[R] working with ordinal predictor variables?

[Alexandra Thorn]

I'm trying to develop a linear model for crop productivity based on
variables published as part of the SSURGO database released by the
USDA.  My default is to just run lm() with continuous predictor
variables as numeric, and discrete predictor variables as factors, but
some of the discrete variables are ordinal (e.g. drainage class, which
ranges from excessively drained to excessively poorly drained), but
this doesn't make use of the fact that the predictor variables have a
known order.

How do I correctly set up a regression model (with lm or similar) to
detect the influence of ordinal variables?

How will the output differ compared to the dummy variable outputs for
unordered categorical variables.

Thanks,
Alex