Hi Kenny, You have an interesting question.  I am not familiar with tropical agriculture.  I have researched some information about plantains and it sounds like they need a few cultural requirements to grow and produce properly.
The first thing that I noticed was that the pH of the soil should be in the pH 5-7 range.  That means that the soil should be on the acid side.  That affects the absorption of nutrients by the plant.  It can be compared to blueberries that have to have an acid soil for the iron and other micro nutrients to be available for the plant to take them up.  If the pH is in the alkaline range (above pH 7), then the iron and other nutrients are not available for the plant to take them up and utilize them in growth.  Plants will turn a sickly yellow and slowly die.
To combat this, when in doubt, soil test the planting area.  This will tell you what the pH is and some testing labs can give you a recommendation for any crop you might want to grow.  
The second thing I found in my short look for info was that FINCATECH and EOS are a couple of groups that advise growers on proper fertilization for growing plantains.  You can google them as well and see if you find any information that will help you. 
I did find that they were very adamant about proper drainage.  That is a very important part of the cultivation of plantains.  If you let the roots get too wet, then they rot and the whole plant will die from the rot taking over the plant growth.  
As for the Ammonium Sulfate recommendation, that would be in agreement with the fact that they need the lower pH to grow properly.  The nitrogen will help in growth and the sulfate will keep the soil in the pH range that is needed for proper growth.  If you had a soil test and the information on proper fertilization for the plantain crop, then they might also suggest other macro or micro nutrients.  The whole idea is "don't guess, soil test!"
OK, I shouldn't recommend a soil test company over the list of them but I worked while going to school many years ago at a soil test lab.  They did soil analysis for farms all over the country.  They were also one of the first to do tissue analysis with a spectrograph.  Samples came in from far and wide to the lab to be tested.  They would then write up a recommendation based on what you put on the form that the crop was desired.  That lab is now called Spectrum Analytic based in Washington Courthouse, Ohio.  They are in a group of labs that send samples to each other to check their work and recommendations, so they are on the up and up.
Again, soil test is the best bet.  Mailing a sample won't be cheap but it is the only way to know what should be done.  (You won't waste money on putting on unneeded fertilizer or the wrong analysis of fertilizer.)  Hope this helps you.  Don

Thank you, Don, For getting back to me.   I am not in Nicaragua right now, but I am leaving on Wednesday. I was going to do a soil test when I got there. There is a university relatively close to me that I can get a soil test done, but I already know the soil has to be in the neutral to acidic since we are fairly close to a volcano so really I can’t find any reason for using only ammonium sulfate.   I just thought there might be a pretty simple answer to it, but clearly there isn’t.   I told them to go ahead and use the ammonium sulfate and that I would add potassium later if needed like I said I will be there Thursday of this week so I will check it then. Thank you again Don have a good one

Kenny

Hi Kenny, thanks for your remarks and have a safe trip to the tropics.  It is going to be cold here.
It might be that volcanic soils already have potassium in high enough levels.  I don't know how it will break down but some soils already have a high enough level to support good plant growth.  I would ask at the university what the normal ranges are for the area.  That might be why things grow so great there.  Don