Written by Mike Hamilton, CCA & President of Turf Dietitian
Sulfur’s an interesting nutrient because it really falls in between a micro and macro nutrient. Like most macro nutrients, sulfur is responsible for structural formations within a plant. In addition, sulfur is responsible for enzyme production and reactions, like most micro-nutrients. Sulfur is often referred to as the micro-nutrient equivalent of nitrogen.
Liebig’s law of minimums teaches us that no one nutrient is more crucial than another. The plant is only as good as the weakest link. However, there ARE nutrients that play a role in more essential functions than others, and sulfur is one of those nutrients.
With that being said, I rank sulfur near the top of that indispensable list. The amount of sulfur required by a plant is very low in comparison to most macro-nutrients. Yet we frequently see sulfur applied at an excessive rate, because of water treatments or because it’s in the majority of fertilizers used in turf.
So, does the application of excess sulfur create plant stress or plant health problems? I’ve yet to find any research that could confirm excess sulfur has any negative affect on plant health. So, there’s no problem applying excess sulfur, right? Not so fast my friend, you can’t forget about the effects of excess sulfur on soil structure.
Like all nutrients in excess, sulfur has the potential to be destructive to soil structure. When soil structure is poor, the likelihood for plant decline increases significantly. Therefore, in my mind applying excess sulfur will eventually cause life-threatening problems for the turf. Most of the time problems associated with excess sulfur is diagnosed as water infiltration issues, excessive organic matter accumulation, or black layer.
How does excess sulfur cause soil structure problems? The simplest explanation is that it’s part of the process that reduces the amount of oxygen in the soil.
The available form of sulfur to a plant is SO4 (sulfate). Therefore, as natural mineralization of sulfur takes place, it’s using up 4 oxygen molecules in the soil. However, that alone usually isn’t enough to deplete the soil of oxygen (unless the soil pH is below 6). Converting sulfur into sulfate is only the start of the oxygen depletion process. As SO4 increase in the soil so do anaerobic bacteria, and they are the main culprit in the depletion of oxygen in soil.
Your soil is a war zone every second of every day. All living organisms in the soil are battling each other to establish the perfect environment for their existence. The good guys need oxygen to thrive, while the bad guys need an oxygen-free environment to survive. The nuclear bomb in the anaerobic bacteria arsenal is sulfuric acid. Anaerobic bacteria use SO4 to produce sulfuric acid H2SO4, in order to kill its enemy (anaerobic bacteria). Once the balance of power shifts, anaerobic bacteria start colonizing the macro-pour space of the soil, clogs them up, and makes them void of oxygen.
Most of the time in higher pH soils, excess SO4 is being supplied by irrigation water and not excess sulfur. Not only is the water providing the necessary means for anaerobic bacteria colonization, it’s already stocked with significant levels of them. Any aerification system you can install into your water supply will help to reduce the levels of anaerobic bacteria in the water source.
It’s never easy or inexpensive to treat water. So, what can you do to remedy excess SO4 in your water source? By going after it in the soil. Be proactive with any cultural practice the increases oxygen in the soil (aerification, flushing, venting, underground ventilation).
The key to maintaining perfect soil, is to balance everything. Not only is it important to balance the 4 major cations, but also the anions, bacteria, organic acids, water, oxygen and much more. If the SO4 is high, switch your bulk applications to carbonates, chlorides, or organic sources.
I know what you’re thinking: carbonates and chlorides are just as bad, right? Absolutely they are, but only if they are in excess. If I were talking about excess carbonates or chlorides, I’d be recommending sulfates to help balance the anions.
Yes, bicarbonates, chlorides, and sulfate can all three be extremely detrimental to soils and plants if they are excessive in your water source or soil. Equally important, all three are essential for healthy plant growth; but only when in balance. All 3 major anions, a Dr. Jeckle and Mr. Hide personality.
