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Dust from agricultural sources may reduce air quality, leading to poorer health among people living and working in affected areas. Baker et al. (1260–1269) report that changing crop management to include conservation tillage practices (e.g., fewer field operations, dedicated traffic lines, and surface residue) can significantly reduce in-the-field dust emissions. Dust concentrations measured in conservation tillage plots over a complete cotton–tomato rotation were lower than in standard tillage plots, although treatments grown with a winter cover crop had an increased organic component in the dust. Reduced dust concentrations in conservation tillage treatments were due primarily to fewer field operations and elimination of several key "dusty" operations, but long-term monitoring is necessary to determine the role of changing soil properties (organic matter content, aggregation) in dust production.
Removing Nitrate from Ground Water
Nitrate removal from ground water can reduce environmental pollution. Denitrification walls are a practical way to remove nitrate, but little is known of their maximum potential because they are normally limited by the amount of nitrate present, carbon substrate, and aeration. Schipper et al. (1270–1276) stimulated denitrification rates in an existing wall by adding extra nitrate. The nitrate removal rate averaged 1.4 g N m–3 of wall material d–1. This was not the maximum rate. Denitrification remained limited by the nitrate concentration, even with additional nitrate. However, these conservative rates can be used by environmental engineers to design denitrification walls for removing nitrate from ground water.
Phosphorus Leaching from Wastewater Application
Land application of wastewater, if not properly managed, presents potential for ground water pollution. Zvomuya et al. (1277–1285) report that applying potato-processing wastewater on seasonally frozen sandy loam soils results in phosphorus leaching past the 1.5-m depth in soils high in soil test phosphorus but not in those that were low in soil test phosphorus. Results from in situ phosphorus breakthrough tests and phosphorus transport simulations with treated wastewater (phosphorus concentration less than 6 mg L–1) suggest that most phosphorus leached from high phosphorus sites because of desorption and dissolution of weakly adsorbed phosphorus from prior phosphorus applications. The authors concluded that in addition to wastewater phosphorus concentration, land-applied wastewater should be regulated based on both soil test phosphorus level and wastewater phosphorus loading.
Nitrate Reduction by Wüstite
Recent strategies to reduce elevated NO–3 concentrations in water use metallic Fe0 as a reductant. Secondary products of Fe0 corrosion include magnetite (Fe3O4), green rust [Fe6(OH)12SO4], and wüstite [FeO(s)]. Rakshit et al. (1286–1292) investigated the reactivity of NO3– in the presence of wüstite under anoxic conditions. Wüstite reduced nitrate rapidly, characterized by stoichiometric conversion of NO3– to NH4+. Reaction rates depended on NO3–, FeO(s), and proton concentrations. The reaction was strongly affected by temperature, and the rate controlling step involves a surface reaction. These findings indicate that wüstite will effectively reduce NO3– concentrations in water.
Biosolid Particles Enhance Metal Migration
Biosolid wastes with heavy associated metals, when applied to land, pose a serious threat to surface water and ground water. Karathanasis et al. (1153–1164) report that metal loads transported in association with biosolid colloid particles migrating through large undisturbed soil monoliths were significantly higher than soluble controls. Aerobically digested biosolid colloids had more metal transporting capacity than poultry manure or lime-stabilized biosolid colloids. Transported metal loads contained both sorbed and soluble forms, following the sequence Cu > Zn > Pb. Soil and colloid metal sorption affinities were not reliable predictors of metal transportability or attenuation. Thus, soils with considerable macroporosity receiving heavy applications of biosolid wastes may pose more risk than anticipated in ground water contamination.
Nickel Distribution in Willow Annual Rings
There is little information available on the spatial distribution and speciation of metals within the tissues of nonhyperaccumulator plants. Punshon et al. (1165–1173) discovered a black willow (Salix nigra L.) growing at the mouth of an eroding former settling pond, containing elevated Ni concentrations localized within an annual ring, and used this as an opportunity to study the micron-scale distribution of Ni and other metals using synchrotron X-ray fluorescence spectroscopy. The distribution of metals, especially Ni, was strongly constrained by the annual ring boundary, with no evidence of post-growth radial translocation. Nickel and Mn were particularly strongly co-associated in Ni enriched regions. X-ray absorption spectroscopy of Ni-enriched areas showed close similarities with a Ni–pectic acid complex, Ni–histidine, and NiSO4.
