Sulphur Fertilizer Application in Crop Production

Sulphur (S) is an essential plant nutrient required by all crops for optimum production. Plants take up and use S in the sulphate (SO4-S) form, which like nitrate (NO3-N), is very mobile in the soil and is prone to leaching in wet soil conditions, particularly in sandy soils.

Sulphur deficiencies are becoming increasingly common in Alberta. Deficiencies can be easily corrected with fertilizers containing sulphate (S04). Generally, S is the third most limiting soil nutrient in cereal, oilseed and forage crop production in Alberta. It is third only to nitrogen (N) and phosphorus (P) in fertilizer use in Alberta.

Background

Oilseed crops, particularly canola, and forage crops, have a higher S requirement than cereal crops. Table 1 provides examples of nutrient uptake and removal by wheat, canola, pea and alfalfa. Sulphur is required in the development of fertile canola flowers and must be present for good nodule development on legume forages such as alfalfa and pulse crop roots such as pea and faba bean.

In Alberta, an estimated 6 to 8 million acres are considered potentially S deficient for optimum canola production, and the potentially deficient areas are increasing due to increased crop yields and increased canola production, which is drawing down S soil reserves.

Soil organic matter is the primary source of plant-available SO4-S in surface soil. Soils that are sandy, low in organic matter and found in upper to mid-slope field positions are particularly prone to S deficiency since only a small amount of SO4-S is released from organic matter and is susceptible to leaching loss.

The subsoil of Brown and Dark Brown soils in southern and south central Alberta often have an abundance of gypsum, which is calcium sulphate (CaSO4). This mineral is an important source of plant-available S in these soils.

Figure 1. Soils map of Alberta

Effect of Sulphur on Crop Growth

Sulphur is essential in the structural and enzymatic components in plants. Sulphur is a key component of some essential amino acids and is needed for protein synthesis. Chlorophyll synthesis also requires S.

Sulphur is not readily translocated within plants, so all plants need a continuous supply of sulphur from emergence to crop maturity. Therefore, in S-deficient plants, older leaves may appear more healthy, while newer leaves and tissue may have stunted growth and a lighter green or even yellow appearance.

A sulphur deficiency at any growth stage can result in reduced crop growth and yield. Adequate S results in rapid crop growth and earlier maturity.

Sulphur Deficiency Symptoms

Sulphur deficiency symptoms vary between crops. In canola, deficiency symptoms may begin as early as the one-leaf stage (Figure 2a), with the newest leaves turning yellowish green with dark vein coloration. Leaves may take on a “cupped” appearance and later, a reddening from the leaf margins (Figures 2b and 2c).

Flowers are usually smaller and paler yellow, and they produce small, underdeveloped purplish-coloured pods (Figure 2d). Under mild S deficiency, there may be good vegetative growth, but flowers and pods will be underdeveloped.

For cereals and forage grasses, the yellowing of newly emerging leaves is a strong indicator of S deficiency (Figure 3). Depending on the degree of deficiency, the leaves may be a shade of light green to entirely yellow.

Yellowing of the new growth occurs because S is immobile in the plant; therefore, newly emerging leaves cannot “scavenge” S from older leaves. This situation is in contrast to nitrogen deficiency symptoms in which the older leaves turn yellow first because nitrogen is translocated or “scavenged” from older leaves to support new growth.

Figure 2. Sulphur deficiency symptoms in canola.

2a. Sulphur deficient seedling.

2b. Leaf cupping.

2c. Dark vein coloration and purpling of leaf margins.

2d. Small pale flowers and poor pod development.

Sulphur deficiency in alfalfa results in shorter plant stature with pale green to yellow leaves (Figure 4a and b). The yellowing is often, but not always, localized to newly emerging leaves. Sulphur deficiency can result in nitrogen deficiency in alfalfa since S is required for health and function of nitrogen-fixing nodules. Sulphur deficiency in alfalfa occurs more commonly and is more severe in Gray Wooded soil.

Figure 3. Yellowing of newly emerging leaves is an indicator of sulphur deficiency in wheat, other cereals and forage grasses.

Figure 4. Sulphur deficiency in alfalfa.

4a. Deficient alfalfa plant on left is shorter in stature with yellowing most severe on young leaves.

4b. In the field, sulphur-deficient alfalfa on the right.

Similar symptoms
Great care must be exercised when diagnosing nutrient deficiencies, and using visual symptoms alone can be very misleading. The diagnosis of problems must include using soil and tissue sampling and the analysis of affected and unaffected areas of a field to confirm a nutrient deficiency.

