In order to help you, we need to know exactly which NPK sensor you have as there are a few variations available and the all look physically very similar. You need the actual user manual / datasheet for your device as that will tell you the register addresses for the various parameters as well as the default device ID (usually 01) and the default baud rate (can be 9600 or 4800).
Once you have that information, you can setup the nitro, phos and pota arrays with the correct values.
You should also be aware that the code that you are using - which I assume you got from one of the various web sites or youtube videos is flawed. For an explanation of the problem see post #115 of this discussion:
Also search these forums for other discussion on NPK sensors to see how to handle the serial comms better and also how to do it properly with a Modbus library.