[R] Sub- or superscript in factorial variable - possible?

[mcg]

Hello R-users,

I would like to use subscript in chemical formulas for the different 
treatments in a boxplot.
Fot title, xlab and ylab sub- and superscript is no problem, but for the 
different treatments of the following example I cannot get subscript.

Example:
weight <-  c(6,5,7,2,7,3,9,4,2,7,8,9,2,3,4,5)
treatments <- as.factor(rep(c('Control', 'P2O5','K2SO4','CaSO4'),4))
data <- data.frame(treatments,weight)
boxplot(data$weight~data$treatments)

If I apply expression(P[2]...) I get  "unimplemented type 'expression' 
in 'HashTableSetup' ".
If there is a solution for this in base graphics or ggplot please let me 
know.

Thanks in advance,
Michael