Several people asked recently about preflood nitrogen sources that contain sulfur. The main two fertilizer forms we’re referring to are ammonium sulfate or AMS (21-0-0-24) and sulfur-coated urea (41-0-0-4).
Many folks like the quick “green up” they see from early V2-V3 applications of AMS on rice. But on silt loam soils, we don’t generally observe other benefits besides the improved green color.
Some people associate this improved color and crop response with the sulfur in AMS. But since most of our soils have adequate amounts of sulfur, this is not the case.
Instead, what you’re seeing is a response to the more readily available N in AMS in the form of ammonium. In contrast, urea must hydrolyze into an ammonium form, and that takes abou 2 days.
It’s a “delayed fuse” effect with urea compared to AMS.
With this quick-greening response in mind, some have moved toward using sulfur-coated urea or 41-0- 0-4 (as it’s often referred to by its analysis). Again, the idea is that sulfur provides a boost or quick green-up. In reality, the sulfur in 41-0-0-4 is elemental sulfur, not sulfate as in AMS, and it takes a long time to become plant-available, So, it can’t provide a kick even if you did need sulfur.
If You Think You Need Sulfur…
Using 41-0-0-4 is also less desirable because it is actually a controlled (delayed) release. This could cause availability issues when rice hits its high-demand for nitrogen at the mid-tillering stage.
So, using 41-0-0-4 is going to cost you more money per unit of nitrogen, the sulfur won’t be available for a long time, and the availability of that nitrogen will be slower. So, this is an option we probably want to avoid.
If you think you need sulfur or want to give a little kick with your preflood, it’s more economical to blend in a little AMS with your preflood urea. Odds are, you don’t need to pay more for the AMS since urea works just fine, but going with a small amount of AMS is a better option than using 41-0-0-4.