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The study examines Chlorpyrifos's aqueous sorption kinetics and equilibria parameters in Teflon Lined Centrifuge Container walls and tropical soils and its persistence in these soils.Varying methanol fractions were used in batch sorption kinetics experiments, and the Solvophobic theory was employed to predict sorption coefficients KWand KOCindirectly.Degradation experiments on ten soils under flooded and field capacity moisture conditions shed light on the influence of biotic and abiotic processes.KWvalue for container walls is 0.19 mL/g, while soil KWvalues range from 46.53 to 56.71 mL/g.KOCvalues range from 1551 to 1890.Degradation half-lives range from 18 to 52 days for sterilized flooded soils, 30–132 days for unsterilized flooded soils, 18–21 days for sterilized field capacity soils, and 32–33 days for unsterilized field capacity soils.The Groundwater ubiquity index demonstrates that Chlorpyrifos does not leach in the selected soils, ranging from 1.013 to 1.536.

Models help to describe and predict complex processes and scenarios that are difficult to understand or measure in environmental management systems. Thus, model simulations were performed (i) to calibrate HYDRUS‐2D for water and solute movement as a possible decision support system for Candler and Immokalee fine sand using data from microsprinkler and drip irrigation methods, (ii) to validate the performance of HYDRUS‐2D using field data of microsprinkler and drip irrigation methods, and (iii) to investigate Br−, NO3−, and water movement using annual or seasonal weather data and variable fertigation scenarios. The model showed reasonably good agreement between measured and simulated values for soil water content (R2= 0.87–1.00), Br (R2= 0.63–0.96), NO3−–N (R2= 0.66–0.98), P (R2= 0.25–0.78), and K (R2= 0.44–0.99) movement. The model could be successfully used for scheduling irrigation and predicting nutrient leaching for both microsprinkler and drip irrigation systems on Florida's sandy soils. The model showed good agreement between measured and simulated values.NO3and Br on Candler fine sand was leached out beyond the 60‐cm depth within <20 d.NO3and Br at Immokalee leached in 20 and 25 d, respectively.The model appears ideal for irrigation scheduling under microirrigation.The model is ideal for predicting nutrient leaching for on Florida's sandy soils.

Phosphorus is among the essential nutrients applied to sugarcane (Saccharum officinarumL.) fields in the form of a fertilizer mixture (N, P, and K) in southwestern Florida. Sorption coefficient is used for modeling P movement, and in this study, we hypothesized that the sorption coefficient determined using fertilizer mixture (N, P, and K) will be significantly different from values determined using KCl and CaCl2, the electrolytes most commonly used for conducting sorption experiments. Supporting electrolytes, 0.01 mol L‐1KCl, 0.005 mol L‐1CaCl2, deionized (DI) water, simulated Florida rain, and fertilizer mixture prepared in Florida rain were used to characterize P sorption. Immokalee (Sandy, siliceous, hyperthermic Arenic Alaquods) and Margate (Sandy, siliceous hyperthermic Mollic Psammaquents) are the dominant mineral soils used for sugarcane production in southwestern Florida; we used the A and Bwhorizons of Margate soil and the A and Bhhorizons of the Immokalee soil for sorption experiments in this study. Freundlich sorption isotherms described P sorption data. The Freundlich sorption isotherm coefficients followed the trend 0.005 mol L‐1CaCl2> 0.01 mol L‐1KCl ≈ fertilizer mixture > simulated Florida rain ≈ DI water. Sorption coefficients were used for modeling P movement with HYDRUS 1D; similar P results were obtained with the 0.01 mol L‐1KCl and fertilizer mixture electrolyte treatments. The sorption coefficient for DI water and simulated Florida rain overpredicted P movement. The P sorption data showed the importance of choosing the appropriate electrolyte for conducting experiments based on the composition of fertilizer. Soil was sampled from two sugarcane fields with Margate and Immokalee soils.Electrolytes used were KCl, CaCl2, water, Florida rain, and fertilizer mixture.The KDof P applied to sugarcane fields of southwestern Florida should be determined using KCl.The kinetics of P applied to sugarcane fields of southwestern Florida can be studied using KCl.Movement of P in sugarcane fields should be modeled using KDdetermined with KCl.

