There are basically three ways to diagnose a sulfur deficiency. Each has benefits and drawbacks.
- Visual assessment (practical and quick, but not very accurate)
- Plant tissue analysis (more complicated, but you get a better idea of which nutrients are limiting)
- Soil analysis (a good baseline and should always include microbial analysis)
How do these diagnoses relate to sulfur (S) deficiencies or excesses? It’s rare to have an excess of sulfur unless you’ve spilled some fertilizer. However, the majority of cultivated fields are now showing some degree of S deficiency due to industrial agricultural practices, leaching, and high-yielding crops that take a lot of S and other nutrients out of the soil.
If you see symptoms of sulfur deficiency, you probably have a severe deficiency that will impact the yield and quality of the crop in that field this season. Chemical analyses of plant tissue and soil show the “hidden deficiency” that isn’t evident to the naked eye. With these tests, you may be able to save your harvest this year.
Because the nitrogen/sulfur relationship is so strong, it’s possible to diagnose S deficiency (or N deficiency) by looking at the ratio of these two elements. (1)
Sulfur Deficiency Symptoms
Different crops have different sulfur requirements. Adequate and balanced S nutrition varies with the developmental stage and species. Good record-keeping will help you determine exactly which nutrients have been issues in the past and should be watched.
In severe cases, sulfur deficiency shows up as chlorosis of the newer plant growth. This symptom is very similar to a nitrogen deficiency. The two nutrients are heavily dependent on each other, so a deficiency in one may well create an imbalance in the other.
The plant root structure should be analyzed for abnormalities. A sulfur deficiency will show up as enhanced root growth in some plants, and decreased root growth in others. What you’re looking for is abnormal root growth for that species. Aboveground symptoms may include stunted growth and bending of the leaf blade. For some crops, such as corn, sorghum, and cotton, the plant will have a reddening of the stems and leaf edges with an overall purplish tone to the plant. Legumes are unable to effectively partner with soil microorganisms to create nitrogen nodules when they are S-deficient.
Many of these symptoms are shared by deficiencies in other nutrients. That’s why it’s so important to have tissue and soil analyses done before adding nutrients.
Plants Need Microorganisms to Break Down Sulfur
Most sulfur is spread as ammonium sulfate or gypsum, calcium sulfate. These products delivers S in the sulfate form that plants uptake. Ammonium sulfate delivers a balance of nitrogen and sulfate sulfur. Gypsum delivers calcium that may or may not be needed. Gypsum is mined or created by a process called calcination. It’s heated to drive off the water content. The result is the powdery form commonly used as fertilizer.
Sulfur is a complex nutrient that is necessary for plant growth and a high nutrient profile. The protein content in soybeans, wheat, sorghum, and other crops won’t be at optimum levels without a balance of nutrients, especially S. Regenerative farmers and ranchers use the free labor of the soil microorganisms to supply S to pastures and crops for optimum protein and amino acids profiles.
Microorganisms break down organic sulfur in the soil into inorganic forms, primarily sulfate (SO42-), which plants absorb through their root systems. This is called mineralization. Up to 98% of total soil sulfur is in organic compounds. (2)
The amount of elemental sulfur in the soil is large and becomes plant-available with the help of microorganisms. Manure, compost, and crop residues all contain sulfur and the activities of microorganisms release the nutrients for crop uptake. Higher soil temperatures, enough moisture, pH neutral or slightly acidic soil, and a good amount of organic matter all contribute to a natural source of sulfur. This is sulfur that is free, and who doesn’t like that economy?
Microbial biomass moves around, but the majority of microorganisms are found in the root rhizosphere. Sulfur is a highly mobile element and can be easily leached out of many soils. When the S is tied up in microbe bodies that die in the direct vicinity of plant roots, S is released in plant-available form. The mineralization of free soil S is only possible if there is a strong microbial population. Regenerative agricultural practices that emphasize no-till or low-till, cover crops, and crop rotations encourage a diverse microbial community.
Sulfur is the fourth most important nutrient for plant growth. When you diagnose a S deficiency, you’re also diagnosing a plant and soil nutrient imbalance. No crop will achieve optimum growth or nutrient profile without a balanced soil profile. Using regenerative agricultural practices is the best way to achieve that balance.
To find out more about sulfur and the other macro and micronutrients that lead to resilient, high-nutrient profile crops visit our website. We are here to help you succeed. When soil speaks, we listen.
