[R] expression() and axis labels

[Thomthom]

Hi everyone!

I tried to look up in the previous topics whether there was something
similar to my question but I think there's nothing corresponding...

Anyway! Here is my problem:

I have a plot (qplot()) in which I want to insert axis labels with the
following text:

kg NH4+ N-equiv. * ha-1 yr-1 (well... 4 smaller and down and + as a power
(the same for the -1) but I am sure you understood what I meant).

So, in my code, I typed : 

qplot(...., xlab="Amount of N-fertilizer" expression(kg NH[4]^+ N-equiv. *
ha^-1 yr^-1))

but I don't get what I expect... everything after the first "^" is
considered as a power, whereas I would like to have only the + and the -1 as
a power.

Is there a way to obtain what I want? somehow to split up the expression so
I don't get everything put at the power of NH4.

I already thank you guys for your help!!!

Thomthom

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