United States Golf Association putting greens are susceptible to nitrogen (N) and phosphorus (P) leaching. Inorganic soil amendments are used to increase moisture and nutrient retention and may influence N and P leaching. This study was conducted to determine whether N and P leaching could be reduced using soil amendments and surfactant‐modified soil amendments. Treatments included a control (sand), sand–peat, zeolite, calcined clay, hexadecyltrimethylammonium–zeolite, and hexadecyltrimethylammonium–calcined clay. Lysimeters were filled with a 30‐cm rootzone layer of sand–peat (85:15 by volume), below which a 5‐cm treatment layer of amendments was placed. A solution of NO3–N, NH4–N, and orthophosphate‐P (2300, 2480, and 4400 μg mL−1, respectively) was injected at the top of each lysimeter, and leachate was collected using an autocollector set to collect a 10‐mL sample every min until four pore volumes were collected. Uncoated amendments, sand, and peat had no influence on NO3–N retention, whereas hexadecyltrimethylammonium‐coated amendments reduced NO3–N leaching to below detectable limits. Both coated and uncoated amendments reduced NH4–N leaching, with zeolite reducing NH4–N leached to near zero regardless of hexadecyltrimethylammonium coating. Pure sand resulted in a 13% reduction of applied orthophosphate‐P leaching, whereas peat contributed to orthophosphate‐P leaching. Surfactant‐modified amendments reduced orthophosphate‐P leaching by as much as 97%. Surfactant‐modified soil amendments can reduce NO3–N, NH4–N, and orthophosphate‐P leaching and, thus, may be a viable option for removing leached N and P before they enter surface or ground waters. Soil amendments did not reduce NO3–N leaching.Modified soil amendments reduced NO3–N, NH4–N, and orthophosphate‐P leaching.Zeolite was most effective at reducing NH4–N leaching.

Advanced citrus production systems (ACPS) are being evaluated in Florida citrus groves for sustainable, profitable citrus production. The ACPS practices provide tight control over water and nutrient‐mediated plant growth and development using irrigation to train the root system into a limited area and fertigate with daily nutrient requirements. Thus, a study was undertaken to (i) compare daily water use using the stem heat balance method on ≤5‐yr‐old citrus trees using two ACPS irrigation methods (using daily drip and microsprinkler irrigation) and conventional irrigation on two Florida sandy soils; (ii) determine relationships between hourly sap flow and time of day, and daily water use vs. total available soil water in the irrigated zone; and (iii) evaluate the effect of Huanglongbing (HLB) disease on water uptake. The sap flow data suggest that intensive drip and microsprinkler irrigation systems resulted in similar or higher water use than conventional irrigation. Water use was greater (P≤ 0.01) with ACPS than conventional microsprinkler practice irrigation in summer 2010, spring 2011, and late summer 2011 at the Entisol site. Water use was 29 to 38% greater using ACPS than conventional irrigation in June 2011 at the Spodosol site, albeit, not significantly different. All irrigation systems showed water contents close to field capacity at both sites, indicating that water was nonlimiting in each irrigation system despite having different irrigation schedules. The high water uptake using intensive irrigation systems is ascribed to frequent irrigation and improved water distribution in the irrigated zone.

Good management of N is critical for improved citrus production and environmental sustainability on Florida sandy soils. Thus, understanding N movement in the root zone is important to devise appropriate nutrient management strategies. Field experiments were conducted on citrus at two sites in Florida to: (i) determine the soil moisture distribution in the irrigated and unirrigated zones as a function of time and depth; (ii) determine NH4+–N and NO3−–N distribution patterns in the irrigated and unirrigated zones as a function of depth, lateral distance, and fertilizer application method; and (iii) determine NH4+sorption in a fertilizer mixture due to cation exchange. Earn 1 CEU in Soil & Water Management by reading this article and completing the quiz at www.certifiedcropadviser.org/certifications/self‐study/655.

Citrus (Citrus sp.) root length density (RLD) can help in understanding and predicting nutrient and water uptake dynamics. A study was conducted at two sites in Florida to investigate root and water distribution patterns among different irrigation and fertigation systems. The results over the 2 years showed that RLD was highest in the 0- to 15-cm soil depth and decreased with depth for all treatments at both sites. About 64% to 82% of the fibrous roots (<1 mm diameter) were concentrated in the irrigated zones of drip- and microsprinkler-irrigated trees and 18% to 36% were found in the nonirrigated zones at the Spodosol site (SS). At the Entisol site (ES), the RLD (<0.5 mm diameter) in the 0- to 15-cm depth soil for intensive microsprinkler or drip irrigation was 3- to 4-fold (nonirrigated zone) and 4- to 7-fold (irrigated zone) greater at the 0- to 15-cm soil depth than that for conventional irrigation system. The trees at SS were symptomatic for Huanglongbing (HLB; Candidatus Liberibacter asiaticus) in the second year, while those at ES were asymptomatic throughout the study. This might have limited the density and extent of root distribution at SS. The water contents remained either close to or slightly above the field capacity. The results showed higher RLD for intensive irrigation and fertigation practices in irrigated zones compared with conventional grower applications suggesting greater water and nutrient uptake potential for the former. [ABSTRACT FROM AUTHOR]

Good management of N is critical for improved citrus production and environmental sustainability on Florida sandy soils. Thus, understanding N movement in the root zone is important to devise appropriate nutrient management strategies. Field experiments were conducted on citrus at two sites in Florida to: (i) determine the soil moisture distribution in the irrigated and unirrigated zones as a function of time and depth; (ii) determine NH4+–N and NO3−–N distribution patterns in the irrigated and unirrigated zones as a function of depth, lateral distance, and fertilizer application method; and (iii) determine NH4+sorption in a fertilizer mixture due to cation exchange. The treatments were (i) a simulated conventional grower practice, irrigated periodically with a large‐pattern microsprinkler; (ii) a simulated open hydroponic system, drip irrigated and fertigated daily in small pulses; and (iii) a simulated open hydroponic system irrigated using a small‐pattern microsprinkler. The experiment was arranged in a randomized complete block design with four replicates. Soil moisture contents varied between 5 and 15%, remaining close to field capacity. Approximately 16 to 100% greater NH4+–N and NO3−–N concentrations were observed in the irrigated zones of the intensively managed drip or restricted microsprinkler irrigation than conventional practice. Ammonium adsorption followed a linear isotherm, with sorption coefficients varying between 1 and 2.2 L kg−1. The results suggest the possibility of promoting nutrient retention and availability with intensive fertigation, thereby decreasing the potential for N leaching beyond the root zone.

