Sulfur is a secondary macronutrient for plants. It is a component of some plant amino acids so it is required for good protein content in plants. It also helps plants to incorporate nitrogen into proteins. Sulfur is required for the production of chlorophyll and a number of enzymes. It also helps legumes form associations with nitrogen fixing bacteria. Interestingly, the flavour and odour of mustard, onion and garlic is a result of sulfur containing compounds.
The availability of sulfur to plants is influenced by a number of factors including:
- Soil type - sulfur is easily washed out of (leached from) sandy soils and such soils are thus generally low in sulfur unless they contain large amounts of organic matter
- Organic matter - organic matter stores sulfur
- Soil temperature - cold soil prevents microorganisms from converting unavailable sulfur into available sulfur that plants are able to use
- Drainage - the microorganisms mentioned above require oxygen which poorly drained soils lack thus poor drainage reduces the availability of sulfur
- Pollution - high levels of pollution in an area can increase the amount of sulfur in the soil though this sulfur may not be in a form available to plants
- Water - water may contain high levels of sulfur but it can also leach sulfur from the soil (particularly sandy soils)
- Ammonium - adding ammonium to soil can improve the uptake of sulfur by plants
Ideally, for healthy and productive soil the you should aim for a sulfur concentration of 10‑20 mg/kg.
Sulfur deficiency is more common in plants grown on cold and sandy soils as well as those that are low in organic matter. Sulfur deficiency is also more likely to occur in areas with high rainfall or pollution.
Symptoms of Sulfur Deficiency
Chlorosis is the main symptom of sulfur deficiency. New leaves are affected first but as the deficiency worsens, older leaves may become affected as well. Chlorosis is uniform across the leaf rather than being blotchy or restricted to interveinal areas. In some cases, affected leaves may be smaller than healthy leaves and the whole plant will usually be stunted. Maturity of the plant may also be delayed.
Treating Sulfur Deficiency
When plants exhibit symptoms of sulfur deficiency, your first steps should be to check (and adjust if necessary) the soil pH and increase the amount of organic matter in the soil. Sulfur is most available to plants when the pH of the soil is 6 or more though most plants should be able to uptake sufficient sulfur if the soil pH is 5.5 or higher. Additional sulfur may be required when the soil pH needs to be less than 5.5 (for example, azaleas and blueberries require more acidic soil).
Soil should have good drainage. If it doesn't, organic matter will help though it takes some time for additional organic matter to take effect. If drainage is quite poor, you may want to transplant your plant to a different position or build a raised bed for it. It's also a good idea to ensure that plants are watered only when required (don't let an automated system provide water if it's just rained for instance).
If symptoms appear early in the season, cloches can be used to warm the soil.
If soil nitrogen levels are moderate to low (according to a soil test), an ammonium containing fertiliser can be applied to the soil to aid the uptake of sulfur.
If symptoms persist, it would be a good idea to have your soil tested. If soil tests reveal a moderate or more severe sulfur deficiency, sulfur can be added to the soil directly but this acidifies the soil, which is undesirable except when growing berries or if the soil is alkaline. The preferred alternative is to use gypsum to provide additional sulfur. Other sulfur containing fertilisers are available but they can upset the balance of other elements if sulfur is the only deficient nutrient. If soil tests reveal soil deficiencies in nitrogen, potassium of phosphorus, then you could apply ammonium sulfate, potassium sulfate or super phosphate respectively. If you chose to do so, remember to always follow the directions on the packaging carefully and keep the dose small. It's much better to have to provide multiple small amounts of a fertiliser than to add a large amount and find that you've added too much of something.
For practical purposes, sulfur toxicity should be considered impossible. Excess soil suffer can prevent the uptake of other elements though - nitrogen for example. If your soil contains excessively large amounts of sulfur, increase irrigation and ensure that fertilisers being applied to the soil do not contain sulfur. The deficient nutrient can also be added to the soil provided it doesn't result in excess levels of that nutrient in the soil, which could result in toxicity once soil sulfur levels drop.
If excess sulfur is from air pollution it can be difficult to control. If your plants are all in pots, you can replace the potting mix when required and wash the leaves regularly. If your plants are in the ground, short of replacing the soil you may just have to monitor plants for induced deficiencies and treat those as necessary.