A chain is only as strong as its weakest link. Often overlooked, sulfur (S) can be that weak link in many soil fertility and plant nutrition programs. As of late, there are several reasons for the increased observance of S deficiencies and increased S needs.
Government regulations now restrict the amount of sulfur dioxide (SO2) that can be returned to the atmosphere from coal-burning furnaces. Most of the S is now removed from natural gas used in home heating and in industry. Also, catalytic converters in new automobiles remove most of the S that was previously returned to the atmosphere when S-containing gasoline was burned in automobiles. In addition, S-free compounds have replaced many of the insecticides and fungicides formerly applied to control insects and diseases in crops. As a result of these government restrictions, less S returns to the soil in rainfall.
Sulfur is supplied to plants from the soil by organic matter and minerals, but it’s often present in insufficient quantities and at inopportune times for the needs of many high-yielding crops. Organic matter ties up most S to the soil, where it remains unavailable to plants until soil bacteria convert it to sulfate (SO4-2) form. That process is known as mineralization.
Just like nitrate nitrogen (N), sulfate moves through the soil and can leach beyond the active root zone in some soils during heavy rainfall or irrigation. Sulfate may move back upward toward the soil surface as water evaporates, except in the sandier, coarse-textured soils that may be void of capillary pores. This mobility of sulfate S makes it difficult to calibrate soil tests and use them as predictive tools for S fertilization needs.
In the field, plants deficient in S show pale green coloring of the younger leaves, although the entire plant can be pale green and stunted in severe cases. Leaves tend to shrivel as the deficiency progresses.
Sulfur, like N, is a constituent of proteins, so deficiency symptoms are similar to those of N. Nitrogen-deficiency symptoms are more severe on older leaves, however, because N is a mobile plant nutrient and moves to new growth. Sulfur, on the other hand, is immobile in the plant, so new growth suffers first when S levels are not adequate to meet the plant’s need. This difference is important in distinguishing between N and S deficiencies, particularly in early stages.
Symptoms of deficiency can vary across crop species, but similarities exist for how nutrient insufficiency impacts plant tissue color and appearance. Nutrient deficiencies are commonly associated with the physical location on the plant
(i.e., whether the symptoms are primarily observed on older versus newly formed plant tissue), but these symptoms can spread as the severity of the deficiency progresses.