Sulfur in the Soil
Most soil sources of S are in the organic matter and are therefore concentrated in the topsoil. Elemental S and other forms as found in soil organic matter and some fertilizers must be converted to the sulfate (SO4- -) form to become available to the crop. This conversion is performed by soil microbes. The sulfate form of S is an anion (negative charge), and therefore is leachable. As a rough rule-of-thumb, it can be considered to leach through the soil profile at about 50% as fast as nitrates (NO3-). In soils with a significant and restrictive clay layer in the sub-soil, it is common to find that sulfate which has leached through the soil over time and become “perched” on the clay layer. This SO4– is available to crops when the roots reach this area of the soil.
Sulfur is essential for many plant functions. Some of them are
- A structural component of protein and peptides
- Active in the conversion of inorganic N into protein
- A catalyst in chlorophyll production
- Promotes nodule formation in legumes
- A structural component of various enzymes
- A structural component of the compounds that give the characteristic odors and flavors to mustard, onion, garlic, etc
Factors Affecting Availability
- Sand: Sulfur is leachable, plus sandy soils are typically low in OM, therefore these soils are often low in sulfur.
- Soil Organic Matter: Organic matter is a reservoir for S
- Cold Soil: The conversion of various forms of S to the available sulfate (SO4- – ) form is a microbial process; therefore low soil temperatures slow this process.
- Poor Drainage: The conversion of various forms of S to the available sulfate (SO4- – ) form is a microbial process requiring oxygen, therefore saturated soil slow this process.
- Pollution: Soil that, over the years, has been subject to high levels of deposition from industrial sources of S.
- Irrigation Water: Irrigation water may contain high levels of S, and excess irrigation of sands can leach S out of the root zone.
- SO4:NH4 Applications: Added NH4 has been shown to appreciably enhance the uptake of SO4.
Other reasons for Sulfur shortages to occur are: (1) strict air pollution standards have cleaned the air of gaseous sulfur compounds, and (2) Sulfur is not present in sufficient quantities in today’s higher-analysis fertilizers. As with nitrogen, sulfur can be converted into unavailable forms, lost as a gas, or leached in irrigation or rain water. Excess sulfur does not normally present an agronomic problem for crops, so the conditions listed below do not necessarily indicate a need for corrective measures.
- Other Anions: Anions tend to compete with other anions in terms of availability and plant uptake. Therefore excess sulfate-S (SO4- -) can reduce the uptake of some anions such as nitrates (NO3-) and the available form of molybdenum (MoO4-). Excessive amounts of nitrates can also reduce the uptake of sulfate-S
- Copper: Sulfur can, in some crops, effectively reduce the possibility of Copper toxicity by creating Cu-S complexes.
- Balances and Ratios
Some people will recommend that sulfur be applied in a particular ratio with N. The origin of this concept appears to be based on an N:S ratio found in protein. While it is entirely possible that many crop producers will get a yield response from applying N and S together, this does not prove that a particular N:S ratio from fertilizer is desirable. Using this approach to recommending sulfur applications is not based on sound agronomic principles and data. For example, it completely ignores the S that is naturally available from the soil. Also, it assumes that both the applied N and S will be taken up in the same amounts that they were applied (thus retaining this “ideal” ratio). If the crop takes up N and S in a particular ratio, the dominant controlling factors will not be the ratio of these elements applied as fertilizer. The very complex soil chemistry, microbiology, as well as variable crop, culture, and environmental factors will inevitably work against the crop absorbing N and S in a particular ratio, simply because that ratio was applied as fertilizer.
Sulfur is a necessary constituent in several amino acids and proteins. Since these are building blocks in the plant, Sulfur becomes fixed into the plant’s structure. Therefore, the classic symptom of deficiency is a paleness of the younger foliage. However, many times all of the foliage has a pale green color, and the difference in “paleness” between the older and younger foliage is not easily noticed. This can lead to a misdiagnosis of N deficiency for S deficiency (Nitrogen deficiency symptoms appear on the older leaves first.). In some cases, the leaf veins may be lighterin color than the surrounding tissue.
Sulfur toxicity for practical purposes should be considered as non-existent. Excessive applications most often result in a depression of soil pH and an increase of the problems that occur with the pH decrease. In fact, sulfur uptake is reduced as the pH of the soil decreases.