Biosolids Improve Mine Tailing Properties
Adding municipal biosolids to mine tailings can enhance revegetation, but concerns about potential environmental impact remain, such as increased leaching of NO3– and metals to ground water. Pond et al. (1293–1301) treated different copper mine tailings with several rates of municipal biosolids and exposed them to artificial weathering conditions. In acid tailings, biosolids appeared to reduce the rate of pyrite oxidation. Adding biosolids either decreased or did not affect metal leaching from the tailings, and nitrate leaching decreased sharply with time. Little evidence of adverse environmental impact occurred from applying biosolids to these Cu mine tailings.
Macropore Flow Affects MCPA Losses at the Catchment Scale
If we could identify areas vulnerable to pesticide losses, we could avoid applying pesticides to these areas and minimize their impact on surface waters. This requires an improved understanding of the factors controlling pesticide losses at field and catchment scales. Lindahl et al. (1174–1185) used Monte Carlo sensitivity analysis to identify the main factors responsible for diffuse losses of MCPA (4-chloro-2-methylphenoxyacetic acid) to a stream at the outlet of an intensively monitored small agricultural catchment in Sweden. The most important factors influencing MCPA losses included soil properties, especially those controlling soil structure and macropore flow, management practices such as the timing of application, and weather conditions following application.
Soil Erosion Measured with 137Cs
The radionuclide, 137Cs, contaminated European soils at high levels after the accident at the nuclear power station in Chernobyl. It is strongly adsorbed to soil particles, especially in clay. Hrachowitz et al. (1302–1310) used this property to develop a model that quantifies soil erosion at cultivated and uncultivated sites. The model takes into account vertical migration processes and dilution of 137Cs. Compared to existing models, the advantage of the suggested method is its accurate simulation of 137Cs depth distribution and its site-independent applicability on Chernobyl-contaminated soils.
Organochlorine Pesticide Residues in the Lower Fraser Valley, BC, Canada
Organochlorine pesticide residues from historical use were detected in crop soils, ditch sediments, and water of several farm areas but not in similar substrates from pristine parks in a study by Wan et al. (1186–1193). Selected organochlorine pesticide (except endosulfan) residues, in varying concentrations, remain prevalent in the soils of several farm areas despite being banned for more than three decades. They are likely a major source of continuing input to the Lower Fraser Valley aquatic environment. Concentrations of these pesticides detected in many farm ditches flowing to fish streams exceeded the acute toxicity threshold levels of benthic invertebrates and salmonid fish. They also occur at levels conducive to both bioaccumulation and bioconcentration in the food chain for years to come.
A New Way to Use Solid-State 13C NMR Spectroscopy to Study the Sorption of Organic Compounds to Soil Organic Matter
The toxicity, persistence, and transport of hydrophobic organic contaminants (HOCs) in soils are largely controlled by their sorption to organic matter. Understanding these sorption processes is greatly complicated by the heterogeneity of organic matter. Smernik (1194–1204) has used a nuclear magnetic resonance (NMR) method, which can identify and quantify different types or domains of organic matter, to determine the relative concentration of 13C-labeled HOC molecules in each domain. For HOC molecules ranging from hydrophilic (benzoic acid) to hydrophobic (phenanthrene), the highest HOC concentration was found in highly aromatic domains believed to consist primarily of natural char or black carbon. Furthermore, partitioning into char domains increased with increasing HOC hydrophobicity, with the concentration of phenanthrene nearly 10 times higher in char than nonchar domains. These findings provide direct proof of the importance of soil char domains to HOC sorption, which had previously been inferred in numerous bulk sorption studies.
Fungicide Dissipation Rate Increases
Repeated application of pesticides to soil may increase dissipation rates and reduce persistence and efficacy as well as increase runoff and leaching and their associated environmental risks. Potter et al. (1205–1213) report that dissipation of the fungicide tebuconazole increased up to eightfold following successive applications to peanut. However, the differences in the values of measured tebuconazole concentration in surface soil in field-treated plots and the values predicted using kinetic data indicated that variation in soil temperature and water content had an equal or greater impact on field dissipation rate than on accelerated dissipation. Thus, extending laboratory-derived data to predict responses in the field should be done with caution.
