pH Of Soil Suspension Research Articles

Coupling soil washing with chelator and cathodic reduction treatment for a multi-metal contaminated soil: Effect of pH controlling

Contamination of soil with heavy metals has become a significant environmental issue worldwide. In previous studies, a technology that couped EDTA-washing and cathodic reduction technology was developed to further remove heavy metals from soil contaminated with electroplating waste compared with single soil washing technology. Although the method showed a synergistic effect in previous studies, the effect of soil suspension alkalization and the mechanism behind it remain unclear. High soil pH will have an impact on target metal removal, energy consumption and further utilization of soil. In this experimental research, we investigated the efficacy of pH control on coupling soil washing and cathodic reduction to remove multiple heavy metals from soil under various current densities. To alleviate the OH− ions generated by the cathode and improve the reduction efficiency of metals, an automatic pH controller was used, resulting in a significant increase in the removal rate of target metals. The experimental results show that controlling the pH of soil suspension can increase the total removal rate of target metals by 3-6 times. Additionally, the removal efficiency of residual Pb in the soil is increased by about 30%. In terms of ecological risk, pH control could reduce the solubility and mobility of metals by 50%, and the ecological risk was greatly reduced, especially for Zn, from about 90 mg/kg to 50 mg/kg. Further research is needed to explore the mechanism of the synergistic effect and improve the efficiency of this technology for the subsequent development of coupling EDTA-washing and cathodic reduction.

Agroecosystem Degradation Evaluation of Broccoli (Brassica oleracea) Farm Using Some Biotic Indices in Batu, East Java, Indonesia

The aim of this study was to evaluate the degradation of broccoli agroecosystem in Batu caused by a longterm of intensive farming and of synthetic chemical exposure using some biotic indices. Evaluation covered on quality of habitat, biodiversity, and environmental services in three organic and intensive farm, each Batu, Bumiaji, and Junrejo Regencies, using five repetitions in each location. Habitat quality was evaluated by measuring the soil organic matter, water irrigation and soil suspension pH, electric conductivity, and water turbidity in situ . Biodiversity quality was evaluated by determining the score of Importance Value Index (IVI), Shannon-Wiener Diversity Index (H’) and Margalef’s Richness Index (R) soil fauna using handsorting within the area of l m 2 , as well as refugial vegetation. Environmental service quality was evaluated by scoring using the Ecosystem Integrated Assessment (EIA). The data were analyzed statistically using Principal Component Analysis (PCA). The result showed that soil organic matter and pH were obviously higher, while the irrigation water was less turbid in organic farming rather than the intensive one. Furthermore, the H’ and R indices proved that biodiversity quality in organic farming field was higher (H’ score 1.4-1.9 and R 3.5-4.0) comparing to those of intensive farming (H’ score 0.6-0.9 and R 1.2-1.6). Whereas, the ecosystem service based on EIA index revealed that the organic agroecosystem showed a fair to good quality with the score 3.0-4.3. It was higher than those of intensive farming which was classified as poor to fair with the score 1.7-2.8. Therefore, the organic broccoli agroecosystem in Batu provided a better habitat for biodiversity conservation and greater the environmental service as a sustainable farming system. Key words : Agroecosystem, Biodiversity, Ecological services, Intensive, Organic

Adsorption and attenuation behavior of 3-nitro-1,2,4-triazol-5-one (NTO) in eleven soils.

NTO (3-nitro-1,2,4-triazol-5-one) is one of the new explosive compounds used in insensitive munitions (IM) developed to replace traditional explosives, TNT and RDX. Data on NTO fate and transport is needed to determine its environmental behavior and potential for groundwater contamination. We conducted a series of kinetic and equilibrium batch experiments to characterize the fate of NTO in soils and the effect of soil geochemical properties on NTO-soil interactions. A set of experiments was also conducted using sterilized soils to evaluate the contribution of biodegradation to NTO attenuation. Measured pH values for NTO solutions decreased from 5.98 ± 0.13 to 3.50 ± 0.06 with increase in NTO concentration from 0.78 to 100 mg L(-1). Conversely, the pH of soil suspensions was not significantly affected by NTO in this concentration range. NTO experienced minimal adsorption, with measured adsorption coefficients being less than 1 cm(3) g(-1) for all studied soils. There was a highly significant inverse relationship between the measured NTO adsorption coefficients and soil pH (P = 0.00011), indicating the role of NTO and soil charge in adsorption processes. In kinetic experiments, 1st order transformation rate constant estimates ranged between 0.0004 h(-1) and 0.0142 h(-1) (equivalent to half-lives of 72 and 2 d, respectively), and correlated positively with organic carbon in the soil. Total attenuation of NTO was higher in untreated versus sterilized samples, suggesting that NTO was being biodegraded. The information presented herein can be used to help evaluate NTO potential for natural attenuation in soils.