Phosphorus and K are critical nutrients in citrus production whose deficiency or excess can affect yield, fruit quality, and water quality. However, no study has been conducted to understand the nutrient distribution in the root zone using intensive fertigation practices in Florida's sandy soils. Thus, experiments were conducted to: (i) compare the performance of intensively managed drip and microsprinkler fertigation systems with conventional grower practices; and (ii) determine P and K adsorption based on recommended fertilizer application rates on Candler (hyperthermic, uncoated Lamellic Quartzipsamments) and Immokalee (sandy, siliceous, hyperthermic Arenic Alaquods) fine sands. Phosphorus and K were applied and tracked with time and distance from point of application. Soil P on Immokalee soil was 27 to 163% higher in the irrigated zone for drip and restricted microsprinkler fertigation than the unirrigated zone and up to 70% greater in the 0‐ to 15‐cm depth of the irrigated zone than conventional microsprinkler practices. Soil K was 5 to 61% greater in the upper 0‐ to 15‐cm of the irrigated zone with drip and microsprinkler fertigation than conventional microsprinkler practices on Immokalee fine sand. Soil P and K on Candler also differed by fertilization method, depth, and between irrigated and unirrigated zones. The linearized P sorption coefficients for Candler were three‐ to fourfold greater than the corresponding depths for Immokalee, while sorption coefficients for K were similar for the two soils. It is unlikely that P or K would present a water quality concern as strictly related to irrigation practice. The P application rate for Candler should be lowered for young trees (<3 yr old).

Land application of wastewater is a common practice. However, coarse‐textured soils and shallow groundwater in Florida present favorable conditions for leaching of wastewater‐applied constituents. Our objective in this study was to determine phosphorus (P) and associated cations (Ca, Mg, K, Na) leaching in a Spodosol irrigated with tomato packinghouse wastewater. We packed 12 polyvinyl chloride soil columns (30 cm internal diameter × 50 cm length) with two soil horizons (Ap and A/E) and conducted 30 sequential leaching events by irrigating with wastewater at low (0.84 cm d−1), medium (1.68 cm d−1), and high (2.51 cm d−1) rates. The control treatment received deionized water at 1.68 cm d−1Leachate pH was lower (6.4–6.5) and electrical conductivity (EC) was higher in the wastewater‐treated columns (0.85–1.78 dS m−1) than in the control treatment (pH 6.9; EC, 0.12 dS m−1) due to the low pH (6.2) and high EC (2.16 dS m−1) of applied wastewater. Mean leachate P concentrations were greatest in the control treatment (0.70 mg L−1), followed by the high (0.60 mg L−1) and low and medium wastewater‐treated columns (0.28–0.33 mg L−1). Leachate concentrations of Na, Ca, Mg, and K were significantly (P< 0.05) greater in wastewater‐treated columns than in the control. Concentrations of P, Na, and K in leachate remained lower than the concentrations in the applied wastewater, indicating their retention in the soil profile. In contrast, leachate Ca and Mg concentrations were greater than in applied wastewater during several leaching events, suggesting that additional Ca and Mg were leached from the soil. Our results suggest that tomato packinghouse wastewater can be beneficially land‐applied at 1.68 cm d−1in Florida's Spodosols without significant P and cation leaching.

Soil organic carbon (OC) content is often estimated by determining organic matter (OM) content via mass loss on ignition (LOI) and assuming that there are 580 g OC kg−1OM. We utilized thermogravimetry (TG), oven‐determined mass LOI, and the Walkley‐Black (WB) wet oxidation method to evaluate the relationship between OC and OM contents of carbonatic and organic soils from Florida and Puerto Rico and Spodosols from Florida. These soils have contrasting components and formed under different ecological settings. The OC content of OM, based on TG‐determined OM mass LOI (TG OM) and WB OC, was higher for Histosols and Spodosols than for carbonatic soils. Regression coefficients between WB OC and TG OM were high for all soils. The factor to convert OM to OC, based on TG OM and WB OC, was 0.56 ± 0.04 (±0.04 is the 95% confidence interval) (R2= 0.93) for carbonatic soils, 0.67 ± 0.06 (R2= 0.94) for Spodosols, and 0.69 ± 0.04 (R2= 0.99) for Histosols. Results show that accuracy in estimating OC from OM content may be improved if the conversion equation is developed from soils within limited compositional and/or biogeochemical ranges. As such, there is no universal conversion factor between OM and OC for all soils. Loss on ignition is not a reliable OM measure if there are mass losses below 200°C that are mistakenly attributed to OM combustion, as in the case of reversible dehydration for Histosols of this study. However, TG enables detection of and accounting for such mass losses. Isothermal TG also confirmed that reversible mass loss between 105 and 200°C was energy—rather than time dependent.

