It is an essential part of some amino acids.
Sulfur is thought to be immobile so deficiencies show up in younger leaves.
New growth is very yellow compared to old growth. Plants can reduce sulfate to sulfide and grow with sulfate as the only source of sulfur. Sulfur is very important in legumes, perhaps more important than phosphorus. Deficiency signs in plants Younger leaves are yellowish with a slight upward roll. Stem become woody.