Application of stabilized Fe0 nanoparticles for remediation of Cr(VI)‐spiked soil

In this study, laboratory batch experiments were conducted on a chromium (Cr)‐spiked soil to evaluate the effectiveness of synthesized starch‐stabilized iron (Fe0) nanoparticles and compared with Fe0 and Fe3O4 with different particle sizes, and also with decreasing water‐extractable Cr(VI). Comparative studies were carried out at a Cr(VI) concentration of 100 mg kg−1 and a Fe materials dosage of 1.5% w/w. Results indicated that stabilized Fe0 nanoparticles had a greater efficiency (100%) to immobilize Cr(VI). The efficiency of the iron materials that we used for immobilization of Cr(VI) was in the following order: starch‐stabilized Fe0 nanoparticles > non‐stabilized Fe0 nanoparticles > Fe3O4 nanoparticles > Fe0 micro‐particles > Fe3O4 micro‐particles. Several factors affecting the immobilization of Cr(VI) by stabilized Fe0 nanoparticles, including reaction time, initial Cr(VI) concentration in soil, Fe0 nanoparticles dosage and soil‐solution suspension pH, were investigated. The overall rate of the Cr(VI) immobilization process was quick and almost 50% of the immobilization was reached within the first 2 minutes of the reaction. Cr(VI) immobilization percentages decreased from 100 to 54% as the initial Cr(VI) concentration increased from 50 to 1650 mg kg−1. Furthermore, increasing Fe0 nanoparticles dosage from 0.5 to 3% w/w caused a 70% increase in the immobilization efficiency. The results indicated that increasing the soil suspension pH from 5 to 9, in both buffered and unbuffered conditions, did not have any significant effect on the extent of water‐extractable Cr(VI).

Zinc release by humic and fulvic acid as influenced by pH, complexation and DOC sorption

The complexation properties of humic substances, the stability of the dissolved humic-metal complex, and the interaction of complex with other soil constituents determine the solubility and mobility of heavy metals. This study investigated the influence of humic and fulvic acid on the release of zinc of soils from Fethiye (Mediterranean Region); Samsun (Black Sea Region); and Izmir (Aegean Region). The time-dependent release of zinc from the soil was evaluated using plots of logarithm of q t / q s versus time. The kinetics of zinc release by humic acid was characterised by two pseudo-first-order reactions. The observed rate coefficients in our model were k 1 = 4.2 × 10 − 3 h − 1 and k 2 = 5 × 10 − 5 h − 1 , which indicated to an initial fast reaction followed by a slower reaction. The release of zinc increased with increasing humic acid concentration representing an approach to plateau formation at the higher humic concentrations. The highest releases of 334.5, 350.8 and 369.4 μmol kg − 1 for Fethiye, Samsun and Izmir soils, respectively, were obtained for a humic acid concentration of 1000 mg L − 1 . A different situation was observed in treatments with 150 mg L − 1 humic acid solutions, where the precipitative removal of humic acid–zinc complexes occurred at about pH 7. The interactive behaviour of humic acid–zinc complexes represented a similar reduction in the zinc concentrations in aqueous solution. A rise in the zinc load caused only a slight increase in release, which indicated that zinc load had little effect on release. Changes in soil suspension pH reflected higher zinc releases. An increase in the suspension pH to approximately 10.5 resulted in an increase from 209.4 μmol kg − 1 to 280.2 μmol kg − 1 (300 mg L − 1 humic acid), which was attributed to the involvement of other functional groups like phenolic groups in the process. On the other hand, decreasing the suspension pH to about 5 revealed to an increase from 89 μmol kg − 1 to 602.7 μmol kg − 1 (100 mg L − 1 humic acid), which was determined to be mainly the contribution of acidity. The influence of humic acid sorption on zinc release was investigated by evaluating sorption behaviour with respect to pH, salt, clay, oxide and organic matter contents of soils. Sorption of humic acid on soils presented a linear behaviour, and as the partition and distribution coefficients imply, was highest for Fethiye Soil, followed by Samsun and Izmir soils. Since this order reflected the reverse order of zinc release from soils, the degree of zinc release from soils was related to the degree of humic acid sorption. Thus, the greatest release of zinc from Izmir Soil was explained by the low humic acid sorption. The release of zinc from soil was lower for fulvic acid solutions, despite its higher acidic functional group content. For fulvic acid concentration of 300 mg L − 1 , a zinc release of 46.7, 85.7 and 148.5 μmol kg − 1 was obtained for Fethiye, Samsun and Izmir soils, respectively. This was linked to the strength of bonds involved in the process, as shown by the stability constants of zinc humic or fulvic acid complexes.

