The experimental design was a 3 × 5 factorial with the three soil types (as described above) and five glyphosate rates: 0× (control), 0.5× (29.5 μg ae (acid equivalent) g−1 dry soil), 1× (59 μg ae g−1), 2× (118 μg ae g−1), and 3× (177 μg ae g−1) the recommended field application rate. The form of glyphosate used in this treatment was a K salt, branded Roundup PowerMax (Monsanto, St. Louis, MO, 540 g ae L−1). The application rate of glyphosate was calculated based on the maximum recommended field application rate of 1.74 kg ae ha−1 and a 2 mm interaction depth in the soil (Haney et al. 2000). For all concentrations, the herbicide was diluted in 1 mL of deionized water to improve evenness of application. For this study the commercial formulation was used, which includes the chemical glyphosate and other ingredients, including surfactants. Many laboratory studies use analytical grade glyphosate, which lacks the additional ingredients found in commercial formulation. The use of the commercial formulation allows for a more accurate representation of field conditions. However, it has been shown that surfactants or other ingredients present in the commercial formulation may have a slight inhibitory effect on microbial respiration when compared to analytical grade glyphosate (Haney et al. 2002).