Sulfur in Plants and Soil – The 4th Macronutrient
Sulfur is an essential nutrient for plant growth. In recent years, sulfur deficiencies have become more frequent and the importance of sulfur in crop production is becoming more and more recognized.
Although it is considered a secondary nutrient, it is now becoming recognized as the ‘fourth macronutrient’, along with nitrogen, phosphorus and potassium.
Oil crops, legumes, forages and some vegetable crops require sulfur in considerable amounts. In many crops, its amount in the plant is similar to phosphorus.
ROLES OF SULFUR IN PLANTS
Sulfur has various functions in plants. Some major roles are:
- It is found in some amino acids, the building blocks of proteins. Most of the sulfur absorbed by plants, about 90%, is used for that purpose.
- Sulfur is essential for chlorophyll formation. It is a major constituent of one of the enzymes required for the formation of the chlorophyll molecule.
- Essential in the synthesis of oils, especially in oil crops.
- Active in the metabolism of nitrogen.
SULFUR DEFICIENCIES
Sulfur deficiencies are more likely in sandy soils with low organic matter (less than 2%) and under high rainfall conditions. However, even in high organic matter soils, often, the breakdown of the organic matter and the mineralization process are not rapid enough to meet the sulfur requirement of the crop. When this occurs, fertilizers or amendments containing sulfur have to be applied.
Sulfur is immobile in plants and does not readily translocated from older leaves to young leaves. Therefore, sulfur deficiency first appears on younger leaves.
Sulfur deficiency symptoms show up as light green to yellowish color. Deficient plants are small and their growth is retarded.
Symptoms may vary between plant species. For example, in corn, sulfur deficiency shows up as interveinal chlorosis; in wheat, the whole plant becomes pale while the younger leaves are more chlorotic; in potatoes, spotting of leaves might occur.
Sulfur deficiency in corn Sulfur deficiency in soybean
Sulfur deficiency in tobacco Sulfur deficiency in wheat
SULFUR IN SOIL
Most of the sulfur in soils is found soil in organic matter . However, it is not available to plants in this form. In order to become available to plants, the sulfur must be first released from the organic matter and go through mineralization process.
The mineralization process is a result of microbial activity. In this process sulfur is converted to the sulfate form (SO4-2), which is readily available to plants.
The process is affected by the C/S ratio, temperature and moisture.
Immobilization of sulfur is the opposite process in which available sulfate is converted back into the organic form.
Due to its negative charge, the sulfate inorganic form (SO4-2) is mobile in soils.
DIAGNOSING SULFUR DEFICIENCY
Plant analysis is the best way to estimate sulfur sufficiency. Combining the plant analysis with soil test result will give the best indication on if and how much sulfur should be applied.
Many times, soil test alone is not reliable enough. Soil tests are valuable only if a correlation was found between the sulfur test level and the crop response to application of fertilizer.
Because sulfur leaches readily, the top soil might test low in sulfur, while subsoil samples will show higher levels of available sulfur.
SULFUR CONTAINING FERTILIZERS
Today, there are various fertilizers containing significant amounts of sulfur. The most common ones are listed in the table below.
| % S | Suitability for fertigation | |
| Ammonium Sulfate | 24 | Suitable |
| Calcium sulfate (gypsum) | 15-18 | Not suitable |
| Magnesium sulfate | ||
| Potassium sulfate | 18 | |
| Superphosphate (normal) | ||
| Potassium magnesium sulfate | 22 |
Including sulfur in a fertilizer program to avoid deficiency is more efficient and less costly than correcting a sulfur deficiency once it occurs.
INTERACTIONS WITH OTHER NUTRIENTS
Sulfur interacts with other nutrients in the plant and in soil. These interactions will be discussed in another article.
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