Mechanisms for high Cd activity in a red soil from southern China undergoing gradual reduction

Cadmium (Cd) activity may vary in flooded soils but the mechanisms underlying these variations have remained conflicting and ambiguous. In this study, a continuous N2 bubbling experiment was conducted using a red soil from the subtropical region of southern China to identify factors controlling Cd activity under reductive conditions. The results showed that Cd solubility, although very low, increased gradually with time during the experiment. From 0 to 14 h, the pH and pe+pH of soil suspension decreased, respectively, from 5.6 to 5.5 and from 11.1 to 9.6. The solubility of Cd was enhanced within this period due to the kinetics of solubilisation of salts or dispersion of colloids upon rewetting, the pH decrease resulting from the hydrolysis of soluble Al and Fe(III) and the competition for soil surface sites from dissolved organic carbon (DOC) and cations, including Al3+, K+, and Mg2+ in the soil solution. From 14 to 114 h, however, the reductive dissolution of Fe and Mn oxides, the DOC consumption by microbial respiration, and the competition of Cd2+ with K+ and Mg2+ for the soil surface sites were the major mechanisms promoting the release of Cd into the soil solution, as the pH of soil suspension increased from 5.5 to 5.8 and the pe+pH continued to decrease from 9.6 to 7.5. On the other hand, more than half of the total Cd was present in the exchangeable fraction during the experiment, indicating that Cd activity remained high. Further, it was observed that Cd transformed from the Fe-Mn oxide-bound fraction to the carbonate-bound fraction and Fe from the oxide- and carbonate-bound fractions to the exchangeable fraction during the experiment. Cadmium redistribution had a close relationship with Fe redistribution, suggesting that the chemical behaviours of Fe oxides affected the physicochemical properties of soil solid phases and further controlled Cd redistribution among soil solid phases. The above results implied that the mechanisms controlling the solubility of Cd were different within the different ranges of pH and pe+pH after rewetting of the air-dried soil, and Fe chemistry might be an important factor inducing the transformation of Cd among several soil solid phases when soil became reducing.

Reductive dechlorination of carbon tetrachloride in acidic soil manipulated with iron(II) and bisulfide ion

Batch and column tests were conducted to investigate the effect of reductant concentration, reductant contact time, and suspension pH on reductive dechlorination of carbon tetrachloride (CT) by soil manipulated with Fe(II) and HS −. Kinetic rate constants for the reductive dechlorination increased as the reductant concentrations increased. Fe(II) was more effective reductant than HS −, resulting in higher rate constants. The contact time of 1 day for the soil with HS − and that of 4 h with Fe(II) showed the highest reaction rates, respectively. The kinetic rate constants increased as the pH of soil suspensions with Fe(II) (5.2–8.0) and HS − (8.3–10.3) increased. Soil column with Fe(II) showed larger bed volumes (13.8) to reach a column breakthrough than that with HS − (4.0). Fe(II) treatment showed better removal of CT in the soil column with the addition of CaO than HS − treatment did. In contrast, HS − treatment not producing toxic products could be considered as an environmentally favorable reductant.

Aggregate distribution, stability and release of water dispersible clay for two subtropical Oxisols

The behavior of a soil regarding the dispersion and aggregation of its particles is very important for the development of environmental and agricultural soil functions. This study was conducted to determine how aggregate distribution and stability are impacted by land uses and how the release of Water Dispersible Clay (WDC) relates to disaggregation in Oxisols from subtropical Brazil. Samples from two Oxisols, collected at three depths from sites under no-tillage (NT), conventional tillage (CT) and native vegetation (NV) land uses were shaken in 250 mL plastic bottles for intervals up to 27 hours. The mass of aggregates was measured in five size classes ranging from 53 to 2000 µm. Most aggregates larger than 500 mm disappeared during the first 7.5 hours of shaking, concurrent with an increase in WDC release and without change in soil suspension pH and electrical conductivity, without increase in smaller aggregates. Therefore, there is no aggregate hierarchy in these soils and the release of WDC was caused by breaking aggregates within the 500 to 2000 mm range. Land uses affect mass of aggregates in each size class, but the aggregate stability depends on its size, not land use.

