COMPARING 2 APPROACHES OF CHARACTERIZING SOIL MAP UNIT BEHAVIOR IN SOLUTE TRANSPORT

Soil maps can be used in different ways to characterize spatial patter ns 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 chara cteristic profile by its behavior with respect to water and solute tra nsport, The first approach was tested against the second with data fro m a SMU in the Netherlands. The use of pedogenetically representative profiles resulted in biased estimations of five studied functional pro perties related to water and solute transport. These properties were ( i) the number of days with good workability; (ii) the number of days w ith 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-is opropylphenyl)-N', N'-dimethylurea]). Soil profiles that did not fit t he definition of the SMU (impurities) were responsible for the occurre nce of extreme values for four out of five properties, which implies t hat impurities must be sampled when risk assessments have to be made. In this case, probability sampling offers a valid approach.