It depends on the crop in question as well as the form of sulfur used. While small grains need sulfur early in the season to promote tiller formation, in the case of canola sulfur is more critical at the time of flowering and seed set; and in the case of modern corn hybrids and soybean varieties, sulfur needs are significant both early in the season as well as post-flowering. Regarding sources, elemental sulfur sources may need up to three seasons to oxidize, depending on environmental conditions, while sulfate sources, like ammonium sulfate, are readily available. At-planting applications of sulfate sources tend to be more efficient than pre-plant applications, since the latter provide more chances for leaching loss. As for in-season applications of sulfate sources, they can be more or less efficient depending on the amount and distribution of rainfall relative to timing of application and crop uptake pattern.