Foliar Application of Sulfur-Containing Compounds-Pros and Cons

Abstract

Sulfate is taken up from the soil solution by the root system; and inside the plant, it is assimilated to hydrogen sulfide, which in turn is converted to cysteine. Sulfate is also taken up by the leaves, when foliage is sprayed with solutions containing sulfate fertilizers. Moreover, several other sulfur (S)-containing compounds are provided through foliar application, including the S metabolites hydrogen sulfide, glutathione, cysteine, methionine, S-methylmethionine, and lipoic acid. However, S compounds that are not metabolites, such as thiourea and lignosulfonates, along with dimethyl sulfoxide and S-containing adjuvants, are provided by foliar application-these are the S-containing agrochemicals. In this review, we elaborate on the fate of these compounds after spraying foliage and on the rationale and the efficiency of such foliar applications. The foliar application of S-compounds in various combinations is an emerging area of agricultural usefulness. In the agricultural practice, the S-containing compounds are not applied alone in spray solutions and the need for proper combinations is of prime importance.

Keywords: foliar fertilization; foliar spraying; sulfur-containing adjuvants; sulfur-containing agrochemicals; sulfur-containing metabolites.

Figure 1

Foliar application of S-containing compounds. These compounds are S-gases [sulfur dioxide (SO₂), hydrogen sulfide (H₂S), carbon disulfide (CS₂), carbonyl sulfide (COS), dimethyl sulfide (DMS)], fertilizers containing sulfate, S-containing metabolites, and S-containing non-metabolites. For the integrity of the approach, in the last group, the S-containing adjuvants and agrochemicals have been added. The arrows indicate the complex journey of the S-containing solute to the action point within the cell which includes several structures to cross, and the characteristics of each one of them is summarized in the text. Towards understanding and handling the effectiveness of such foliar applications, the nature and mode of action of these compounds, along with some characteristic case studies, are discussed.