Phosphorus Release Linked to Degree of Soil Saturation
An understanding of the relationships between various soil P determinations and potential P release to water may help in identifying sites for implementing pasture management strategies to limit P loss via overland flow. Mathews et al. (1214–1223) describe these relationships for 12 Hawaiian pasture soils with contrasting soil chemical properties. They found a satisfactory predictive relationship between soil test P and dissolved reactive P desorbed from soil in a water extract (DRPWE) when data for the soil orders with medium to high DRPWE (two Mollisols and an Inceptisol) were pooled separately from those with low DRPWE (five Andisols, three Ultisols, and an Oxisol). The oxalate P saturation index (PSIOX) procedure was the best predictor of DRPWE across soil orders, and there was little DRPWE until PSIOX exceeded 6 to 8% when calculated using oxalate extractable reactive P or total P. A less tedious and more routinely applicable dilute-acid P saturation index procedure showed some utility in predicting DRPWE and was related to the PSIOX procedures. These findings show the benefits of considering the percent saturation with P when attempting to assess potential P release to water across a range of tropical pasture soils derived from volcanic materials.
Quantifying Water Use Efficiency in Northeastern China
Water use efficiency is an important ecophysiological characteristic of plants, especially in semiarid and arid regions. Yu et al. (1311–1318) analyzed intrinsic water use efficiencies of grasses and shrubs along the western part of the Northeast China Transect and their relationships with plant functional types (C3 versus C4) and site variables, including annual rainfall, degradation status, and vegetation type. Photosynthetic pathway played a very important role in determining species intrinsic water use efficiency; the mean intrinsic water use efficiency of C4 plants was almost double that of C3 species. The mean frequency of C4 occurrence at degraded sites was more than double that at nondegraded sites. Consequently, mean site water use efficiency, based on species' intrinsic water use efficiency and frequencies of species occurrence at each site, at degraded sites was more than double that at nondegraded sites. Thus, the level of degradation was a critical indicator of ecosystem structural and functional dynamics along the transect.
Compost Hydrocarbon Content Affects Nitrogen Uptake in Seedlings
Co-composted drilling wastes contaminated with hydrocarbons may be used as growth media in land reclamation and further remediated. Choi et al. (1319–1327) report that, in a pot experiment using 1-, 2-, 3-, and 4-yr-old compost (hydrocarbon concentrations and C to N ratios decreased with age), seedling growth was affected by hydrocarbon concentrations. Plant N uptake increased with compost age, which also corresponds with an increase in indigenous mineral N concentration. Using the carbon-13 technique, they provided evidence that limitations on water uptake caused by the residual hydrocarbon might have been the predominant factor limiting seedling growth in the compost media.
Phosphorus Loading from Field and Nonfield Areas
Phosphorus (P) loss from the upland landscape is controlled by the interaction of P source potential and hydrologic transport factors. On dairy farms in the New York City watershed, soils with high P source potential, as measured by soil nutrient testing, are associated with near-barn areas and historically well-manured fields. Hively et al. (1224–1233) used the National P Project rainfall simulator to characterize surface runoff from nine locations in a dairy farm landscape. Concentrations of dissolved P in runoff correlated well with soil test P, which was highest in frequently manured fields, intensively grazed pastures, and heavily manured near-barn areas. In contrast, forested and extensively managed areas had low soil test P and low runoff P concentrations. Interestingly, nonfield areas produced runoff more quickly than field areas under dry summer conditions (first day of rainfall simulation), indicating that near-barn areas of small spatial extent but high P source potential may play an important role in determining summertime stream water P concentrations. Under wet conditions (second day of rainfall simulation), runoff production was more uniform, indicating that P leaching from high-P field soils and manured areas, along with dilution by clean runoff from forested areas, contributes to streamwater P loading when hydrologic source areas are large. Barnyard and cowpath sampling sites exhibited both quick runoff production and high P source potential, indicating that they are critical management areas.
Bioluminescent Bacteria Indicate Coastal Contamination
As coastal communities grow, threats to the environmental quality of estuarine systems increase. However, except for direct measures of specific contaminants, there are few biologically relevant general indicators of coastal water quality for assessing the effect of human activities on the functioning of coastal ecosystems. Frischer et al. (1328–1336) report an evaluation of a simple and low cost indicator system that relies on naturally occurring populations of luminescent bacteria. Laboratory and field studies demonstrated that the ratio of luminescent bacteria to the total number of cultivatable bacteria could be used to identify coastal waters affected by a wide range of chemical contaminants. However, because there is considerable natural variability in this parameter associated with temperature and salinity, seasonal baseline data sets will have to be developed to effectively use luminescent bacteria as a reliable coastal water quality indicator assay.
