How to add exponent value to x title

Having issue adding space,
I am using the following to name my x and y title.

labs(y = "% biomass", x = expression(paste("Mulch amount", tha^{-1})))

It’s resulting in no space between mulch amount and tha (making it Mulchamounttha(-1).

Does anyone know how to add space in between using the same code style?

>Solution :

From ?plotmath

‘x ~~ y’ put extra space between x and y

plot(0:1, 0:1, xlab = expression("Mulch amount"~~tha^{-1}))

Or you could include the space in your string

plot(0:1, 0:1, xlab = expression("Mulch amount "*tha^{-1}))

(since the * operator does juxtaposition, I often use it as a shortcut for paste())