Strongly hydrophobic organic chemicals (SHOCs) can be defined as neutral organic chemicals that have soil organic carbon (OC) normalized sorption coefficient (KOC) >10,000. Sorption isotherms of SHOCs are normally measured in aqueous systems to determine KOCSince SHOCs can adsorb on container walls leading to overestimation of KOC, we used mixed solvent systems to characterize this potential error. Sorption coefficient (KM) and percent recovery (%RM) of anthracene, DDT, and dieldrin during sorption on centrifuge tubes made of polytetrafluoroethylene (PTFE), polycarbonate (PC), polypropylene copolymer (PPCO), and glass high pressure liquid chromatography vials (HPLCV) were measured in volume ratio‐varied methanol‐water mixtures until 100% recovery of the sorbate was achieved. The data were evaluated using the Solvophobic theory. The KMvalues of the entire test SHOCs decreased exponentially with increasing fraction of methanol (fc). For sorption on PTFE, 100% recovery of the three chemicals was at fc> 0.45. However, 100% recovery of DDT and anthracene from PC and PPCO was at fc> 0.90. The 100% recovery of dieldrin from HPLC vials was at fc> 0.70. In water the calculated recoveries of DDT, dieldrin, and anthracene from PTFE were 32, 43, and 48%, respectively. However, the recoveries of dieldrin from HPLC vials and DDT and anthracene from PC and PPCO ranged from 2 to 14%. The data demonstrate that sorption on container walls is a source of error that can reduce the integrity of the analyte and might be one of the causes for the large variability in literature KOCvalues for SHOCs.

Sorption of a representative ionizable nitrogen heterocycle, quinoline (pKa= 4.92), was investigated to determine the relative contributions of the neutral and protonated species to the overall process. Batch sorption experiments were conducted on surfactant‐modified clays that were synthesized from the exchange of hexadecyltrimethylammonium cations for resident sodium cations on a specimen smectite (Swy‐2) at 0, 60, 80, and 100% of the clay's cation exchange capacity (CEC). Hexadecyltrimethylammonium exchange creates highly effective organic partitioning domains within the clay interlayers in proportion to their coverage on the exchange complex. The fractionally exchanged clays, therefore, provided discrete exchange and organic partitioning domains for the protonated and neutral species of quinoline. Data were described by a combined Langmuir‐linear isotherm that permitted independent characterization of both sorption components. Results indicated that cationic sorption dominated but that the neutral species can contribute substantially given sufficient organic carbon content relative to the CEC and at pH above the pKaof quinoline. The data obtained in this study for quinoline demonstrated that the combined isotherm (including cation exchange and hydrophobic partitioning terms) describes sorption data and compares favorably with the purely empirical Freundlish isotherm.

Composts are applied to carbonatic soils in south Florida to improve their physical characteristics and increase water retention. Blends of biosolids and municipal waste are often combined to increase the nutrient content of the compost. However, the high P content of some compost has led to concerns about the potential for P movement into shallow groundwater. Studies were conducted to determine the potential for P leaching in soil amended with biosolids, clean organic waste, and Bedminster composts. Bedminster was the most suitable of the composts used in terms of a lower potential for P leaching as a result of P sorption in the amended soil. Each compost-amended soil demonstrated a slight decrease in P leaching at 1 pore volume after simulated rainfall (21 cm). Pore volume was defined as the total volume in a column less the volume of solids. The high P content of the composts made it unlikely that additions of these materials to soil would improve P sorption capacity. However, Bedminster and clean organic waste did not significantly increase P leaching above that of the soil. Caution should be exercised when applying these composts because materials themselves contain an enormous amount of P that could be eventually transported into the groundwater.

Nile tilapia and Nile perch samples from Lake Victoria were analyzed for lindane (gamma‐1,2,3,4,5,6‐hexachlorocyclohexane), aldrin (1,2,3,4,10,10‐hexachloro‐1,4,4a,5,8,8a‐hexahydro‐1,4:5,8‐dimethanonaphthalene), α‐endosulfan (6,7,8,9,10,10‐hexachloro‐1,5,5a,6,9,9a‐hexahydro‐6,9‐methano‐2,4,3‐ benzo(e) dioxathiepin‐3‐oxide), dieldrin (1,2,3,4,10,10‐hexachloro‐6,7‐epoxy‐1,4,4a,5,6,7,8,8a‐octahydro‐1,4,5,8‐dimethanonaphthalene), DDE (p,p′‐1,1‐dichloro‐2,2‐bis‐(4‐chlorophenyl)ethylene), and DDT (p,p′‐1,1,1‐trichloro‐2,2‐bis‐(4‐chlorophenyl)ethane). No significant difference (α = 0.05) in the residue levels between fish types for lindane, α‐endosulfan, p,p′‐DDE, p,p′‐DDT, and dieldrin was observed. The aldrin levels in Nile perch (Lates niloticus) were significantly higher than the levels in Nile tilapia (Oreochromis niloticus). No difference was observed in the distribution of residues in the different parts of Nile tilapia, although a difference for p,p′‐DDE was observed in the Nile perch. No significant difference was observed in the average fat content of the tissue of Nile perch and Nile tilapia; however, the distribution of fat was significantly different in the different parts of the fish, with the abdominal portion having the highest amount of fat. There was no correlation observed in this study between fat content and organochlorine concentration. Lower p,p′‐DDT residues levels compared with the p,p′‐DDE levels observed in this study indicate that DDT is no longer in use. The levels of organochlorine pesticide residues found in fish samples in this study were below the FAO, U.S. FDA, Australian, and German extraneous residue limits and maximum residue limits. The concentration of organochlorine residues in surface water within the Napoleon Gulf of Lake Victoria was below detection limit (0.1 μg L−1).

