'data.frame':   120 obs. of  5 variables:
 $ Block       : Factor w/ 2 levels "1","2": 1 1 1 2 2 2 1 1 1 2 ...
 $ Drought     : Factor w/ 5 levels "0","1","2","4",..: 1 1 1 1 1 1 1 1 1 1 ...
 $ Tillage     : Factor w/ 2 levels "Conv","ST": 2 2 2 2 2 2 2 2 2 2 ...
 $ Nutrients   : Factor w/ 2 levels "Background","Fertilized": 2 2 2 2 2 2 1 1 1 1 ...
 $ PlantBiomass: num  2.62 4.48 3.71 1.82 1.52 ...

split.mod<-aov(PlantBiomass ~ Drought + Tillage + Drought:Tillage + Block +
                 Error(Drought:Tillage:Block)+
                 Nutrients + Drought:Nutrients +
                 Tillage:Nutrients +
                 Drought:Tillage:Nutrients,
                 biomass)
summary(split.mod)

#Test homogeneity of variance assumption:
leveneTest(PlantBiomass ~ Drought,data=biomass)
leveneTest(PlantBiomass ~ Tillage,data=biomass)
leveneTest(PlantBiomass ~ Nutrients,data=biomass)

#Levene test result for drought:
Levene's Test for Homogeneity of Variance (center = median)
   Df F value    Pr(>F)
group   4  11.206 1.034e-07 ***
      115
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1