R – ggplot2 positive and negative values different color gradient

I am using R (3.0.1), and ggplot2 (0.9.3.1). I have data that contains both positive and negative values, and I want to obtain a geom_tile plot in which there are different color scale gradients for the positive and the negative numbers (e.g., positive numbers go from red to yellow, negative numbers from blue to cyan). I am able to approach what I want using scale_fill_gradientn (see below), but this results in one continuous gradient that includes "white" color at the midpoint (0). I need to avoid these midpoint colors, and therefore somehow need to "break" the gradients at zero. In other words, I need two continuous gradients (one for positive values, one for negative values) that have a discrete break at zero. Any help?

dat <- data.frame(Row = rep(x = LETTERS[1:5], times = 10)
 , Col = rep(x = LETTERS[1:10], each = 5)
 , Y = runif(n = 50, min = -1, max = 1))

p <- ggplot(data =  dat, aes(x = Row, y = Col)) + 
  geom_tile(aes(fill = Y)) +
  scale_fill_gradientn(colours=c("blue","cyan","white", "yellow","red"), values=rescale(c(-1,0,1)))