A batch reactor was used to determine sorption kinetic parameters (k2, F, and K*) and the equilibrium sorption coefficient (K). The two‐site nonequilibrium (TSNE) batch sorption kinetics model was used to calculate the kinetic parameters. Two probe organic pesticides, atrazine [2‐chloro‐4‐ethylamino‐6‐isopropylamino‐s‐triazine] and diuron [3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea] were studied using three carbonatic soils from South Florida (Chekika, Perrine, and Krome), one noncarbonatic soil from Iowa (Webster), and one organic soil (Lauderhill) from South Florida. Carbonatic soils contained more than 600 g kg−1CaCO3Sorption is initially very fast up to 3 h and then slowly reaches equilibrium. All soil–chemical combinations reached sorption equilibrium after about 24 h and all sorption isotherms were linear. The sorption kinetics data were well described by the TSNE model for all soil‐chemical combinations except for the marl soil data (Perrine–Atrazine), which were better described by the one‐site nonequilibrium (OSNE) model. Diuron, with higher K, undergoes slower sorption kinetics than atrazine. The Lauderhill soil containing organic carbon (OC) of 450 g kg−1exhibited slowest sorption kinetics for both pesticides. An inverse relationship between k2and Kwas observed for atrazine and diuron separately in Chekika, Webster, and Lauderhill soils but not in Perrine and Krome soils. The sorption kinetic parameters were used to distinguish the sorption behavior between atrazine and diuron and to identify differences between soils. Normalizing the sorption coefficient (K) to OC showed that atrazine and diuron had Kocvalues in carbonatic soils that were a third of reported literature values for noncarbonatic soils. Using existing literature Kocvalues in solute transport models will most likely underestimate the mobility of atrazine, diuron, and other neutral organic chemicals in carbonatic soils.

Sediments may act as both a carrier for and a potential source of contaminants such as toxic organics in aquatic environments. This study investigated the spatial distribution of the pesticide DDT [1,1,1‐trichloro‐2,2‐bis(p‐chlorophenyl)ethane] in sediments from the Cedar and Ortega Rivers located in the lower St. Johns River basin, Florida, USA, using field measurements and three‐dimensional kriging analysis. High DDT concentrations were found near the junction of the Cedar and Ortega Rivers and at the north end of the Ortega River in the upper 0.5 m of the sediments, indicating that the sediment was enriched with DDT in the top layer although use of this chlorinated compound was banned in 1972. Further study revealed that the influence of sediment grain size or texture on DDT contamination was negligible in this river system and no linear correlations existed among DDT and its metabolites such as DDD [1,1‐dichloro‐2,2‐bis(p‐chlorophenyl)ethane] and DDE [1,1‐dichloro‐2,2‐bis(p‐chlorophenyl)ethylene]. Comparison of three‐dimensional distribution of DDT content to the Florida sediment quality assessment guideline or probable effect level (PEL) showed several “hot spots” in the Ortega River sediments, where DDT contents exceeded the PEL value of 4.78 μg kg−1Such contamination may pose a significant hazard to aquatic life.

A low-charge Na-montmorillonite (SWy-2) was exchanged with hexadecyltrimethyl-ammonium (HDTMA) at levels equal to 20, 40, 60, 70, 80, 90, 100, 150 and 200% of the cation exchange capacity (819 mmol(+)/kg) to determine the nature of adsorption and the ionic composition of the clay interlayers. In contrast with earlier work with smaller aliphatic cations, which suggested random interstratification of interlayers occupied by either organic or metallic cations, there was no evidence of cation segregation into homogeneous interlayers. Instead, X-ray analysis indicated that the organic cations assumed two dominant configurations which were roughly equivalent in prevalence at ~70% coverage of the CEC. Below 70% exchange the organocations existed predominantly in heterogeneous monolayers with Na+, attaining basal spacings of between 1.41 and 1.44 nm which were sensitive to changes in relative humidity. Relative humidity effects indicated that Na+and HDTMA occupied functionally discrete domains within the interlayer as shown by the free interaction of water and a neutral organic solute, naphthalene, with Na+and HDTMA, respectively. At greater levels of HDTMA exchange (up to 100% of the CEC), the organocations assumed a predominantly bilayer configuration. Transition to a fully-developed bilayer indicated by a 1.77 nm d-spacing at 100% coverage was gradual, suggesting some interstratification of the monolayers and bilayer configurations between 70 and 100% exchange. Sorption of naphthalene to the organoclays within this range of coverage was well correlated with clay organic carbon content, consistent with relatively unimpeded interlayer access of neutral organic molecules.

