Sulfur Management

This element is a structural component of proteins, enzymes, and antioxidants.

Sulfur in the Soil

Sulfur (S) is an important macronutrient which is becoming increasingly deficient in agricultural settings. About 90% of soil sulfur is found in organic forms, but these organic forms of S are unavailable to plants, and inorganic S typically makes up only 5-10% of total sulfur. Understanding the sulfur cycle and analyzing the sulfur in your soil system is critical for managing your fertilizer applications and balancing nutrients long-term.

The organic S pool is a heterogeneous mixture of soil organisms, plants, and animals in varying states of life, death, and decomposition. Although organic S is unavailable to plants, soil microbes process and mineralize it, slowly releasing the bioavailable sulfate as a product. Sulfate is the most common form of inorganic sulfur. In reducing conditions, other forms of inorganic sulfur may be found as well. However, these reduced sulfur compounds must first be oxidized to sulfate to be available for uptake by crops.

Sulfate released from organic S is not always retained in the soil. Adsorption of sulfate usually increases with the clay content of soils as well as Al and Fe oxide content. Above pH 6.5, adsorption is negligible and most sulfate is found in soil solution, whereas sulfate is adsorbed more strongly under acid conditions. Thus, sandy soils with heavy rainfall may experience severe sulfur leaching.

The carbon/sulfur (C/S) ratio of the soil determines whether inorganic sulfur is released from the organic pool, or immobilized by microbes as organic S. A C/S ratio < 200:1 results in a release of sulfur and a ratio > 400:1 results in immobilization. A C/S ratio between 200 and 400 results in little change in S availability.

Sulfur Deficiency

When sulfur is deficient, plants are unable to produce sulfur-containing amino acids, which inhibits protein synthesis and causes other down-stream effects. The symptoms of S deficiency first appear in the young leaves of the plant. Different types of plants have different symptoms of S deficiency, but they are all characterized by the reduced height and chlorosis of leaves (especially new leaves).

Figure from Eriksen, J., Murphy, M. D., and Schnug, E. (1998). The soil sulphur cycle. In E. Schnug (Ed.), Sulphur in agroecosystems (pp.39-73). Springer.

Sulfur in the Plant

Sulfur (S) is considered the fourth most important plant nutrient, after nitrogen, phosphorus, and potassium, although sulfur requirements vary considerably among crops. Plant leaves can absorb gaseous SO2 and H2S, but their primary source of S is sulfate from the soil. Once taken up by the plant, the amino acid cysteine is the first organic product synthesized from S. Cysteine is the precursor to a wide range of metabolites with critical biological functions.

Sulfur is an integral part of vitamins, proteins, enzymes, phytohormones, and antioxidants which often provide the signature taste or smell of a plant. Glutathione (GSH) is a sulfur-containing antioxidant compound produced by plants which scavenges reactive oxygen species, preventing oxidative damage. Glutathione and other sulfur-containing compounds help plants defend against disease, pathogens, and other stresses.

Sulfur Toxicity

Sulfur toxicity is rare and is typically not harmful to plants. Plants are able to transport excess S to cellular compartments such as the vacuole where it can be stored. However, high concentrations of S in the soil can inhibit the uptake of other essential nutrients.

Further Reading

Bouranis, D.L., Malagoli, M., Avice, J.-C., and Bloem, E., (2020). Advances in plant sulfur research. Plants 9, 256. doi:10.3390/plants9020256

Capaldi, F.R., Gratão, P.L., Reis, A.R., Lima, L.W., and Azevedo, R.A., (2015). Sulfur metabolism and stress defense responses in plants. Tropical Plant Biology 8, 60–73. doi:10.1007/s12042-015-9152-1

Dick, W. A. (2008). Availability of sulfur to crops from soil and other sources. In J. Jez (Ed.), Sulfur: A Missing Link between Soils, Crops, and Nutrition. Wiley.

Eriksen, J., Murphy, M. D., and Schnug, E. (1998). The soil sulphur cycle. In E. Schnug (Ed.), Sulphur in Agroecosystems (pp.39-73). Springer.

Kopriva, S., et al. (2016). Editorial: Frontiers of Sulfur Metabolism in Plant Growth, Development, and Stress Response. Frontiers in Plant Science.

Nakai, Y. and Maruyama-Nakashita, A., (2020). Biosynthesis of sulfur-containing small biomolecules in plants. International Journal of Molecular Sciences 21, 3470.. doi:10.3390/ijms21103470