# We first start by creating vectors.
> site_id <- c("A1.01", "A1.02", "B1.01", "B1.02")
> soil_pH <- c(5.6, 7.3, 4.1, 6.0)
> num_sp <- c(17, 23, 15, 7)
> treatment <- c("Fert", "Fert", "No_fert", "No_fert")

# We then combine them to create a data frame with the data.frame function.
my_df <- data.frame(site_id, soil_pH, num_sp, treatment)

# Visualise it!
my_df