So how do you start developing a balance of anions in the soil? It all starts with soil testing. For many years the standard for superintendents was to have their soil tested one to two times per year. However, what we have discovered over the past 10 years, is that the anion ratios change throughout the year due to changes in weather and the environment. I recommend minimal soil testing at least once per month for one to two years, until you have established enough data to be able to predict the changes in anion levels in your soil.
Because sulfur leaches quickly most deposits are in the organic matter. Sulfur itself is not available to plants. Sulfur must convert to the sulfate (SO4–) to become available for plant uptake.
Excess sulfur is generally thought to cause black layer. Black layer is deadly to plants when in it converts to hydrogen sulphide. However, for black layer to form; sulfur, sulfates, and anaerobic bacteria must all be present in the soil. Typically, the symptoms of black layer begin with saturated soils, followed by surface algae, rotten smelling soil, turf decline, followed by turf death.
The function of Sulfur in the plant:
- A structural component of proteins and peptides
- Assists with the conversion of inorganic N into a protein
- Sparks chlorophyll production
- A structural component of various enzymes
- Sulfur acts as a soil conditioner that helps reduce the drying sodium content of soils.
- Sulfur aids in the plant’s resistance to disease.
Sulfur Uptake by Plants
For plants to be able to successfully uptake sulfur, it must be in the sulfate form. For years superintendents only monitored cations. However, because we can now measure the available nutrients in soil solution, it’s important to monitor and manage for anions for plant and soil health. Anions tend to compete with other anions in this regard, therefore any excess sulphate can reduce the uptake of other anions such as nitrates, carbonates, chlorides, and phosphorus.
Sulfur Deficiency Symptoms
We’ve already established that without sulfur in your soil, your turf will be lacking in amino acids and proteins, enzymes, and vitamins. Obviously, without these building blocks plants will suffer and die.
Sulphur deficiencies will be more likely in sandy soils with a low organic matter component and have high rainfall conditions. A low organic matter for turf is considered to be less than 2%. It’s important to note that even in high organic matter soils, deficiencies can occur if the mineralization process isn’t rapid enough to meet the plants’ requirement.
As sulfur is fixed into plant’s the deficiency symptoms can become quite striking. In fact, the first sign of deficiency is a paleness in the younger foliage. As this paleness is common in both young and older foliage, it becomes harder to notice. It can lead to a misdiagnosis of a nitrogen deficiency. However, it’s important to note that if there is a nitrogen deficiency, the symptoms will start to appear on the older leaves first.
Another sign would be a lightening of leaf vein color that can be compared to the surrounding tissue. You may also note that plant growth is small and stunted.
Sulfur deficiencies may occur when the following factors affect its availability:
- Sand: As sulfur is leachable and that sandy soils are generally low in OM, sandy soils will be lower in sulfur.
- Low Soil Organic Matter: The amount of organic matter in your soil acts as a reservoir for sulfur. The lower the OM the lower the levels of sulfur will be.
- Soil Temperature: Cold soil will cause a slowing of the mineralization process.
- Drainage: Poor drainage causes saturated soil. The saturated soil will hinder the microbial process as it reduces the oxygen levels in the soil.
Sulfur toxicity rarely occurs. Excessive applications of sulfur mostly result in a depression of soil pH and an increase in problems that follow with a pH decrease.
There are many products available to help superintendents manage and prevent any sulfur deficiencies.
Plants can readily take up sulfur in the sulfate form. It’s immediately available to help develop the roots and get your plants off to a faster start. Monitoring your soil’s general health is a good management tool to help maintain balance in the soil.
Choosing the Most Efficient Method of Sulfur Management
I cannot stress this enough; plant, soil and water analysis are a key method to maintain turf health. Even more so for analyzing the amount of sulfur and its ratio to other anions in the soil.
By doing more frequent monitoring of plant tissue, soil and water, it will allow you to be proactive to seasonal changes, and more efficient with your applications.
Correcting any nutrient problem is difficult, but it becomes easier when you understand how they behave throughout the year. It’s hard to correct problems if you don’t know what they are in the first place. Regular testing gives you solid data to make the best choices for your turf.