A field study was conducted to determine the fate of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) within the root zone (0 to 90 cm) of a sandy soil cropped with sorghum [Sorghum bicolor(L.) Moench] in Gainesville, Florida. Atrazine was uniformly applied at a rate of 1.12 kg a.i. ha-1to a sorghum crop under moderate irrigation, optimum irrigation, and no irrigation (rainfed), 2 d after crop emergence. Bromide as a tracer for water movement was applied to the soil as NaBr at a rate of 45 kg Br-ha-1, 3 d before atrazine application. Soil water content, atrazine, and Br-concentrations were determined as a function of time using soil samples taken from the root zone. Atrazine sorption coefficients and degradation rates were determined by depth for the entire root zone in the laboratory. Atrazine was strongly adsorbed within the upper 30 cm of soil and most of the atrazine recovered from the soil during the growing season was in that depth. The estimated half-life for atrazine was 32 d in topsoil to 83 d in subsoil. Atrazine concentration within the root zone decreased from 0.44 kg a.i. ha-12 days after application (DAA) to 0.1 kg a.i. ha-126 DAA. Negligible amounts of atrazine (˜5 µg kg-1) were detected below the 60-cm soil depth by 64 DAA. Most of the decrease in atrazine concentration in the root zone over time was attributed to degradation. In contrast, all applied bromide had leached past the 60-cm soil depth during the same time interval.

Soil hydraulic conductivity (K)-water content ()-pressure head (h) relationships (K--h) are key parameters for crop growth, irrigation, drainage, and modeling water flow and chemical transport through the soil. Several laboratory methods have been used to determine these parameters and to extrapolate to field conditions. However, it is essential to determine these parameters under field conditions to minimize the effect of spatial variability. The objective of this study was to use the Instantaneous Profile method in combination with capacitance probes, tensiometers, the Guelph permeameter, and the van Genuchten hydraulic functions to determine the hydraulic conductivity-water content-pressure head relationships of a Candler fine sand (hyperthermic, uncoated, Typic Quartzipsamments) soil. The soil was flooded for more than 2 h to achieve a constant water content through the profile and was then covered with a plastic sheet to prevent evaporative loss or addition of rain water. The capacitance probe and tensiometer readings were taken simultaneously at 0.1-, 0.2-, 0.4-, 0.7-, and 1.1-m depths. Both water content and hydraulic conductivity values decreased substantially as water redistributed through the soil profile after saturation. Saturated hydraulic conductivity, measured with the Guelph permeameter for the five soil depths, varied between 6.1 and 10.0 m day−1. More than 50% of the soil water was drained from the soil profile within 8 to 10 h following saturation. The analytical RETC (RETension Curve) model was used to extend soil water release curves beyond the limited range of tensiometer suction measurements. Results of this study demonstrate that the Sentek capacitance probe is a practical tool that can be used in combination with the Guelph permeameter and analytical fitting software (RETC) to determine in-situ soil water characteristic curves, saturated hydraulic conductivity, and k() at difference soil depths.

A technique was developed to analyze three beta‐emitter isotopes (45Ca, 36Cl and 3H2O) in a sample, using a two channel liquid scintillation spectrometer. This technique was further successfully used to analyze the concentration of these isotopes in the effluent during miscible displacement studies through columns packed with an Oxisol soil. The isotopic triple labeling approach was used to investigate the effects of physical and chemical properties of the porous medium during solute transport. Tritium was used to study the effects of the physical properties of the porous medium (aggregate size and flux), while 45Ca (a cation) and 36Cl (an anion) were used to study the effects of the chemical properties (solution pH and concentration) of the same porous medium. This is a promising approach to determine how the physical and chemical properties of a soil interact to influence solute transport.

Effluent breakthrough curves (BTCs) for four tracers (3H2O, 14CH3OH, 36Cl, and 45Ca) were measured over a wide range of porewater velocities (3 to 164 cm/hour) using water‐saturated columns of Ione soil, an aggregated Oxisol. Increasing asymmetry of the measured BTCs for all tracers with increasing pore‐water velocity was attributed to decreasing residence time for solute transfer by diffusion between inter‐ and intra‐aggregate regions. The measured BTCs for 36Cl and 45Ca illustrated the role played by the pH‐dependent soil surface charge in determining the retention and transport of ionic species. Measured BTCs were compared with those calculated using a simulation model where (i) convective‐dispersive solute flow was limited to the inter‐aggregate regions, (ii) solute transfer into and out of equivalent spherical aggregates was described by Fick's second law of diffusion, and (iii) isotopic exchange was considered to be an instantaneous reversible process and was described with a linear exchange isotherm. For the slightly sorbed solutes (3H2O and 14CH3OH), all model parameters were independently estimated. Owing to the strong pH‐dependence of isotopic exchange and the inability to exactly duplicate in batch experiments the chemical environment (pH and ionic strength) of the soil columns, exchange coefficients (KD) for 36Cl and 45Ca were estimated by curve‐fitting the model to BTCs measured at the low pore‐water velocity (3 cm/hour). These KDvalues and all other independently estimated parameters were then used to simulate 36Cl and 45Ca BTCs for the high pore‐water velocity (120 cm/hour). Good agreement was found between measured and calculated BTCs for all tracers. These data and simulations demonstrated the coupling between physical processes (convective‐dispersive flow, diffusive mass transfer) and chemical processes (ion exchange, pH‐dependent charge) in determining the shapes and positions of BTCs obtained from water‐saturated aggregated soils with a variable surface charge.

