Glasshouse experiments were conducted at the USDA-ARS Forage and Range Research Laboratory in Logan, UT, USA. Evaporative coolers and radiant heaters maintained a temperature about 20 °C from May to December 2007. Natural light was augmented with 14 h of light each day from sodium lamps. Pots were watered regularly to prevent water stress and minimize leaching. LBH589 Pots were rotated every 2 weeks so that all plots spent equal time in each location on the glasshouse bench. Plant biomass by species and soil type were compared using two-factor anovas. Transformations to meet assumptions of homogeneity and normality were used as necessary. A Tukey–Kramer adjustment was used to compare differences among means. To determine if PSF and competition model predictions differed from observed values, we conducted chi-squared tests for each plant community. Two tests were conducted to determine if the PSF model improved upon competition model predictions. First, a Student’s t-test on the absolute difference between observed and predicted values for the PSF and competition models was conducted. Secondly, predicted and observed results were regressed to determine the goodness-of-fit for the predictions from each model. Significant differences were accepted at P < 0.05. All statistical analyses were conducted using sas v9 for Windows (SAS Institute, Cary, NC, USA). The non-native N-fixer Medicago sativa had greater above-ground biomass and Agropyron cristatum had lower above-ground biomass than other non-native species (Table 2; F3,262 = 137.18, P < 0.001). The native N-fixer Lupinus sericeus had greater above-ground biomass than other native species (Table 2; F3,260 = 186.19, P < 0.001). Mean differences in plant biomass among species was 0.29 ± 0.06 g, which was larger than mean biomass of plants (0.28 ± 0.12 g), which reflected large differences in biomass between N-fixing species and other species. Plant biomass differed among soil types for Hesperostipa comata, Koeleria cristata, A. cristatum, Bromus tectorum and M. sativa (Table 2). When calculated as the difference in plant biomass on different soil types, mean differences among soil types were 0.05 ± 0.02 g, or 20% of plant growth (Table 2). In no case was the difference in plant growth between two soil types greater than the difference in plant growth between the two species that created those soil types. At the end of the experiment, all pots contained all three planted species (i.e. competitive exclusion was not observed in any pot). The PSF model correctly predicted rank order abundance in all four plant communities while the competition model correctly predicted rank order abundance in only two communities (Fig. 3). Despite improving predictions of rank order abundance, PSF model predictions for three of four communities were different from observations (d.f. = 2; N1: χ = 6.13, P = 0.05; N2: χ = 22.38, P < 0.001; X1: χ = 6.29, P = 0.04; X2: χ = 52.29, P < 0.001).