Sulphur deficiency can be similar to and confused with many other stressors that cause symptoms in canola similar to those caused by S deficiency. For example, Group 4 herbicides such as 2,4-D or dicamba (Banvel) at low, sub-lethal rates from wind drift, tank contamination or even precipitation can cause leaf cupping in canola (Figure 5a). However, the leaves in this figure lack the dark vein coloration associated with S deficiency.

Like S deficiency, Group 2 herbicides can cause purpling of canola leaves, but unlike the leaves of S-deficient canola, the purpling does not develop from the leaf margins.

Figure 5b shows Muster (ethametsulfuron-methyl) injury on canola. Muster is a registered herbicide (Group 2) on canola, but injury could occur if a heavy rainfall occurs shortly after spraying, facilitating the uptake of the herbicide via the roots.

Figure 5a. Canola leaf cupping from Banvel injury.

Figure 5b. Purpling of older leaves resulting from Muster sprayed just prior to heavy rainfall.

Other Group 2 herbicides such as Ally (metsulfuron-methyl) or Pursuit (imazethapyr) can carry-over in the soil and cause purpling of the older canola leaves. If Group 2 herbicides are sprayed directly on canola as a tank contaminant, purpling of the youngest leaves may occur.

Boron deficiency can produce thin purple pods in canola, which are hard to distinguish from S-deficiency symptoms. Fortunately, boron deficiencies in Alberta soils are rare. Sun scald produces purpling, but the discoloration is confined to the “sunny” side of stems and pods.

Indirect Causes of Sulphur Deficiency

Sulphur deficiency in canola can occur despite the adequate application of S fertilizer. Any stressor causing root pruning or restricted root growth can reduce the plant’s ability to take up nutrients, which can potentially induce S and other nutrient deficiencies.

Herbicide carry-over, acidic soil or a shallow hard pan soil layer can cause restricted root growth and restrict sulphate-S uptake, leading to deficiency symptoms. Root rot diseases, some of which are preventable with seed treatment, can cause the development of sulphur deficiency in canola (Figure 6).


Figure 6. Canola grown from bare seed (left) versus Vitavax-treated (right ).


Inset: Close-up of canola grown from bare seed showing sulphur deficiency resulting from seedling blight.

Soil Testing for Sulphate-Sulphur

Plant-available S in soil is determined by measuring the soluble sulphate-sulphur content. This soil test provides a reliable basis for making S fertilizer recommendations as long as representative soil samples can be obtained from the field.

Ideally, samples should be taken just before seeding. However, late fall sampling is usually satisfactory to provide reliable fertilizer recommendations when samples are taken after the surface soil temperature drops below 5° C (generally after mid-October).

A soil sampling probe should be used to obtain samples. Take soil samples from 0 to 6, 6 to 12 and 12 to 24 inches (0 - 15, 15 - 30 and 30 - 60 cm) depths. Fertilizer recommendations for phosphorus and potassium are based on the 0 to 6 inch depth.

Soil testing laboratories will provide fertilizer recommendations for S based on a 0 to 6 inch or 0 to 6 plus 6 to 12 inch depth; however, this approach may provide a higher than reasonable recommendation because available S in the subsoil is not taken into consideration. With each extra soil sample taken from a greater soil depth (6 - 12 inch, 12 - 24 inch), the fertilizer recommendations for S become more precise.

Even with proper sampling depth, obtaining a representative sample from the field can be a challenge. Studies across all soil zones have found the level of crop-available S can vary greatly within a field.

For example, grid sampling (0 - 60 cm) within a field near Stettler found sulphate levels varied from 9 to 6,330 ppm with the average being 588 ppm. Based on a random composite sample, no fertilizer recommendation for S would be made for this field. However, the mode (the value of most frequent occurrence) was 11 ppm. Based on this number, S fertilizer would be recommended. If the soil test results from the composite sample were followed, the majority of this field would be lacking adequate S. Obtaining a representative sample from fields highly variable in available S is difficult.

The benchmark or topographic system of soil sampling can help with fertilizer decisions. For benchmark sampling, the location selected must be representative of the field; if it is not representative, very misleading information could lead to over or under application of sulphur fertilizer.

With the benchmark system, several representative sites in a field should be selected, and each site should be about 100 feet by 100 feet (30 by 30 m). Each year, 15 to 20 sample cores are taken from each benchmark site. Provided the benchmark sites are representative, this system gives a good indication of the nutrient status of the field and how it varies from year-to-year.

Finding good benchmark sites may be difficult. However as a general rule, top slope and sandy soils are often associated with S deficiency and should be sampled separately from low relief or heavy textured soils.