Residue and Manure Reduce Runoff and Sediment Losses
Sediments and associated contaminants transported in agricultural runoff degrade surface water. Grande et al. (1337–1346) report the effects of increased residue cover, achieved by high-cutting silage, and manure application on runoff and sediment losses from 3-m2 plots subjected to 76 mm h–1 (1 h) of simulated rainfall. Surface-applied manure reduced spring runoff by 71 to 88% compared to plots receiving no manure. The combination of higher residue cover and spring-applied manure reduced sediment loads by 85 to 97% compared to conventional silage plots receiving no manure. Residue level differences between the two silage treatments had no effect on the aggregate size distribution, but recently applied spring manure enriched the clay-sized fraction of runoff sediments. Results suggest that recent manure additions combined with higher residue levels can substantially reduce sediment losses in spring runoff when soil is most susceptible to erosion.
Model Predicts Manure Phosphorus in Runoff
Dissolved P in runoff from agricultural soils is an environmental concern. Computer models rarely simulate P in runoff from surface-applied manures. Vadas et al. (1347–1353) developed a model to predict P in runoff from manures and tested it using field-plot and soil-box data from three published studies. The model accurately predicted runoff P for soil boxes but underpredicted runoff P for field plots. Underpredictions were caused by runoff to rain ratios used to distribute P into runoff or infiltration. Vadas et al. updated the model with new P distribution fractions to replace runoff to rain ratios. Using a new, independent field-plot runoff data set, Vadas et al. found that the updated model accurately predicted P in runoff from surface-applied manures. Future work should test the model at larger field or watershed scales and at longer time scales.
Manure Use and No-Tillage Are Compatible
How will the combined use of conservation tillage and manure affect water quality in well-drained karst soils? To evaluate leaching of nitrate and herbicide, Stoddard et al. (1354–1362) established a lysimeter study in continuous corn that varied tillage, fertilization, and timing of manure application. The benefits and problems of manure use applied equally to the no-tillage and chisel-tillage systems in this study. Herbicide leaching immediately after application was most affected by macroporosity. Fertilizer N added in spring caused higher nitrate levels in leachate than manure applications, but after three years, nitrate in manured soil leachate exceeded that of soil amended with N only from fertilizer. Manure can be used in no-till corn systems but can substantially affect vadose-zone water quality if N mineralized from previous and current applications exceeds crop requirements.
Manure Favors Runoff and Antibiotic Losses
Recently, veterinary antibiotics have been found in soil as well as surface and ground waters, raising concern about increased antibiotic resistance and the negative effects of increased resistance on ecosystems. To help increase understanding of the fate of veterinary antibiotics in the environment, Burkhardt et al. (1363–1371) performed experiments on how manure influences the transport of sulfonamide antibiotics on grassland. Manure increased the runoff volume up to sixfold and enhanced sulfonamide losses to an even larger extent. Total sulfonamide losses varied between 0.2 and 2.2% of the applied amounts. This range is comparable to losses reported in the literature for other agrochemicals like phosphorus or pesticides.
Estrogen Moving and Changing in Soil
Estrogens are naturally present in animal manures, which are typically applied to the soil with little or no treatment. The parent estrogen, 17ß-estradiol, is very potent and has the potential to disrupt aquatic organisms at extremely low concentrations. Casey et al. (1372–1379) designed batch and column studies to observe the sorption, transformations, and transport of this labile compound. Their experiments, together with inverse modeling, indicated that sorption was hydrophobic and rate-limited. The major daughter product of 17ß-estradiol was estrone, both of which had similar fate and transport characteristics. Under natural conditions, estrogens would not likely persist or be mobile in soils because of their degradation rate and the strength and rate of sorption.
