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Nitrogen fertilization for corn (Zea mays L.) production has relied extensively on yield-based recommendations that were developed to represent regional averages, yet are routinely applied to individual fields, on the assumption that fertilizer N serves as the major supply for crop N uptake. Using data from 102 on-farm N-response studies, an evaluation was conducted of the Illinois proven-yield (PY) method for accuracy and economic profitability on a site-by-site basis. As additional objectives, the Illinois soil N test (ISNT) was evaluated for detecting whether N fertilization was economical, and for quantifying crop response to N fertilization relative to soil and management factors. For 18% of the site-years studied, N recommendations by the PY method were accurate to within 20 kg [ha.sup.-1], whereas 13% were underfertilized by 25 to 129 kg [ha.sup.-1] (60 kg [ha.sup.-1] on average) at a current cost of $5 to $170 [ha.sup.-1] ($75 [ha.sup.-1] on average), and 69% were overfertilized by 21 to 235 kg [ha.sup.-1] (103 kg [ha.sup.-1] on average) at a cost of $12 to $130 [ha.sup.-1] ($57 [ha.sup.-1] on average). The latter group included 30 site-years that were completely nonresponsive to N fertilization, all but two of which were predicted by site-average ISNT values assuming a critical test level of 230 mg [kg.sup.-1] . This level was exceeded fur 19 of 69 responsive site-years, mostly during 2001-2003 when corn followed soybean (Glycine max L. Merr.) with high plant populations. A higher critical test level would have been required under such conditions, owing to more extensive residue inputs that would promote microbial N immobilization, and increased crop uptake of mineralized soil N. The ISNT was significantly related to crop N requirement, and was the most powerful predictor of error in PY recommendations (P < 0.001).

A systematic analysis of the effect of displacement length, soil texture, and steady state water flux (q) on solute transport was performed on saturated repacked loam and sandy loam soil columns to identify the simplest possible description that was consistent with the observations across all of the breakthrough curves (BTCs) for each data set. All 39 BTCs obtained from loam and 21 from sandy loam soils, with column lengths of 10, 20, and 30 cm, and a two-order magnitude variation in measured pore water velocity ([v.sub.m]), were fitted simultaneously and global estimates of parameters [dispersivity ([lambda]) and molecular diffusion coefficient ([D.sub.0])] were obtained. We showed that a two-parameter global dispersion relationship (D = [lambda][v.sub.m] + [D.sub.0], where D is the apparent diffusion coefficient) accurately represents the spreading process for all 39 loam soil BTCs ([r.sup.2] > 0.99), whereas a single global parameter (D = [lambda][v.sub.m]) was all that was required for the sandy soil ([r.sup.2] > 0.98). The globally fitted [lambda] was independent of displacement length and [v.sub.m]. D remained independent of [v.sub.m] in the lower velocity ranges; however, a linear relationship between [V.sub.m] and fitted D was obtained for [v.sub.m] > 0.1 cm [h.sup.-1] The results of this study illustrated the importance of molecular diffusion. In addition, we identified a nonlinear relationship between [v.sub.m] and average solute velocity ([v.sub.s]), which suggested that the anion exclusion volume decreased with increasing [v.sub.m].

Three solutes were applied in series under steady, unsaturated flow to determine the spatial scale dependence of transport. Twenty-seven days after solute application, a 2 by 2 by 2 [m.sup.3] field plot was excavated using two measurement scales, 22.2 by 22.2 by 10 [cm.sup.3] (scale L) and 7.4 by 7.4 by 10 [cm.sup.3] (scale l), providing two sets of 81 samples for each 0.1-m layer to a depth of 2.0 m. For each chemical and for each scale, vertical moment analysis was performed on the data for each of the 81 vertical columns. Evidence of enhanced local horizontal mixing with increasing mean travel time was observed in two ways: (i) a decrease in mass recovery variance at scale L for [Mathematical Expression Omitted] (center of mass at 80-cm depth) vs. [Br.sup.-] (46-cm depth of center of mass), and (ii) a horizontal correlation range in solute concentrations in individual 0.1-m-thick layers, which was lowest between 70 and 130 cm deep. Point variograms of the vertical moments for each solute were estimated by simultaneous deconvolution of the two observed (at scales l and L) area-averaged variograms. Dispersion variance analysis (based on these point variograms) characterized the variability within the plot of mass recovery and center of mass depth as a function of sample size. Several approaches for characterizing plot- or field-scale transport features from soil-core scale measurements were evaluated. The horizontal dependence of the vertical variance increased with mean travel time. For each tracer, this dependence was reasonably characterized as the sum of the dispersion variance in the center of mass and the average L-scale vertical variance.

The reduction of hydraulic functions needed to characterize water flow through heterogenous soils using evolved scaling systems is discussed. The concept of water content scaling was extendedto nonuniform soil profiles, and tested of its effectiveness for reducing apparent spatial variability as the unsaturated hydraulic functions were estimated in two instantaneous profile experiments. Heterogenous soil hydraulicproperties were successfully coalesced into unique functions for both water retention and hydraulic conductivity by water content scaling.