kruskal wallis post hoc test in R

Hello, 
I need nemenyi-test or any other post-hoc test for kruskal-wallis, but I
just can't find out how to implement this in R. 
My data set is nitrite concentrations in four different groups which I
intend to compare. The kruskal-wallis-test showed significance, but I
still don't know between which groups these significances are. 
Can anybody help me with this?
Thanks