Lagoon-Soil Chemical and Physical Characteristics
The downward movement of lagoon-liquor constituents into the soils underlying animal-waste lagoons has the potential degrade ground water quality. DeSutter et al. (1234–1242) sampled soils beneath three cattle-waste retention lagoons and one swine-waste lagoon to determine the concentrations and extent of movement of constituents normally found in animal waste lagoon liquor. Their research concluded that NH4–N concentrations were highest in the upper 0.5 m of soil under all four lagoons with concentrations ranging from 275 to 1139 mg kg–1. Organic N made up between 39 and 74% of the total N beneath all lagoons with virtually no NO3–N present beneath any of the lagoons. Nitrogen concentrations decreased with increasing depth beneath the lagoons, but concentrations of N were higher than expected background levels, indicating that the sampling did not reach the bottom of the N plume. Determining the depth of movement below lagoons and concentrations of lagoon-liquor constituents in the underlying soil should be addressed when lagoon closure/remediation plans are implemented.
Phosphorus Leaching at Cold Temperatures
Land application of wastewater in the northern-tier United States during winter months may be an alternative to building costly storage lagoons. However, nutrient leaching, especially P, may compromise ground water and surface water quality when large quantities of wastewater are applied during winter months, when plants are not actively growing. In a laboratory study using packed soil columns, Mamo et al. (1243–1250) observed that P leaching can occur past the surface sandy soil when potato wastewater is applied at cold temperatures. Phosphorus leaching was most evident when the soil P level was high and the wastewater P level was low. Most of this P leaching was from soil-adsorbed P. Results showed that, depending on P concentration in the wastewater relative to soil P concentration, the soil could serve as a sink or as a source of P.
Linking Phosphorus Forms in Dairy Diets, Feces, and Manures
Effective manure management to efficiently use organic wastes without causing environmental degradation requires a clear understanding of the transformation of phosphorus forms from diet to manure. Toor et al. (1380–1391), using nuclear magnetic resonance spectroscopy, found that phytic acid concentrations were two times higher in high phosphorus diets than low phosphorus diets. Microbial phytase present in dairy cow rumens converted much of the phytic acid to inorganic orthophosphate. By contrast, phosphonates, phospholipids, and deoxyribonucleic acid increased from diets to feces, suggesting excretion of microbial residues. Phytic acid, deoxyribonucleic acid, and phospholipids decomposed during manure storage, thereby enriching the manures with inorganic orthophosphate. Understanding phosphorus forms in a continuum from dairy diets to feces to manures may provide strategies for manipulating animal diets to increase absorption of phosphorus by animals, thus reducing phosphorus solubility in the resulting manures.
Maintaining Organic Matter in Arid Soils
Applying agricultural or urban biosolids is an option in maintaining adequate levels of organic matter in arid soils. Fares et al. (1392–1403) compared two agricultural biosolids, obtained by aerobic composting and anaerobic processing of farm manure, and municipal waste compost. Chemical characteristics of the manure products were comparable; however, anaerobic processing was much faster (25 d) than aerobic composting (90 d). With 30% ash and 50 mg Pb kg–1, manure products seemed superior to municipal waste compost that contained 65% ash and 108 mg Pb kg–1. On the other hand, the latter had higher C content, lower acidity, and higher exothermic peaks than manure products, thus showing more humification. Municipal waste compost was also more resistant to decomposition in arid soils than manure products, but the observed differences were too small to dismiss any of the products as a viable material for improving soil quality by applying it to arid soils.
Secondary Precipitates Enhance Radionuclide Immobilization
Caustic radioactive wastes have leaked from underground storage tanks at the Hanford site near Richland, Washington, since the 1950s, resulting in potential risks to human health. However, Um et al. (1404–1414) discovered that secondary precipitation following mineral dissolution after contact with simulated caustic wastes not only modified the flow path of the leaking waste plume but also increased the capacity of natural sediments to retard radionuclide mobility. Based on batch equilibrium sorption results, the sorption affinities of radionuclides of major concern at the Hanford site (129I, 79Se, 99Tc, and 90Sr) were much higher on the reacted sediments, including the dominant secondary precipitates (cancrinites), than they were on the unreacted sediments. X-ray microtomography images of a reacted quartz column also revealed that secondary precipitates cemented quartz grains together and modified pore geometry in the center of the column. These findings suggest that secondary precipitates might behave as a sink, preventing radionuclide transport and providing an explanation for the observed pronounced lateral migration of contaminant plumes and the concentration of many contaminants in the sediments within 10 m of the waste tank bottoms and sides.
