widths (= c(300,700,1000,2000): Width of soil layers (in mm).
clay (= 25): Clay percentage for each layer (in %).
sand (= 25): Sand percentage for each layer (in %).
om (= NA): Organic matter percentage for each layer (in %).
bd (= 1.5): Bulk density for each layer (in g/cm3).
rfc (= c(20,40,60,85)): Percentage of rock fragment content (volume basis) for each layer.