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UAN 28% and 32% are both formulations of the same thing, urea-ammonium nitrate, with a 28% nitrogen on one and a 32 % nitrogen on the other. The reason they have both forms is that 32% is how it's produced as a full concentrate and it's used more in southern climates. Then as it gets more in northern climates, especially if it's going to face a winter season, then they dilute it with water because the freezing point is less for the 28%. So the 32% more easily freezes and then it may drop off and salt up.
28% is a very good nitrogen and a pretty clean nitrogen, something we recommend because it's basically a nitrogen that's coming out of the air. It's a better source of nitrogen than anhydrous ammonia which is very hard on soil structure, it's very hard on all sorts of biology. But the 28% is a very good nitrogen. It's very important for corn production and we normally would always mix in 5-10 lbs of sugar when we're using that because you want to add in soluble carbons to slow-release the 28%.
One very nice formula that really goes good to make the nitrogen soil release is if you're using 10 gals. of 28% or 32%, you would add in your 5-10 lbs of dissolved sugar, then you could add in 1-2 gals of Thiosol and 1-2 gals. of ammonium sulfate and you could probably throw in a quart or two of liquid humates (RL37). That has all the components of an amino acid--nitrogen, carbon, hydrogen, oxygen, calcium, sulfates--and it really works good to hold the nitrogen throughout the season that way instead of losing it so quickly.
28% (or 32%) by itself is going to fairly quickly be lost to the atmosphere or leeched, so the nitrogen doesn't stay around so long. But when you put in the sugars and add some of the other components it will stay around much better and work for the plant.
Ammonium sulfate is a good product to use as a broadcast in conjunction with 28% or 32%. About 200 lbs of ammonium sulfate is pretty good and then take the rest of the nitrogen needs using 28% or 32%.