Exchangeable cations and CEC determinations of some highly weathered soils

Abstract

Eight methods to determine exchangeable cations and cation exchange capacity (CEC) were compared for some highly weathered benchmark soils of Alabama. The methods were: (1) 1N NH₄OAc at pH 7.0 by replacement (for CEC only), (2) 1N NH₄OAc at pH 7.0 (summation of basic cations plus 1N KCl extractable Al), (3) 1N NH₄OAc at pH 7.0 (summation of basic cations plus exchangeable H⁺), (4) 0.1M BaCl₂ (summation of basic cations plus exchangeable Mn, Fe and Al), (5) Mehlich 1 (summation of basic cations plus 1N KCl extractable Al), (6) Mehlich 1 (summation of basic cations plus exchangeable H⁺), (7) Mehlich 3 (summation of basic cations plus 1N KCl extractable Al), and (8) Mehlich 3 (summation of basic cations plus exchangeable H⁺). The 0.1M BaCl₂ was chosen as the standard method for the highly weathered soils and the other methods compared to it. The results indicated that the 1N NH₄OAc replacement method gave significantly higher CEC values compared to the summation methods. This was probably due to the overestimation of the field CEC caused by measurement of pH dependent cation exchange sites in these soils. There was, however, close agreement between the BaCl₂ method and the summation methods that included extractable Al. The generally good agreement between these summation methods suggests that the Mehlich 1 and Mehlich 3 extractants, commonly used to determine available nutrients in the southeastem USA, may also be used to measure effective CEC of some acid‐rich sesquioxide benchmark soils of Alabama. However, 1N KCl extractable Al as opposed to exchangeable H⁺ should be included in the computation.     

Cation exchange capacity measurements

Abstract

Soil cation exchange capacity (CEC) measurements are important criteria for soil fertility management, vaste disposal on soils, and soil taxonomy. The objective of this research was to compare CEC values for arable Ultisols from the humid region of the United States as determined by procedures varying widely in their chemical conditions during measurement. Exchangeable cation quantities determined in the course of two of the CEC procedures were also evaluated. The six procedures evaluated were: (1) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity; (2) N Ca(OAc)₂ (pH 7.0) saturation with Mg(OAc)₂ (pH 7.0) displacement of Ca²⁺; (3) N NH₄OAc (pH 7.0) saturation with NaCl displacement of NH₄ ⁺; (4) N MgCl₂ saturation with N KCl displacement of Mg²⁺; (5) compulsive exchange of Mg²⁺ for Ba²⁺; and (6) summation of N NH₄OAc (pH 7.0) exchangeable Ca, Mg, K, and Na plus N KCl exchangeable AJ. The unbuffered procedures reflect the pH dependent CEC component to a greater degree than the buffered methods. The compulsive exchange and the summation of N NH₄OAc exchangeable cations plus N KCl exchangeable Al procedures gave CEC estimates of the same magnitude that reflect differences in soil pH and texture. The buffered procedures, particularly the summation of N NH₄OAc exchangeable cations plus BaCl₂ ‐ TEA (pH 8.0) exchangeable acidity, indicated inflated CEC values for these acid Ultisols that are seldom limed above pH 6.5. Exchangeable soil Ca and Mg levels determined from extraction with 0.1 M BaCl₂ were consistently greater than values for the N NH₄Oac (pH 7.0) extractions. The Ba²⁺ ion is apparently a more efficient displacing agent than the NH₄ ⁺ ion. Also, the potential for dissolving unreacted limestone is greater for the Ba₂ ⁺ procedures than in the NH₄ ⁺ extraction.     

Determination of exchangeable potassium in soil using ion-selective electrodes in soil suspensions

Summary Methods of determining exchangeable K⁺ of soil by mixing extracting solutions and analysing the soil suspension with ion‐selective electrodes were developed and evaluated on 30 samples of soils. From preliminary comparisons of the K⁺ extracted by BaCl₂ and NH₄OAc solutions and by batch and leaching treatments of soils, we established that suspensions of 5 g soil in 100 ml 0.5 m BaCl₂ and single batch treatments of 1 h should be used. The exchangeable K⁺ was determined with a K‐selective, valinomycin‐based PVC membrane electrode and electrochemical cells that did or did not include a liquid junction (the reference electrode being a double‐junction reference electrode assembly with a 10 m LiOAc salt bridge solution or a Cl‐selective electrode, respectively). The Ba‐exchangeable K⁺ values were sensibly the same whether a liquid junction was involved or not, a result that can be attributed to the beneficial effects of the salt bridge and the ionic strength of the extractant. Comparisons of these Ba‐exchangeable results with those obtained by various combinations of batch or leaching treatments, BaCl₂ or NH₄OAc extractants and filtrate analysis by the ion‐selective electrode method or atomic absorption spectrometry showed they were all highly correlated (r≥ 0.996). The selectivity of the K⁺‐selective electrode (k^pot_KNH4 = 0.004) significantly reduced the interference from indigenous soil NH₄⁺ in the BaCl₂ suspensions. Overall, the results show potentiometric measurements of K⁺ in soil suspensions can provide a simple, rapid, and reliable means of determining exchangeable K⁺ in soils.     

