Dear Sir,thank you very much for suggestions.

Dear Michael,

Are you saying that you have observed toxic effects or is this hypothetical? 

Soluble sulfates can cause toxicity by pushing either the EC or the soluble Al concentration above the critical threshold for a crop.

Paul.

Sulfur requirement for optimal growth varies between 0.1 and 0.5% of the dry weight of plants. On average, proteins from legumes contain less sulfur than proteins from cereals like wheat, the N/S ratio being 40:1 and 30:1 respectively ( Dijkshoorn and Wijk, 1967). Sulfur may affect root hydraulic conductivity, stomatal aperture and net photosynthesis (Karmokerbet al., 1991). In response of sulfur sufficiency, proteins of high sulfur content are synthesized, which is most evident in the cytoplasm of leaf cells. It may affect the baking quality of wheat flour (Etwart, 1978). It also decreases the acceptability of the animal feed.

Very interesting question , besides some equally fascinating responses....here is an abstract of some interesting work..

Symptoms of sulphur deficiency and toxicity on greenhouse tomatoes (Lycopersicon esculentum Mill.) and cucumbers (Cucumis sativis L.) were produced experimentally in sand culture and are described in detail. The principal morphological effect was a general depression of growth and fruit production. A S content of less than O .25Vo in any plant tissue was associated with severe deficiency. The distribution of S in various plant tissues is shown. On a provoqu6 artificiellement des sympt6mes de toxicit6 et de carence en soufre sur des tomates (Lycopersicon esculentum Mill.) et des concombres (Cucumis sativis L.) de serre cultiv6s dans du sable. L'effet morphologique principal a 6t6 une r6duction g6n6rale de la croissance et de la production des fruits. Une teneur en soufre inf6rieure d 0,254o dans les tissus v6g6taux a 6t6 reli6e ir des symptOmes de carence grave. L'auteur donne la distribution du soufre dans les divers tissus v6e6taux. Unfortunately, its not in english , but could be useful , those who can easily understand it..

PDF enclosed fro further reading...

Dear all, thank you very much.

I answer to Paul - no, there were not clear toxicity symptoms on wheat. The experiment was carried out on alluvial moderately fine and fine soils, very fertile. Among the factors of exepriments was N and N+S treatments. N treatment with ammonium nitrate, N+S with commercial fertilizer produced from mixing of ammnonium nitrate and ammonium sulfate (i.e.- the form of N was not exactly the same in compared tratments). When speaking about yields, they were slightly smaller (sometimes significantly, sometimes not) in N+S comparing with N. For this reason one person interpreted this as "S toxicity". Actually, I cannot agree with such interpretation, because, in my opinion, lower yields do not imply toxicity. I think other factors could cause such situation (for example form of  N, perhaps in relation to weather conditions etc.) or other. Certainly, S as any other nutrient may be toxic (many thanks to Anoop), but probably not in this case, because, the S dose did not exceed clearly estimated S demand of the crop.

With regard to possible efeect of sulphates on soil EC. I think in conditions of Poland, with humid climate (precipitations exceeding potential evapotranspiration) such effect probably rarely occurs (natural salinity of soils rather does not occur in Poland, with exception of some effects of human activity or neighbourhood of the sea). In case of our wheat such "salinity effect" would result from application of all fertilizers, but rather not S treatment (the amount of N added in both treatments was greater, than amount of S, both in readily soluble form).