Some interesting copy 'n' pasted extracts...
Because the soil organic matter is the main store of both sulfur and nitrogen, however, sulfur released by the decomposition of organic matter declines in proportion to the decrease in release of nitrogen. When all of the nitrogen available for use by a crop is supplied from decomposition of the soil organic matter, the sulfur released at the same time is adequate for crop needs. But if the supply of soil nitrogen is supplemented with heavy applications of sulfur-free nitrogenous fertilizer, the amount of nitrogen available for crop use may be excessive in relation to the sulfur.
Under such high-nitrogen and low-sulfur conditions, plant-growth processes are disrupted, and plants develop symptoms of sulfur deficiency. Sometimes total growth has been reduced by fertilization with nitrogen alone, whereas combined applications of nitrogen and sulfur have given the normal yield increases expected from the nitrogen application.
Deficiencies of sulfur may result from combined additions of nitrogen fertilizers and high-energy, low-sulfur organic materials. Then the available sulfur is tied up by the resulting large increase in the microbes in the soil. This effect has been noted more commonly in greenhouses than in experiments in the field.
The transformation of sulfur in the soil organic matter into forms available for plant use is largely a microbial process. If the soil is well aerated, the organic sulfur is oxidized to sulfates, which plants can use directly.
An interesting application is the use of a commercial product that is fortified with an efficient sulfur-oxidizing microflora and sold as inoculated sulfur. Inoculation hastens the oxidation process. Inoculated sulfur sometimes is more effective than the elemental sulfur.
If the soil is waterlogged or if for other reasons anaerobic conditions prevail, the oxidation is retarded, and even added sulfates may be reduced to elemental sulfur, hydrogen sulfide, or related products.
Sulfates are usable by plants and are mobile within the soil profile.