Sulfur is one of the few elements that plants need in larger quantities. Along with nitrogen, potassium, phosphorus, calcium, and magnesium, sulfur rounds out the six macronutrients that plants need in sufficient amounts to maintain good health and achieve high yields. After nitrogen, potassium, and phosphorus, sulfur is the fourth most-needed nutrient plants require.
Like so many other nutrients, sulfur has a plethora of roles in proper plant function. It is a component of chlorophyll and necessary for protein synthesis, which helps plants to regulate the photosynthesis processes.
Sulfur is also needed for nitrogen fixation in plants that can pull nitrogen from the environment, meaning legumes and other nitrogen-fixing plants are particularly susceptible to sulfur deficiencies.
Sulfur and nitrogen are further linked in plant physiology as sulfur is responsible for the activation of key nitrogen enzymes. It is therefore not surprising that sulfur deficiencies are usually always addressed concurrently with nitrogen deficiencies.
Symptoms of Sulfur Deficiency in Plants
Sulfur is highly abundant in the soil. It falls to the earth from a wide range of both natural and man-made processes, such as volcanic activity and pollution. It is not, however, widely available to plants until it is mineralized. Sulfur is also easily leached out of soil during heavy rains or when exposed to irrigation. For this reason, it is not unusual for sulfur deficiencies to occur in plants.
Correctly identifying a sulfur deficiency can be difficult as it can look like other nutrient deficiencies and symptoms may not appear until the deficiency is quite severe. Sulfur is not highly mobile in a plant’s vascular system.
If a deficiency occurs, it will first present itself in the new growth. If the deficiency continues, it will then continue to the older growth. Plants affected by insufficient sulfur are hard to distinguish from plants deficient in nitrogen. Both cause yellowing of the leaves, but nitrogen deficiencies begin with the older leaves first.
To further complicate proper diagnosis, a plant may well suffer from both nitrogen and sulfur deficiencies concurrently. However, if caught before they spread through the plant, the tell-tale sign of dual problems will be a healthy-looking mid-section with chlorotic growth above and below. If not caught in time, it would be nearly impossible to distinguish the two without a lab-performed tissue analysis or by conducting a fertilizer test.
For a lab analysis, it is important to collect proper samples. In the absence of specific instructions from the lab, select up to 10 new-growth leaf samples and wash them off with distilled or deionized water before sending them. This way, the analysis won’t be tainted by any sulfur residue that may have fallen on the leaves from rain or other atmospheric conditions.
To perform a fertilizer test on a crop to determine if the deficiency is due to a lack of nitrogen or a lack of sulfur, multiple plants are needed. Obtain a fertilizer containing both nitrogen and sulfur, such as ammonium sulfate, and one that is just nitrogen, such as urea.
Apply an appropriate amount of the nitrogen-only fertilizer on one section of plants and an appropriate amount of the nitrogen plus sulfur fertilizer on the other. If both sections of plant respond favorably to the treatment, it was a nitrogen deficiency. If only the section treated with the nitrogen plus sulfur fertilizer responds, it was a sulfur deficiency.
Other signs that point to a sulfur deficiency include slow growth and delayed maturity of leaves. Sulfur-deficient plants may appear spindly and have thinning stems. These symptoms could be concurrent with the yellowing leaves or appear separately, depending on the plant species.
How Sulfur Deficiencies in Plants Occur
Across North America, the amount of available sulfur in cropland and the environment is generally declining. Some of this is by design and some is unintended. For example, the Clean Air Act and comparable policies reduced the amount of sulfur that fell back to earth as sulfur dioxide by reducing factory and power plant emissions.
Some modern agricultural practices, such as the reduced use of pesticides in crop reduction, has contributed to the higher incidence of sulfur deficiencies. Many pesticides, especially fungicides, contained sulfur. The higher level of refinement, which removes sulfur, in modern fertilizers has also potentially reduced the amount of available sulfur.
Other hypothesized contributors to the declining amount of sulfur in soils include intensive agricultural practices. Lands that are intensively cropped or that are not allowed to be fallow have lower levels of sulfur as a rule.
Also, higher-yielding crop varieties, whether created by genetic modification or hybridization, pull more sulfur out of the soil than heirloom and open-pollinated crops. Higher yields tend to create more crop residue, which is low in sulfur. The burning of crop residue or burning the vegetation on an area that is to be converted to crop production will likely cause a sulfur deficiency.
Sulfur in the soil is released into the atmosphere as a gas when burned. The advent of no-till crops and earlier plantings also contribute to this deficiency.
Soils that are over-fertilized with phosphorus tend to be deficient in sulfur as well, since abundant phosphorus can displace the sulfur. In general, soils that are too acidic, too sandy, and too silty, or have low levels of organic matter, tend to be deficient in sulfur too, as are soils at higher elevations.
How to Correct a Sulfur Deficiency in Your Garden
There are many products on the market to correct sulfur deficiencies. These range from elemental sulfur to any other product ending in “sulfate” or “sulfide,” including potassium sulfate, potassium magnesium sulfate, ammonium thiosulfate, ammonium polysulfide, ammonium sulfate, potassium thiosulfate, magnesium sulfate (aka Epsom salts), and zinc sulfate.
It is important to consider certain factors when selecting the appropriate formulation to correct a sulfur deficiency, however. Elemental sulfur (which is 90-95 per cent sulfur with a clay binder) is the least expensive and most effective solution, but only if sulfur is the only nutrient lacking.
Before applying any other type of sulfur amendment, however, make sure to perform a soil analysis so you don’t potentially add any unneeded nutrients to your soil. Of course, this can be beneficial. For example, if it is discovered the soil is also deficient in calcium, gypsum (calcium sulfate) can be applied to the area to address both the sulfur and calcium deficiencies.
For those that do not wish to apply a granular or manufactured fertilizer to their crops, there are other viable alternatives to treating sulfur deficiencies. Manure often contains high quantities of sulfur, as well as other beneficial nutrients.
Make sure that it has composted or aged before applying, though, as it will burn crops if applied fresh. Other forms of organic matter like composted leaves or food scraps should also contain sufficient levels of sulfur to correct a deficiency.
Each one per cent of organic matter in the top few inches of soil contains up to 100 pounds of sulfur, which is one of the reasons why soils low in organic matter often are deficient in this element.
Unlike many other nutrient deficiencies, sulfur deficiencies can sometimes correct themselves. If they occur at the beginning of the season when soil temperatures are still relatively low or if they appear on young plants, there is the possibility that it can pull enough mineralized sulfur from the soil to correct itself as the plant’s root system reactivates or matures.
This is because mineralization is performed largely by the microbial activity in the soil, which is limited during colder months.
If, however, symptoms of sulfur deficiency appear mid-season or on mature plants, it will not likely correct itself.