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Claims have been made recently that glyphosate-resistant (GR) crops sometimes have mineral deficiencies and increased plant disease. This review evaluates the literature that is germane to these claims. Our conclusions are: (1) although there is conflicting literature on the effects of glyphosate on mineral nutrition on GR crops, most of the literature indicates that mineral nutrition in GR crops is not affected by either the GR trait or by application of glyphosate; (2) most of the available data support the view that neither the GR transgenes nor glyphosate use in GR crops increases crop disease; and (3) yield data on GR crops do not support the hypotheses that there are substantive mineral nutrition or disease problems that are specific to GR crops.

Smectites may strongly influence the fate of pesticides in soils due to their large surface area and abundance in agricultural soils. This research was undertaken to determine the effects of smectite properties on the affinity of smectitic clays for atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1, 3, 5 triazine). Samples of the <2‐µm fraction were separated by sedimentation from 13 reference smectites and treated with H2O2 for removal of organic matter. A soil smectite sample was separated (<0.02‐µm fraction) from a H2O2 and dithionite‐citrate‐bicarbonate‐treated Webster (fine‐loamy, mixed, mesic Typic Haplaquoll) soil by a dispersion‐centrifugation‐decantation technique. All 14 samples were Ca saturated, dialyzed free of excess electrolyte, and freeze dried. The mineralogy of each sample was evaluated by a combination of x‐ray diffraction and chemical analysis. Values for cation‐exchange capacity (CEC), surface charge density (SCD), surface area (SA), and percent tetrahedral charge (TC) were based on structural interpretations of the chemical analyses. Freundlich adsorption isotherms for atrazine adsorption on each sample were determined using the batch‐equilibration method. Smectite was the dominant mineral in 13 of the clay samples; the 14th sample was a mixture of interstratified smectite‐illite, quartz, and kaolinite. For the smectitic clays, atrazine adsorption ranged from 0 to 100%, and the logarithm of the Freundlich adsorption constant [log(Kf)] decreased linearly with the CEC (r2 = 0.82) of the clays. Stepwise multiple‐regression analysis indicated that log (Kf) values were correlated with SCD and SA values (r2 = 0.83). Inclusion of TC and suspension pH as independent variables in the regression analysis did not significantly improve the correlations. The results indicate that the affinity of smectites for atrazine decreases with increasing SCD, which suggests that atrazine is primarily adsorbed by smectites as a neutral species.

In soils low in organic matter, pesticide adsorption and desorption by clay minerals may strongly influence the fate of pesticides in soil environments. Atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) adsorption‐desorption was determined on 11 reference smectites (<2‐µm size fraction), and a soil smectite (<0.02‐µm size fraction from the Ap horizon of a fine‐loamy, mixed, mesic Typic Haplaquoll). For each clay sample, adsorption and desorption isotherms were determined using batch equilibration. Atrazine adsorption on the clays decreased with increasing surface change density (SCD) of the smectites. The desorption isotherms indicated that adsorption was generally reversible. This suggests that atrazine is primarily adsorbed on smectite surfaces through relatively weak van der Waals or H bonds. However, a small positive hysteresis was observed with some clays, and the magnitude of the hysteresis, evaluated using the ratio of the Freundlich isotherm coefficients for adsorption and desorption, increased with SCD. This suggests that atrazine is retained by stronger binding mechanisms on smectites with high SCD, in spite of its lower adsorption capacity. On clays with high atrazine adsorption coefficients, the amount of atrazine desorbed was larger than would be predicted from the adsorption isotherms, resulting in a “negative hysteresis” whereby the desorption isotherm slope was greater than the adsorption isotherm slope. “Negative hysteresis” can be explained if atrazine is assumed to be excluded from interlayer or intraquasi‐crystal water, where only external or interquasi‐crystal water would be available during the desorption process.

Sorption−desorption of imidacloprid [1-[(6-chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolidinimine] and metabolites 1-[(6-chloro-3-pyridinyl)methyl]-2-imidazolidinone (imidacloprid−urea), 1-[(6-chloro-3-pyridinyl)methyl]-4,5-dihydro-1H-imidazol-2-amine (imidacloprid−guanidine), and 1-[(6-chloro-3-pyridinyl)methyl]-1H-imidazol-2-amine (imidacloprid−guanidine−olefin) in three soils was determined using the batch equilibration technique with initial concentrations for the four chemicals ranging from 0.05 to 1.5 μg mL-1, which corresponds to a field application rate of 0.2−1.0 kg ha-1. Calculated slopes of the Freundlich sorption isotherms were significantly less than 1. The order of sorption (K f) was imidacloprid−guanidine > imidacloprid−guanidine−olefin > imidacloprid > imidacloprid−urea in the three soils. Average K f-oc values were 203, 412, 2740, and 3200 for imidacloprid−urea, imidacloprid, imidacloprid−guanidine−olefin, and imidacloprid−guanidine, respectively. Desorption was hysteretic for all chemicals in all soils. Greatest hysteresis was observed with imidacloprid−guanidine and imidacloprid−guanidine−olefin. Sorption−desorption of imidacloprid determined at half the solubility (250 μg mL-1) (K oc = 77) greatly overpredicts potential leaching compared to K oc determined at field application rates (K f-oc = 411). Keywords: Imidacloprid; metabolites; sorption; desorption; hysteresis