The Sulfur concentration in a citrus tree is approximately 10 times less than the Ca concentration, and it is about equal to the P concentration. The uptake and assimilation of S and N by plants are strongly interrelated and depend on each other because of their mutual occurrence in amino acids and proteins. Sulfur is an essential constituent of many proteins, vitamins, and some plant hormones. As a result, protein synthesis and amino acid and chlorophyll production are retarded in S-deficient plants. Sulfur is also known to enhance the development of nodules and N fixation by legumes, indicating its importance in root growth and development as well as in root vigor and hardiness. Sulfur also affects carbohydrate metabolism. Sulfur is a major component of soil organic matter and becomes available to plants as organic matter decomposes. Sulfur is also present in some irrigation water sources.
Many producers often forget sulfur as a needed element and critical nutrient. The problem of S deficiency may be widespread but not known because lab analysis does not target this nutrient. In general, plant tissue analyses are important to diagnose nutritional deficiency or sufficiency of all nutrients. Unfortunately, tissue analysis has not been routinely used to check the S status of citrus trees. The effects S nutritional status has on yield and quality are well documented for numerous crops, but not for citrus. Sulfur controls certain diseases and insect pests in many crops and improves plant tolerance to heavy metal toxicity.
Because S is associated with forming proteins and chlorophyll, deficiency symptoms resemble those of N deficiency, but the symptoms first appear on new growth (Figure 3). Such chlorosis in citrus is worse on new growth because S does not move readily from old to young leaves like N. Plants are stunted and pale green to yellow in color. Visual diagnosis of S deficiency is not easy to identify in citrus production. Accurate diagnosis should involve tissue analysis.
These leaves have sulfur deficiency symptoms because they show chlorosis (pale green to yellow in color) similar to N deficiency. S deficiency symptoms appear on new growth because S does not move readily from old to young leaves like N.
Sulfur deficiency occurs most commonly with high N fertilizer rates. If the supply of N is not supplemented with adequate S, the N available for crop use may be excessive in relation to S. Under high N and low S conditions, plant growth processes are disrupted and plants develop symptoms of S deficiency. Sometimes total growth is reduced by fertilization with N alone, whereas combined applications of N and S provide normal growth and yield.
Sulfur deficiencies have become more common in the past few decades with the increased use of fertilizers lacking S, such as ammonium nitrate, potassium nitrate, urea, concentrated superphosphate, monoammonium phosphate (MAP), and diammonium phosphate (DAP). Decreased use of S-containing pesticides and fungicides may also contribute to the more common occurrence of S deficiency. Sulfur deficiency in citrus can easily be corrected by soil application of S-containing fertilizers such as ammonium sulfate, potassium sulfate, or magnesium sulfate. Applying gypsum is an inexpensive option that can also correct S deficiency and supply Ca. Using manures may also be a good management strategy to increase S availability to citrus trees.
The information presented above is part of document SL382, one of a series of the Department of Soil and Water Science, UF/IFAS Extension. A version of this article originally was published in Citrus Industry magazine. Original publication date: July 2013. Please visit the EDIS website at http://edis.ifas.ufl.edu.
Mongi Zekri, multicounty citrus UF/IFAS Extension agent; and Tom Obreza, professor and interim associate dean for Extension; UF/IFAS Extension, Gainesville, FL 32611. Photo Dr. R. C. J. Koo