You do need an ammonia sourxe. Rotting plant roots will only go so far. Rotting fishfood or shrimp really smell which can take a long time for the smell to dissipate form the tank. It can also foster mold.
Pure ammonia, that does NOT foam when shaken is preferred for fishless cycling. Clean and controllable.
Well, you're just cycling a 5g for a Betta, right? First see how long it takes for the ammonia to decline. Watch for nitrite to go up then down. You may have enough ammonia to do a Betta-sized cycle already. Once the nitrite starts to decline, you can put a fish or snail in there to keep the ammonia-oxidizing bacteria fed...and you're home free.
Let us know how it goes.