Comparing two approaches of characterizing soil map unit behavior in solute transport

Soil maps can be used in different ways to characterize spatial patterns of soil behavior, A first approach is to establish the behavior of a pedogenetically representative profile from each soil map unit (SMU); a second is to subsample each SMU and select from the sample a characteristic profile by its behavior with respect to water and solute transport, The first approach was tested against the second with data from a SMU in the Netherlands. The use of pedogenetically representative profiles resulted in biased estimations of five studied functional properties related to water and solute transport. These properties were (i) the number of days with good workability; (ii) the number of days with sufficient aeration; (iii) the elapsed time to 10% breakthrough of an inert tracer (Cl-); (iv) the percentage breakthrough after 1 yr of an adsorbing, inert contaminant (Cd); (v) the percentage breakthrough after 1 yr of an adsorbing, degrading herbicide (isoproturon [N-(4-isopropylphenyl)-N', N'-dimethylurea]). Soil profiles that did not fit the definition of the SMU (impurities) were responsible for the occurrence of extreme values for four out of five properties, which implies that impurities must be sampled when risk assessments have to be made. In this case, probability sampling offers a valid approach.