Sorption, degradation, and mineralization of carbaryl (1‐naphthyl N‐methylcarbamate) alone and in mixtures with atrazine(2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) and diuron (N′‐(3,4‐dichlorophenyl)‐N,N‐dimethylurea) were characterized for two topsoils and their respective subsoils. In general, differences in sorption and transformation between single and multiple‐pesticide systems were less than differences due to soil type and between topsoil and subsoil. For a given soil, the topsoil containing more organic carbon (OC) than the subsoil exhibited a larger value for the Freundlich sorption coefficient (Kt). Similarly, a larger value for the soil solution degradation rate coefficient (k1) was obtained from topsoil than subsoil, likely due to greater microbial activity in the topsoil. However, for all soils, sorption of carbaryl on soils considerably increased the magnitude of the degradation half‐life (t1/2) for carbaryl in either single‐pesticide or multiple‐pesticide systems. In soils that contained <10 g kg−1OC content, the effect of multiple‐pesticide systems was to enhance the mineralization of carbaryl. This was attributed to cometabolism. This study shows that carbaryl chemodynamic parameters (Kocand t1/2values) can be used to adequately analyze the fate of carbaryl in the environment even when carbaryl exists in a mixture with atrazine and diuron.

Sorbent‐sorbate interactions heavily retard the movement of P during water flow in most soils. The effect of soil/solution ratio on sorption kinetics and movement of P during unsteady unsaturated water flow were investigated. A series of batch experiments with soil/solution ratios ranging from 0.1 to 6.4 Mg m‐3were conducted to obtain sorption rate coefficients. Aqueous P solutions (100–800 g m‐3) were applied at two constant fluxes (1.4 × 10‐6and 6.9 × 10‐6m s‐1) to columns of an air‐dry spodic soil. The experimental data were simulated with a parallel two‐site nonlinear, nonequilibrium transport model during unsteady, unsaturated water flow. Phosphorous sorption followed Freundlich‐type reversible kinetics with a very fast reaction occurring in Type I sites and a slow reaction occurring in Type II sites. The sorption reaction of P in batch experiments was satisfactorily described by the model but the rate coefficients varied with the soil/solution ratio. Experimentally determined rate coefficients described P movement in column experiments at an influx rate of 6.9 × 10‐6m s‐1, but not for the slower influx rate of 1.4 × 10‐6m s‐1, hence calibration of rate coefficients was necessary for describing P movement during the slower water influx. We developed a technique for determining rate coefficients under water saturated‐unsaturated conditions that provides a way to validate P transport models during transient, unsaturated water flow.

A miscible displacement technique was used to investigate the mobility of P in columns of Bh soil materials from two dairies in the Lake Okeechobee Basin. Phosphorus-laden solutions with four (5, 10, 50, and 100 g m−3) initial concentrations (C0) were applied to soil columns. An increase in the value of C0increased P mobility as determined by BTCs. Shapes of BTCs were characteristic of kinetically-controlled nonlinear sorption reactions, i.e., early breakthrough, clockwise rotation, and tailing.

A theoretical approach, based upon the assumption of predominance of solvophobic interactions, was formulated to quantitatively describe the sorption and transport of hydrophohic organic chemicals (HOC) from aqueous and aqueous‐organic‐solvent mixtures. In the theoretical approach, solvent‐sorbate interactions (solubility) are specifically considered in order to predict sorbate‐sorbent interactions (sorption). For HOC sorption from a single solvent, the HOC sorption coefficient was shown to increase loglinearly with the hydrocarbonaceous surface area (HSA) of the sorbate. For HOC sorption from aqueous‐organic binary solvent mixtures, the sorption coefficient is predicted to decrease exponentially as the fraction of organic cosolvent increases. This is a direct consequence of increased HOC solubility in the binary solvent. Because sorption and mobility of HOC are inversely related, a decrease in sorption coefficient leads to an enhanced HOC mobility as the fraction of organic cosolvent is increased. A preliminary verification of the theory was performed by an analysis of published data for (i) HOC sorption by soils and sediments from water, (ii) HOC retention by reversed‐phase chromatographic sorbents during isocratic elution with methanol‐water binary solvent mixture, and (iii) HOC mobility on soil‐TLC plates eluted with ethanol‐water mixtures.