Sulphur Fertilizer Types

Sulphur fertilizers are most commonly available in either the soluble sulphate form or elemental forms (S°), which are relatively insoluble. In Alberta, products containing ammonium sulphate (21-0-0-24; 20-0-0-24) are commonly used and are excellent on deficient soils. Other products such as potassium sulphate can be used to correct both potassium and sulphur deficiencies but this fertilizer product is not commonly available.

Products comprised of elemental So, such as 0-0-0-90 (Tiger 90CR) and 0-0-0-95 (SulFer 95), are becoming increasingly popular, but elemental products must be carefully managed to be effective. Other newer products contain both sulphate-sulphur and elemental S such at the MicroEssentials® S15™ (13-33-0-15S).

Provided there is adequate soil moisture, ammonium sulphate fertilizers dissolve quickly to release SO4-S, the sulphate form readily available to plants. As mentioned, SO4-S is quite mobile in the soil and is the only form of S directly utilized by plants. Plants cannot take up elemental S. These fertilizers must be converted to sulphate-S first before plant uptake can occur.

Application Methods

Because ammonium sulphate fertilizers are so rapidly available to crops, they can be applied to crops in various ways. Normally, the uptake efficiency of soluble sulphate fertilizer is better when banded in soil versus broadcast and incorporation.

When sulphate-S is sided banded near the seed or seed-placed, uptake efficiency tends to be best. Further, banded S fertilizer may favour crop growth versus weed growth, since this method places the fertilizer close to the crop roots.

Normally broadcast application onto snow is not recommended. Some sulphate loss due to redistribution in runoff may occur if ammonium sulphate fertilizers are broadcast on snow deeper than 4 inches (10 cm) covering frozen ground, particularly on slopes greater the 4 per cent.

If detected early enough, S deficiency can be corrected in many crops by broadcasting ammonium sulphate. In the case of canola, broadcasting ammonium sulphate until the early flowering stage may help correct an S deficiency provided there is enough rain to dissolve and move the SO4-S into the root zone (Figure 7). The greatest risk from using soluble sulphate fertilizers occurs on sandy soils subjected to heavy rainfall. Under these conditions, SO4-S may leach below the rooting zone.

Figure 7. Canola on the left was rescued from sulphur deficiency by applying 100 lbs/ac of 20-0-0-24 applied at early flower.

For fertilizer manufacturers, elemental sulphur (S°) has the advantages of lower production costs and because of its high analysis, lower transportation costs. In addition, the application of S° does not add any extra nitrogen, which could reduce yields of legumes such as alfalfa or pulse crops such as peas. High soil nitrogen can reduce legume yields by interfering with proper nodule formation and nitrogen fixation.

Elemental S° fertilizer must be oxidized by soil microbes to SO4-S before it is available to crops. Typically, it takes considerably more time for S° to become available compared to soluble sulphate forms of fertilizer. The rate of conversion from S° to plant-available SO4-S depends on two factors: the particle size to which the product degrades and the method of application.

Alberta research has determined that S° granules that break down into particles smaller than 150 Microns (10-6 m) convert more quickly to sulphate, since particles of this size or smaller provide sufficient surface area for microbes to act on and solubilize S. After two simulated wet/freeze cycles and one rainfall, SulFer 95 (0-0-0-95) particles are all <74 microns whereas only 25 per cent of Tiger 90 particles were <150 microns.

The evolution of elemental S° products has been rapid, and a number of elemental S° fertilizers are now available. Industry is continually striving to improve the dispersing nature of their S° products, making it difficult to give current comparisons between these various S° products.

For best management, regardless of brand, all S° products should be surface broadcast without incorporation at least a year before benefits are needed. Elemental S° placed on the soil surface will easily disperse on the surface and is more easily oxidized by soil microbes because rainfall and freeze/thaw cycles break down and disperse the S° granules.

Banding or immediate broadcasting followed by incorporation is an inefficient method of applying S° and is not recommended. Banding in soil or incorporating S° immediately after broadcasting will prevent S° granules from breaking down and dispersing. The surrounding soil would keep the S° granule intact, reducing the exposed S° surface area and vastly reducing the conversion rate to SO4-S. This situation would greatly reduce the microbial oxidation process and slow the release of S° to plant-available sulphate. Even with broadcast applications, the rate of conversion from S° to SO4-S can vary from less than a year to several years or more.

Producers must use care in deciding whether to use ammonium sulphate fertilizer versus elemental S°. Various Alberta studies have found canola yields to be significantly greater when fertilized with a spring-banded application of ammonium sulphate (20-0-0-24) compared to spring or even fall broadcast Tiger (Figure 8).