Cadmium Sorption–Desorption by Soils in the Absence and Presence of Phosphate

Batch experiments were carried out to determine the effect of phosphate anion (P) on the sorption–desorption of cadmium (Cd). The soils selected for the study were two kaolinitic Alfisols and two Andisols, which differ in their variable charge components. At the natural soil pH, Cd sorption in the absence of P was related to the net negative surface charge of the soils. Sorption of P increased the negative charge of the soils but the increase per unit amount of P added decreased with increasing P sorption. Both pH and soil characteristics influence the effect of P on Cd reactions. In the Alfisols, P and Cd were sorbed in similar amounts and the effect of increasing P rates on Cd sorption–desorption reactions was negligible. In the allophanic Andisols, whose capacity to bind P exceeds several times their capacity to retain Cd, the sorption of Cd was clearly enhanced with increasing P rates. In these soils, two distinct effects of P addition on Cd retention were observed. The first one arose from variations in the soil-suspension pH and the concomitant changes in soil surface charge. The second effect was directly related to the amount of sorbed P. Most of the Cd sorbed in the four soils was strongly bonded. In the Andisols, increasing P sorption on the surfaces restricted Cd desorption.

Soil pH and Anion Abundance Affects on Copper Adsorption

Copper (Cu) input to agricultural soils results from Cu containing pesticides and/or that in soil amendments, such as manure or sewage sludge. Soil and soil solution properties influence the adsorption and desorption of Cu by the soil, which in turn determines its plant availability and/or phytotoxicities. Effects of different anion enrichment in the equilibrium solution on Cu adsorption by different soils (pH range of 6.2–9.9) were investigated in this study over a range of Cu concentrations. With Cu concentrations in the range of 0–100 mg L−1 in the equilibration solution, 95–99% of applied Cu was adsorbed by all three soils. The adsorption of Cu was similar regardless of using either 0.01 M CaCl2 or Ca(NO3)2 as the equilibration solution. When the Cu concentration in the equilibration solution was further increased in the range of 500–2000 mg L−1, the adsorption of Cu decreased from 60 to 24% of applied Cu in two soils with pH 6.2–7.9. In a high pH soil (pH = 9.9), the Cu adsorption decreased from 77 to 34%. Addition of incinerated sewage sludge (ISS) to a Palouse silt loam soil (pH = 6.2) increased the Cu adsorption as compared to that by unamended soil. This was, in part, due to an increase in the soil suspension pH with ISS amendment.

The response of humic substances to soil acidification and freezing

The effects of freezing and acidification on humic substances (HS) of tundra, soddy podzolic, and grey forest soils have been studied in a laboratory experiment. Samples were acidified to decreased the initial pH of soil suspension by a 1 pH unit and were kept at +9 or −12°C for two months. HS were sequentially extracted by two cation exchange resins (fractions HS-1 and HS-2) in distilled water, followed by alkali (HS-3). HS functions, as well as humic and fulvic acids isolated from them, were examined by fluorescence spectroscopy in the emission, excitation, and synchronous-scan excitation modes. Electrophoretic mobilities and E 4/E 6 ratios of HS-1, 2, and three fractions were analysed. On the basis of C HA/C FA and E 4/E 6 ratios and electrophoretic and fluorescence behaviour, HS fractions from the same sample showed distinct differences while corresponding fractions of different origin displayed evident similarities. HS of soils studied differed in their response to acidification and freezing. HS of treated and untreated soils were distinguished by fraction composition, peak wavelength, and intensity of fluorescence in excitation and synchronous-scan excitation spectra.

