No abstract

Saturated hydraulic conductivity (KSAT) is an important soil property that is difficult to measure. Positive‐head tension infiltrometer (TI) and single‐ring pressure infiltrometer (PI) techniques show promise for measuring KSAT, but there have been few field tests or comparisons with other methods. The TI, PI, and classical undisturbed soil core (SC) methods for measuring KSAT were compared on single‐grain sand, structured loam, and cracking‐clay loam soils under conventional tillage (CT), no‐tillage (NT), and native woodlot (WL) managements. Of the 27 between‐method correlations (3 methods × 3 soils × 3 managements), only four were significant (P < 0.05). The TI method yielded lower KSAT values under high‐permeability conditions (KSAT ≥ 10−4 ms−1) relative to the other methods, as evidenced by lower geometric mean KSAT (KGM), lower maximum KSAT (KMAX), and lower minimum KSAT (KMIN) values. The 0.10‐m diam. by 0.10‐m‐long SC method cores may have been too small to yield representative estimates of KSAT in the cracking‐clay loam and in the NT and WL managements of the sand and loam, as indicated by high coefficients of variation (CVs), inconsistent KGM values, or high KMAX values relative to the other methods. Erratic KMAX and KMIN values, along with high CVs, suggest that the 0.10‐m‐diam. PI ring may have been too small to adequately sample the cracking clay loam soil under CT and NT management. Further work appears warranted for developing KSAT measurement methods, interpreting KSAT results, and determining appropriate KSAT methods for various soil types and conditions.

Mobility and persistence of commercial formulations of alachlor, metolachlor and isazofos were studied under two moisture regimes using 75 × 15 cm field lysimeters. Formulated atrazine was also applied to each lysimeter for reference purposes. Alachlor‐ and metolachlor‐treated lysimeters were packed with Plainfield sand, while Honeywood silt loam was used to pack isazofos‐treated lysimeters. Effluent was monitored for each chemical, and selected cores were sectioned (7 × 10 cm) and analyzed to determine mobility profiles and persistence at weeks 1, 2, 4, 8, 12 and 21. The 50% disappearance times (DT50) for alachlor, atrazine, isazofos and metolachlor were: <1.5, 4, 1.5 and 3 to 4 weeks, respectively. Water application during week 1, following pesticide application, created ponding on Honeywood soil cores, transporting atrazine and isazofos to a maximum 50‐cm depth. While isazofos moved no further after week 1, atrazine and desethylatrazine (Des‐Atr.) exhibited considerable mobility throughout the study. Ponding on Honeywood silt loam cores produced greater atrazine and Des‐Atr. movement than in Plainfield sand cores. Des‐Atr. production increased with soil moisture content. Relative mobilities in Plainfield sand were: Des‐Atr. ≥ atrazine > metolachlor > alachlor; in Honeywood silt loam: Des‐Atr. > atrazine > isazofos.

This study was conducted to investigate the influence of the saturating cation on the adsorption of parathion, methyl parathion, fenitrothion, aminoparathion, and paraoxon by montmorillonite suspensions. In all cases the saturating cation distinctly influenced the Freundlich-type adsorption, with adsorption decreasing in the following sequence: Fe 3+-> Ca 2+-> Na +-montmorillonite. Adsorption of parathion, methyl parathion, fenitrothion and aminoparathion at low concentrations varied inversely with their water solubilities in Na+-and Ca 2+-montmorillonite suspensions, i.e. parathion > fenitrothion > methyl parathion > aminoparathion > fenitrothion > methyl parathion > aminoparathion adsorption. Paraoxon adsorption was slightly greater than the compound with the next lower solubility, aminoparathion. The water solubility of parathion at 20ºC was 12.9 µg/ml. Aminoparathion was more than 99.9% adsorbed from solution by Fe 3+-montmorillonite, suggesting the possibility of protonation of the-NH 2 group by the acidic clay surfaces. In Na +-and Ca 2+-montmorillonite suspensions, there was some conversion of paraoxon to p-nitrophenol. Parathion adsorption-desorption exhibited greater hysteresis effects in Fe 3+-montmorillonite than in Ca 2+-montmorillonite. The desorption pathway was dependent only on the initial concentration. The volume of solution removed in each cycle did not alter the desorption pathway, but only the rate at which the desorption proceeded down the desorption isotherm.

This investigation was undertaken to determine whether desorption hysteresis effects were dependent upon methods used in obtaining desorption data. Adsorption‐desorption isotherms were obtained for two organophosphorus insecticides in aqueous suspensions of a clay, a sandy loam, and an organic soil. Desorption isotherms were obtained using both the consecutive desorption method and the dilution method. In the consecutive method, the same sample (after adsorption step) was put through a series of equilibration‐centrifugation steps in which, following equilibration, the adsorbent was thrown down by centrifugation, and part of the supernatant pesticide solution was replaced by distilled water before reequilibration. In the dilution method a series of replicate samples (same adsorbent weight), after the adsorption step, were diluted to different volumes with distilled water, reequilibrated, then centrifuged to separate the phases. With the exception of one insecticide‐soil system (where both methods produced minimal hysteresis), hysteresis effects were considerably reduced by using the dilution method. Repeated centrifugation appeared to be associated with the appearance of hysteresis effects observed using the consecutive desorption method. A short discussion is included on the improper use of desorption data to construct “single point desorption isotherms,” which has created confusion in the literature.