Have been sidedressing everything since '01, had some of the wettest springs on record since then and have got every acre covered every year, not always easy but it's always gotten done.  We use NH3 since it's cheaper and I think after a wet spring like we've been having running a knife through the ground to open it up helps get some air down there.  In my opinion the key to making NH3 work for sidedressing it to make sure you have good closers, you want them mounted so the disks run beside the shank to catch the dirt coming off the knife.  We run knives on 60" centers, so our 12 row toolbars have six knives each but each knife has two delivery tubes and two hoses running to it.  The double tubes keeps the pressure down and then if one hose does plug your still getting at least a half rate to that knife. 

As for rates on corn after soybeans we have found with a split application of N (10 gpa of 32% with the planter and sidedress NH3) our most profitable N rate is 140# and corn on corn is 160#.  We have been doing on the farm nitrogen plots since '01 and it seems like regardless of where corn and N prices are at the time those rates are the most profitable.  We've seen lots of 250+ bpa corn on the yield monitor even long term continous corn with these nitrogen rates so under most conditions I don't think nitrogen is a yield limiting factor for us.  One nice thing about side dressing is we can vary our rate based on what the corn looks like at the time, if we have a great stand we can increase it some or with a poor stand reduce how much we put on. This is a picture of our better sidedress bar built out of an IH 183 cultivator and this video was taken in '08 in some of the first corn we sidedressed that year.