Soil Redox and pH Effects on Methane Production in a Flooded Rice Soil

Methane formation in soil is a microbiological process controlled by many factors. Of them soil redox potential (Eh) and soil pH are considered as critical controls. A laboratory incubation experiment was conducted to study the critical initiation soil Eh, the optimum soil pH, and the interaction of Eh and pH on methane production. A small decrease in pH resulting from the introduction of acidic materials significantly decreased methane production. However, a slight increase in soil pH (about 0.2 unit higher than the natural soil suspension pH) resulted in an enhancement of methane production by 11-20 percent and 24-25 percent at controlled Eh of -250 mV and -200 mV, respectively. Results suggest that decrease in methane emission could be obtained by a small reduction in soil pH in Crowley soil.

Neutralization of Soil Acidity and Release of Phosphorus and Potassium by Wood Ash

AbstractWood ash is a residual by‐product being produced at an annual rate of 1.5 to 3.0 million t by wood burning power plants in the USA. Currently, much of this ash is being landfilled. Landspreading of wood ash, which is an alternative liming agent, on both agricultural and forest soils is being proposed as a means of disposal that is both environmentally and economically attractive. As with any soil amendment, there is a potential of adverse environmental impact from the landspreading of wood ash. An experiment was conducted to evaluate the initial rate of soil acidity neutralization by wood ash and the release of P and K into suspensions of 10 acid soils. The wood ash had a CaCO3 equivalence of 480 g kg−1. Changes in soil suspension pH, which were used to quantify the rate of acid neutralization by wood ash, and concentrations of P and K of wood ash‐amended soil suspensions were measured at reaction periods ranging from 30 to 300 s. Soil acidity neutralization rates were strongly correlated with soil pH and decreased 41% per unit pH increase in the pH 3.8 to 6.2 range. After the initial release of P into suspension, sorption of released P resulted in decreasing P concentration in the suspension with increasing reaction time for 7 of the 10 soils. Sorption of released P was greater at lower soil pH values, suggesting precipitation as Al and Fe phosphates. In contrast, the solubility of wood ash K was very high. The increase of suspension K levels resulted from the release of wood ash K as well as from the displacement of K on soil exchange sites by Ca and other exchangeable cations released directly from wood ash into the soil suspension. The increase in suspension K was equivalent to 55% of the total K content of the wood ash. The net K content of the suspensions remained near the same level during the reaction period suggesting that soil solution levels of K would immediately increase upon landspreading of wood ash.

Comparison of indirect methods for the determination of percent exchangeable aluminium in acid soils of Galicia (NW Spain)

Abstract Ten easily determined soil parameters related to soil acidity were evaluated for their potential to provide simple, indirect estimates of the exchangeable aluminium percentage (PAL). These parameters were correlated with the PAL in 0.6N BaCl2 extracts of soils collected from lime trials on 4 different parent materials. Six rates of lime (0, 0.75, 1.5, 3, 6, 12 t/ha) were added 3–4 years prior to collection of the samples. The soil parameters examined were the pH of soil suspensions with SMP, Woodruff and New Woodruff buffer solutions; the pH of 1, 5 and 10mM CaCl2 and 0.6N BaCl2 soil extracts, and the aluminium content of 1, 5 and 10mM CaCl2 soil extracts. The best correlations with the exchangeable aluminium percentage for all soils considered, were those based on the aluminium content of the 5mM CaCl2 extract (r=0.976, n=24) followed by the pH of the 0.6N BaCl2 extract (r=0.945, n= 24). Both these methods are suggested as reliable indirect estimates of soil exchangeable alumimium percentage where availability of staff or equipment restrict direct determinations.

Glass Microelectrode Techniques for In Situ pH Measurements

AbstractGlass pH‐sensitive microelectrodes with lengths < 0.1 mm and diameters < 0.05 mm have many potential uses for in situ soil pH measurements. Physically stable glass microelectrodes which responded rapidly to pH were constructed and their response to soil chemical and physical influences measured. The pH of nine inorganic salts and organic acid solutions measured with the glass microelectrode were compared to those measured with a glass macroelectrode. A slope of 0.92, an intercept of 0.35, and regression coefficient of 0.99 indicated good agreement. Soil suspension pH measured with glass micro‐ and macroelectrodes was compared for eight soils. A slope of 1.26, an intercept of −1.19, and regression coefficient of 0.96 indicated a slight variation. These differences were partly attributed to unavoidable electrode placement problems. In general pH measured with macro‐ and microelectrodes compared very well. The influence of soil water pressure gradients in the range of −10 to −1500 kPa near the pH and reference electrodes on microelectrode pH measurements was studied. No significant differences in pH values due to moisture gradients were observed when the reference electrode was placed in soil at water pressures greater than −33 kPa. Standard deviations in pH where the glass microelectrode was placed in soil with water pressures from −10 to −1500 kPa varied from 0.07 to 0.17 indicating that moisture gradients have little influence on measured pH. Results indicate that glass pH microelectrodes are suitable for use in soil systems.

