in soil containing less than two percent calcium carbonate (lime), which is likely to be found in the eastern Prairies when pH levels are above 7.5.

Amidas (40-0-0-5)

Amidas is a homogenous granule containing urea and ammonium sulfate. Because its nitrogen to sulfur ratio matches most crop use, Amidas is an ideal product for banding. It should not be used for broadcasting unless treated with a urease inhibitor.

Sulfate of potash-magnesia (0-0-22-10.5Mg-22S)

This was mentioned as a potassium product in the last column. It is mined from deposits in New Mexico and is commonly referred to as K-Mag or Sul-Po-Mag.

Potassium-magnesium sulfate has a higher cost per unit of sulfur than ammonium sulfate. It also contains 10.5 percent magnesium and 22 percent sulfur in water-soluble form. It can be useful when all three nutrients are required.

Sulfate of potash (0-0-50-17S) K2SO4

Manufactured by combining muriate of potash (0-0-60) with sodium sulfate (K2SO4) it contains both potassium and sulfate. It is used mainly on crops sensitive to chloride, such as tobacco, potatoes and some vegetable crops.

Ammonium phosphate sulfate (16-20-0-13)

A homogenous granule containing monoammonium phosphate (MAP) plus ammonium sulfate (AMS), this product has been used for years in high-sulfur-demanding crops such as canola. Now mainly used in turf, it occasionally shows up in agricultural applications.

40 ROCK (12-40-0-6)

This is a homogenous granule containing ammonium phosphate sulfate and zinc sulfate. It has many of the advantages of the products already listed.

Many metal-based micronutrients such as iron, copper manganese and zinc are commonly sold as sulfate products. These forms are plant available and will provide some sulfates, but because they are used in very small amounts, the amount of sulfate applied is inconsequential.

The other group of sulfur products available are classified as elemental (SO), which is an inert product.

In order to become plant available, it has to undergo oxidation. Since the oxidation of sulfur to SO4 is a biological process mainly accomplished by thiobacillus bacteria, conditions must be favourable for growth of the organisms in order for oxidation to proceed at optimum rates.

The following environmental conditions have been shown to have an influence on the rate of sulfur oxidation:

  • Particle size — The finer the particle size, the larger the surface area exposed to soil micro-organisms and the more rapid the oxidation process. This has been shown to be the largest single factor on the rate of sulfur oxidation.
  • Temperature — Optimally between 12.5 and 35 C.
  • Soil moisture and aeration — Sulfur-oxidizing bacteria require oxygen, and any condition that reduces the oxygen supply in soil will reduce the activity of sulfur-oxidizing bacteria. Oxidation of sulfur is most efficient at moisture levels close to field moisture capacity. Both waterlogging and excessively dry conditions greatly reduce the rate of oxidation.
  • Presence of thiobacillus organisms — Effective oxidation of sulfur in years following the initial application show that 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 sulfur 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 higher sulfur oxidation rates.

There are a variety of sulfur products available. Most contain 85 to 95 percent finely ground sulfur combined with bentonite clay, making up the other five to 15 percent. This clay swells when wetted, forcing the fine sulfur particles to disperse, making them more available for oxidation by the bacteria.

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

The third group of sulfur products are combinations of the two groups mentioned previously in this column in various formulations and combinations. Some of these were discussed previous in the phosphate column.

Ammonium thiosulphate (12-0-0-26) or ATS H8N2O3S2

This product is the standard fluid or liquid sulfur source. It contains both sulfate and elemental portions of sulfur in approximately a 1:1 ratio. This can offer both advantages and disadvantages. The oxidation rate of elemental sulfur from a liquid application of ATS would depend on the same factors affecting the oxidation of powder elemental sulfur.

In sandier, lighter soils, this can mean that only the sulfate portion of sulfur is susceptible to leaching in a heavy rain event. However, it also means that only half of the sulfur 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 to the atmosphere. It should be noted that this effect is small compared to a product such as Agrotain.

