[R] ANOVA define as factor or not

[HronnE]

Hi all

This is probably a simple problem but somehow I am having much trouble with
finding a solution, so I seek your help!

I have a data-set with continuous response variables. The explanatory
variably is 4xpH treatments (so 8.08, 7.94, 7.81 and 7.71)  so  also
continuous and not technically factorial. 

However I have decided to do Anova's (as well as regression) to explore the
effect of pH.

What I don't understand is why the anova done in such a way:

summary(aov(BioMass~pH))

... gives me completely different p-values if I define the pH as factor or
not. And what would be the correct approach?

Help on this subject would be much appreciated!

Hronn


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