Forest Soil Organic Horizon Acidification: Effects of Temperature, Time, and Solution/Soil Ratio1

A forest soil organic horizon (Oe) was acidified with HNO3 (0–5 cmol kg−1) in the laboratory at three solution/soil ratios (10, 50, 100), three temperatures (4, 14, 30°C), and for up to 162 h to compare effects of added H+ on pH, soluble Ca, Al, and C with effects of environmental conditions likely to vary seasonally in the forest floor. Soil suspension pH reached a steady state within 48 h, regardless of quantity of H+ added, solution/soil ratio, or temperature. Calcium concentrations also reached a steady state with soil solid phases quickly, and additions of H+ increased quantities of Ca (mmol kg−1) in solution as did increasing solution/soil ratio. Shaking time and temperature had smaller effects on Ca than did H+ additions and solution/soil ratio. In contrast, reactions controlling dissolved organic carbon (DOC) and total soluble aluminum (Alt) were slower than those for pH and Ca. Quantities of DOC and Al going into solution remained nearly constant, so that increasing the solution soil ratio diluted concentrations. Dissolved organic C and soluble Al increased with longer shaking times and higher temperature. The results have implications for designing realistic laboratory experiments on forest floor acidification, for evaluating seasonal and yearly variations in soil solution composition in the field, and in developing accurate and reliable simulation models of H+ reactions in forest soils.

Alternative use of the SMP‐buffer solution to determine lime requirement of soils

Abstract A close relationship was found between the pH of soil suspensions in the SMP buffer solution (pHsmp) and the potential acidity of soils (H + Al) extracted by a neutral calcium acetate solution (r = 0.98), for twenty six soil samples of the State of Sao Paulo, Brazil, This relationship was represented by the equation lnY = 7.76 ‐ 1.053X, which allowed for the calculation of H + Al directly from the values of pHsmp. With the values of H + Al and the sum of bases, calcium, magnesium and potassium, the cation exchange capacity (CEC), and the base saturation (V) were calculated. Relationships between the base saturation of the soils and the active acidity of soil suspensions were close, both for pH determined in water (r=0.94) and pH determined in 0.01M CaCl2 solution (r ‐ 0.97). Thus the lime requirement (LR) of soils could be calculated, for given values of pH or base saturation, using the equation LR = CEC (V2 ‐ V1)/100, in which V1 is the base saturation of the soil and V2 is the expected value up...

The adsorption characteristics of soils and sorption of copper, manganese and zinc

Abstract Adsorption chracteristics of soils like cation exchange capacity , oxides of Al and Fe, free calcium carbonate, etc, were correlated to the sorption of copper, manganese and zinc in 27 mineral soils. The sorption of these element was followed for 50 days. None of the soil adsorption characteristics studied was significantly correlated to the sorption of Cu and Zn. The soil saturated paste pH and soil suspension pH were strongly correlated to Cu and Zn sorption. The sorption of manganese was significantly correlated to soil adsorption characteristics in the initial stages of this experiment. With the passage of time, the sorption of Mn gradually became mare independent of these parameters. The cation exchange capacity of soils was poorly correlated to Cu, Mn and Zn sorption. The results of this experiment suggest that the guidelines for determining the safe sludge load on soils, which are mainly based on cation exchange capacity, should be revised

An investigation on laboratory and field methods of determining the pH of soil suspensions

AbstractA comparison was made between the pH values of soil samples, measured with a pH meter in the laboratory, and those obtained by using colorimetric indicator solutions in the field. Of the indicators tested, BDH Soil Indicator and bromocresol purple provided the closest overall comparison with laboratory results for soils with pH values from 3.8 to 7.8. Bromocresol green was useful only over a limited range (less than 5.2). Differences in procedure and analysis sometimes produced compensatory errors but the results did not often agree exactly and the magnitude of the discrepancy for any given soil was predictable only within fairly broad limits—the average standard deviation was 0.34 units and there was about a 75% chance of the result determined in the field being within 0.3 units of the result determined in the laboratory.