trt <- rep(1:6,each=4)
blk <- rep(1:4,times=6)
nitrate <- c(34.98,41.22,36.94,39.97,
             40.89,46.69,46.65,41.90,
             42.07,49.42,52.68,42.91,
             37.18,45.85,40.23,39.20,
             37.99,41.99,37.61,40.45,
             34.89,50.15,44.57,43.29)

nitrogen <- data.frame(trt,blk,nitrate)

attach(nitrogen)

trt <- factor(trt)
blk <- factor(blk)

tapply(nitrate,trt,mean)
tapply(nitrate,blk,mean)

nitrogen.mod <- aov(nitrate ~ trt + blk)
summary(nitrogen.mod)
TukeyHSD(nitrogen.mod,"trt")