Soil profiles were described on a grid at 60‐m intervals to assess the variability of soil morphological properties in 60 ha of Florida flatwoods. Depths from the soil surface to the spodic and argillic horizons were mapped using field data supplemented with kriged values. In addition, a 120 by 120 m subarea was described at 15‐m intervals. The occurrence of a single deep spodic horizon at a few peripheral locations influenced the structure of variograms and contributed to anisotropy. The soil profiles adjacent to these locations exhibited two spodic horizons, one much deeper than the other. The variance structure model for spodic horizon depth in the overall 60‐ha site accurately described the variance structure for the subarea. At some sampling locations, neither a spodic nor an argillic horizon was observed. Absence of the argillic horizon was more common than absence of the spodic horizon. Areal discontinuity in the argillic horizon was more extensive than in the spodic horizon. Such discontinuity in the argillic horizon was observed near and between depressional areas. Although the variance structure of depth to the argillic horizon could be described using an anisotropic variogram model, better estimates of depth to the argillic horizon were obtained using variograms of residuals and drift functions.

The Ione Formation occurs as isolated exposures along the western foothills of the Sierra Nevada. Presence of an intensely weathered soil profile (an Oxisol) on remnants of the Eocene Ione Formation is direct pedologic evidence for a tropical climate during or subsequent to that time. This is the first detailed description of an exhumed Oxisol in the continental United States. The pedon described here has a thin Al horizon, a red and yellow ironstone horizon over a thick oxic horizon. Below the oxic horizon is a thick deposit of kaolinite clay. The oxic horizon has 11.4% F2O3, 0.04% organic C, 10.9 meq/100 g clay CEC (Cation Exchange Capacity), 3.5 meq/100 g clay KCl extractable Al, and a pH in H2O of 3.3. Kaolinite is the only clay mineral present. This relict soil pedon does not meet criteria below the order level in Soil Taxonomybecause of moisture regime limitations. Data such as these illustrate the usefulness of pedology in the study of Tertiary paleoclimatology.

Nutrient redistribution in grazed pastures occurs primarily via deposition of excreta by animals. Because this redistribution is generally not uniform, a composite sample of randomly collected soil cores may not reflect the nutrient status of much of the pasture. To address this limitation, zonal soil sampling, with zones determined by distance from shade, water, and supplemental feeders (lounging areas), has been proposed as an alternative sampling strategy. This experiment was conducted to evaluate the potential of zonal soil sampling for monitoring extractable K distribution in grazed pastures. Utility of the zonal approach was evaluated by comparing it with a more rigorous, but less practical transect sampling procedure. Rotationally and continuously stocked bermudagrass [Cynodon dactylon(L.) Pers. ‘Callie’] pastures were used in the study. Ranges of extractable K within a pasture were as great as 13‐fold (14‐181 mg kg−1) after 2 yr of grazing. Extractable K concentration was greatest in the one‐third of the pastures closest to lounging areas regardless of grazing method. Data from zonal soil sampling described this distribution of soil K. Kriged contour maps constructed using data from transect soil sampling supported the conclusions made using zonal sampling. We conclude that zonal soil sampling is a useful and practical tool for describing K redistribution in pastures and for improving the utility of soil test results and fertilizer recommendations for grazed swards.

In many fertilizer trials, the amount of nitrate-nitrogen in soil solution must be quantified frequently because nitrate is easily leached. Because pesticides are generally applied to cropland with fertilizers, quantitative information is needed on the concentration of these chemicals still available in the soil. Information on nitrite, nitrate and pesticide concentrations in food, water and environmental samples is essential because of their toxicity and potential for groundwater and surface water contamination. Most of the methods currently used for nitrate determination also account for nitrite, because nitrite and some organics act as interferences. Some of the existing analytical methods require sample reduction or derivatization, complex solvent mixtures or large sample volumes which make analysis times long. A High-Performance Liquid Chromatography (HPLC) method has been developed for the simultaneous determination of nitrate, nitrite and organic pesticides in soil solution samples and extracts using a multidimensional separator column with ultraviolet detection at 220 nm. The method is rapid and requires small sample volumes (20 μL). It is a sensitive method which is suitable for routine analyses of up to 100 samples per day. A comparison of this method with standard ion chromatography with conductivity detection showed very good agreement between the two methods for the analysis of NO3- and NO2-

A high performance liquid chromatography (HPLC) method with UV detection was developed to analyze paraquat (1,1′‐dimethyl‐4,4′‐dipyridinium dichloride) herbicide content in soil solution samples. The analytical method was compared with the liquid scintillation counting (LSC) method using 14C‐paraquat. Agreement obtained between the two methods was reasonable. However, the detection limit for paraquat analysis was 0.5 mg L−1by the HPLC method and 0.05 mg L−1by the LSC method. The LSC method was, therefore, 10 times more precise than the HPLC method for solution concentrations less than 1 mg L−1In spite of the high detection limit, the 12C (nonradioactive) HPLC method provides an inexpensive and environmentally safe means for determining paraquat concentration in soil solution compared with the 14C‐LSC method.