> tapply(nitrate,trt,mean)
      1       2       3       4       5       6 
38.2775 44.0325 46.7700 40.6150 39.5100 43.2250 
> tapply(nitrate,blk,mean)
       1        2        3        4 
38.00000 45.88667 43.11333 41.28667 
> 
> nitrogen.mod <- aov(nitrate ~ trt + blk)
> summary(nitrogen.mod)
            Df Sum Sq Mean Sq F value   Pr(>F)   
trt          5 201.32  40.263  5.5917 0.004191 **
blk          3 197.00  65.668  9.1198 0.001116 **
Residuals   15 108.01   7.201                    
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 
> TukeyHSD(nitrogen.mod,"trt")
  Tukey multiple comparisons of means
    95% family-wise confidence level

Fit: aov(formula = nitrate ~ trt + blk)

$trt
       diff         lwr          upr     p adj
2-1  5.7550  -0.4097239 11.919723853 0.0741633
3-1  8.4925   2.3277761 14.657223853 0.0048516
4-1  2.3375  -3.8272239  8.502223853 0.8149970
5-1  1.2325  -4.9322239  7.397223853 0.9849215
6-1  4.9475  -1.2172239 11.112223853 0.1552755
3-2  2.7375  -3.4272239  8.902223853 0.7024907
4-2 -3.4175  -9.5822239  2.747223853 0.4934365
5-2 -4.5225 -10.6872239  1.642223853 0.2225996
6-2 -0.8075  -6.9722239  5.357223853 0.9978438
4-3 -6.1550 -12.3197239  0.009723853 0.0504756
5-3 -7.2600 -13.4247239 -1.095276147 0.0168331
6-3 -3.5450  -9.7097239  2.619723853 0.4558882
5-4 -1.1050  -7.2697239  5.059723853 0.9907491
6-4  2.6100  -3.5547239  8.774723853 0.7402085
6-5  3.7150  -2.4497239  9.879723853 0.4078918