Fig. 1. Experimental layout superimposed on the March 1999 shallow (approximately 0–30 cm depth of measurement) apparent electrical conductivity map of the 250-ha site. Field numbers, followed by cropping treatments—winter wheat (W), corn (C), proso millet (M), and fallow (F)—for 1998 and 1999 (in parenthesis) are shown in the upper left-hand corner of each field.