MES – MicroEssentials S15 (13-33-0-15S); SZ 1 (2-40-0 10S 1Zn): S10 12-40-0-10S

It is comprised of monoammonium phosphate (MAP) plus ammonium sulfate (AMS) and elemental sulfur in equal proportions as a sulfur and phosphate fertilizer source. This product is promoted as an all-in-one alternative to a MAP and ammonium sulfate (AmSul) blend. While safer in the seed row versus such a blend, it is not as seed row-safe as MAP alone. Again, the advantages and disadvantages discussed above exist.

Tiger 50 (12-0-0-50)

It is a physical blend of sulfur in the form of 0-0-0-90 and ammonium sulfate (21-0-0-24) in a sulfur to sulfate ratio of approximately three to one.

One of the selling points of this product is that because such a high percentage of sulfur is in the elemental form, seed row safety is greatly improved. However, the banding or seed row application of sulfur products greatly reduces conversions of the sulfur to plant useable forms.

The right rate

Like phosphorus and potassium, the right rate is going to depend on three factors.

The crop that is being grown is critical to the discussion. However, while canola is a heavy user of sulfur and it responds aggressively to sulfur applications, there are other crops that require it, albeit at rates lower than canola. The exception is alfalfa, which is also a large user of the nutrient.

The rate depends on soil test results, but sulfur is quite variable in many glacial till soils in Western Canada. Till soils often have sand or gravel areas below the surface, which over time may have reduced the amount of sulfur in the soil through leaching.

Sulfates are also found in many salts in the soil, including the two dominant salts in our soils: sodium and magnesium sulfates. These two salts account for the dominant 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 sulfates can result in a high sulfur result, even when much of the field is deficient. Always suspect your sulfate results if your electrical conductivity rating is above four deci-Siemens per metre.

Gypsum

This soft, sulfate mineral is composed of calcium sulfate with the chemical formula CaSO4-2H2O. It often is observed to precipitate out as crystals in our arid soils. A couple of these crystals can impact a composite sample.

Because of the variability, I usually use the following decision tree when making sulfur recommendations.

  • If the soil test recommendation is in the deficient level for sulfur, I follow the higher-end of the recommendation, especially with canola.
  • If the soil test recommendation is in the marginal level for sulfur, I recommend treating it as if it was deficient.
  • If the soil test recommendations show sufficient levels of sulfur, I look for reasons such as electrical conductivity to show why the test is showing high. I also talk to the grower to discuss the possible past applications of manure or sulfur products. I 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 electrical conductivity levels above four dS. per metre, I usually recommend sulfur applications.

Often this is 15 to 30 lb. of sulfate per acre when I have a 60 bushel per care canola crop targeted. I usually routinely make a recommendation of eight to10 lb. of sulfate per acre for other annual crops.

Alfalfa is a heavy user, and a shortage can result in the early demise of a strong stand. Apply 50 lb. of sulfate and 100 lb. of elemental sulfur the fall before seeding. Apply 50 lb. of either product every second year.

The right place for sulfur fertilizers will depend on the product. Sulfate fertilizers are very flexible. Broadcast and banded products work equally well and, within safe rate restrictions, seed row placed products do as well. With elemental sulfur, surface broadcast is the correct application. Banding these products, especially in reduced or no-till fields, may severely reduce the conversion to plant-available sulfate forms.

The right time for sulfur fertilizer application is very flexible. Except for sandy soils where leaching may occur, fall broadcasting or banding work as well as spring operations. Finally, top-dressing up to late vegetative stages works surprisingly well if rainfall or irrigation occurs shortly after applications. Elemental sulfur is completely different. To be most effective, these products should be applied by broadcasting in the fall without incorporation and leaving it exposed to the elements. Any other timing will compromise the product’s performance, except on established forages.

I have encouraged many farmers to move to an every-field-every-year approach with sulfur. Cereal crops such as wheat and oats benefit from sulfur, as do pulse crops, which also have a fairly high demand. One of the key functions of sulfur 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 sulfur application. I personally like to see an application of eight to10 lb. of sulfur with every crop and possibly doubling this rate with canola.

Thom Weir, C.C.A. is a former agrologist in the Yorkton, Sask., area. He can be reached by emailing thom.weir@farmersedge.ca.

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