i came apon the following in the Mole Valley Farmers current news letter, in a section on fertiliser for grassland it says "historically the crops sulphur requirement has been met from atmospheric deposition, but this source in 1999 was only 15% of that in 1980." the plants use sulphur with nitrogen to make amino acids and hence protein. if sulphur is in short supply the crop may not be able to make use of all the nitrogen available in the soil.
the article doesnt actually give the current level of sulphur deposition but it could be something to bear in mind if crops dont respond properly to nitrogen fertilisers.