Soil Amendments Reduce Phosphorus Losses
The environmental impact of nutrient loss from animal waste applied over the long term to grass continues to be a concern. Torbert et al. (1415–1421) evaluated whether the chemical materials of gypsum, lime, or ferrous sulfate could be applied with animal waste on pastures and turfgrass to reduce soluble P that is potentially lost in a runoff event. Runoff events over turfgrass were conducted in runoff boxes that had been amended with composted dairy manure and other soil amendments. Gypsum and lime additions partly reduced soluble P losses moving through the thatch. Adding ferrous sulfate to the sod surface was so effective at reducing the soluble P that the treatment was indistinguishable from no manure application. Further, in subsequent run-over events, no difference was observed from the treatments that received 13.5 Mg ha–1 of manure and those that received none when ferrous sulfate was added as well. This indicates that ferrous sulfate has potential to reduce initial P losses to the environment when applied with manure to pasture, turfgrass, and filter strips.
Wetlands Reduce Agricultural Phosphorus Pollution
Artificial wetlands are increasingly constructed to reduce agricultural phosphorus pollution and to help restore lakes and coastal waters suffering from eutrophication. Retention efficiency varies considerably in wetlands treating agricultural drainage water with an episodic flow regime and highly variable phosphorus concentrations. Reinhardt et al. (1251–1259) show that both water residence time and phosphorus inlet concentration determine phosphorus retention efficiency. Most of the annual phosphorus load is related to high discharge events, so minimum water residence time during these periods is the key factor for phosphorus retention. The studied wetland in central Switzerland retained 23% of the phosphorus load, corresponding to a surface-related retention of 1.1 g P m–2 yr–1.
Evaluating Wetland Soils Using Light
Evaluating soils is expensive, but their variability in space and time makes it necessary to evaluate large numbers of samples in effectively managing the environment. Cohen et al. (1422–1434) report using an optical method to evaluate soils in wetland ecosystems. This optical technique, which involves collecting detailed signatures of reflected light from the soil surface using a spectrometer, has been shown to be useful for upland soils but not for wetland soils. For many of the typical measures of wetland soil quality (e.g., soil organic matter, pH, soil nutrients, microbiological activity), this research shows that the technique provides adequate accuracy for many applications. The advantages of the technique are that it is low cost and very rapid, allowing many more samples to be processed. Therefore, this tool can be used to improve our understanding of soil processes in time and space and eventually improve the way that we manage our soil resources.
Moderate Electrical Potential Dechlorinates Solvents in Microcosm
Electrochemical reduction systems have been investigated as a means to enhance dechlorination reactions to clean up the environment. In a soil system, however, an extreme pH shift resulting in electrical resistance elevation was a major drawback in field application. Shimomura and Sanford (1435–1438) demonstrate that applying a constant potential and ground water recirculation to a sand reactor system could mitigate this problem. Tetrachloroethene in this electrolysis reactor had a half-life of 6.8 d, with the pH of the pore water maintained at less than 9.4. Ethane and ethene were the main dechlorination products in the reactor. Since complete dechlorination can be achieved under moderate pH, this type of electrolysis technology is attractive as a remedial method for subsurface chlorinated solvent contamination.
Canola and Sunflower Diet Amendments Affect Cattle Manure
Cattle producers can replace part of the traditional diet of barley grain/silage with high lipid supplement such as sunflower seeds (SS), alfalfa hay, or canola meal/oil to enhance the conjugated linoleic acids (CLA) content in milk and meat. Hao et al. (1439–1445) reported that steers fed high lipid diets with SS produced manure with a lower pH and higher propionic, isobutyric, and isovaleric content. In traditional diets, nitrogen (N) loss after excretion increases with pH and N levels in both feed and manure. However, diets combining SS with barley silage resulted in a lower volatile fatty acid and ammonia content in manure and should be a better option to manage N nutrient cycles and reduce ammonia and odor problems. The pH of feed and manure should be one factor to consider when determining feed mix rations.
Colloids Can Seriously Disturb Phosphorus Determination
Colloids (particles with a size between 1 nm and 1 µm) are often present in soil solution, soil water extracts, and runoff water, even after filtration over 0.45 µm. Koopmans et al. (1446–1450) determined the P concentration in water extracts of a heavy clay soil, known to be P fixing. The concentrations were unexpectedly high, and this appeared to be due to colloids present in the filtrates. After adding NaCl, shaking, and refiltration over 0.45 µm, measured concentrations were reduced by more than 90%. The colloids had absorbed light during determination of P concentration, thus causing overestimation of the concentration. After removing colloids by flocculation with NaCl and filtration, this effect disappeared.
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