Physical Cycle Sites
Henderson's Dictionary of Biological Terms, by Eleanor Lawrence. 10th ed.
Biology of Plants, by Peter H. Raven, Ray F. Evert, and Helena Curtis. 2nd ed.
Proteins are not only made from carbon and nitrogen, but many important proteins also contain sulfur. Sulfur is also an important component of coenzyme A, which is used to produce energy in cellular respiration. So the availability of sulfur is essential to maintaining life.
Just as plants can not convert N2 into something useful in the nitrogen cycle, neither can plants use elemental sulfur 2S. Again, plants are depended upon bacteria, in this case chemoautotrophic bacteria, which oxidizes elemental sulfur to sulfates, as in the following formula
2S + H2O + 3O2 ---> 2H2SO4.
Once in the form of sulfate (2H2SO4), plants can then incorporate the sulfur into proteins.
Sulfur shares other characteristics to the nitrogen cycle. 2H2SO4 can be converted into hydrogen sulfide (H2S) by sulfur bacteria, as can proteins when were broken down by decomposers.