An evaluation of cation exchange capacity measurements for soils in the tropics

Abstract

Comparisons of CEC and exchange acidity neasurements were made on a group of selected West African soils using three commonly used analytical procedures, namely, neutral‐acetate displacement, BaCl₂‐TEA leaching at pH 8, and unbuffered KCl extraction.

The three methods gave large differences in CEC values which followed the order of BaCl₂‐CEC>>NH₄OAc‐CEC> KCl‐CEC. Results of exchange acidity also followed the same order. The high exchange acidity values obtained by the BaCl₂‐TEA (pH 8) method were mainly due to changes in surface charge characteristics of Fe and Al oxides and hydrous oxides. The effective CEC method is recommended for routine soil analysis for highly weathered soils in the tropics.

Regression analysis of the base saturation values obtained from the three methods indicated the data followed a curvilinear relationship. The acetate method was more highly correlated with the effective CEC method than with the BaCl₂ method.     

A rapid method to determine cation exchange capacity and exchangeable bases in calcareous,gypsiferous, saline and sodic soils

Summary

A simple, single‐step extraction with LiEDTA for the estimation of CEC and exchangeable bases in soils has been developed. Multivalent cations are stripped from the soil adsorption sites by the strongly chelating agent EDTA, and are replaced by Li. In soils without CaCO₃ or water soluble salts, exchangeable divalent cations (Ca, Mg) are chelated by EDTA and exchangeable monovalent cations (Na, K) are replaced in a single extraction step using 0.25–2.5 g of soil and 10.0 ml of extractant.

In calcareous soils the CEC can be determined in the same way, but for the extraction of exchangeable Ca and Mg, another separate extraction is needed because dissolution of calcite by EDTA is unavoidable. This extraction is done with as much NaEDTA as needed to extract only exchangeable Ca and Mg in a 1:2 (m/V) soil/alkaline‐50% (V/V) aethanolic solution to minimize dissolution of calcite.

In gypsiferous soils gypsum is transformed into insoluble BaSO₄ and soluble CaEDTA by LiBaEDTA thus avoiding interference of Ca from dissolution of gypsum, which renders the traditional methods for determining CEC unsuitable for such soils. To determine exchangeable Ca and Mg, Na₄EDTA is used as for calcareous soils.

In saline/sodic soils replacement of Na by Li is incomplete but the Na/Li‐ratio at the complex after extraction is proportional to the molar Na/Li‐ratio in the extracts, so that the CEC and original exchangeable sodium (ESP) content can be calculated. Additional analysis of Cl and, if necessary, SO₄ in the extracts of saline soils can be used to correct for the effect of dissolution of the salts on the sum of exchangeable cations.

This new method is as convenient as the recently developed AgTU (silverthiourea), but is better suitable for calcareous and gypsiferous soils.     

Release of Al by hydroxy-interlayered vermiculite and hydroxy-interlayered smectite during determination of cation exchange capacity in fine earth and rock fragments fractions

The fine earth (<2 mm) and rock fragments (>2 mm) fractions of two soils derived from Oligocene sandstone have been examined to assess the origin of the discrepancies between cation exchange capacity (CEC) and effective CEC (ECEC). The soils differ in terms of acidity: soil A is more acid than soil B. When the A samples are treated with BaCl₂, the solution became sufficiently acid (pH < 4·5) to dissolve and to maintain Al in solution. From these samples more Al is released than base cations. Aluminium was continuously replenished even after 192 h, so that the ECEC was always larger than the CEC. Samples from soil B contain less H and Al ions, and the BaCl₂ solution could not lower the pH below 5·0. In these samples little Al is released, and the base cations dominate the exchangeable pool of ions. This Al can be considered to be exchangeable, and a good agreement exists between the ECEC and the CEC. The source of non-exchangeable Al in the A samples is the OH-Al polymers of the hydroxy-interlayered vermiculite (HIV) and hydroxy-interlayered smectite (HIS) that tend to dissolve during the BaCl₂ treatments. In the less acid B samples the Al polymers are not affected by BaCl₂ treatment. Different results were obtained when the clays, extracted from an Na-dispersed suspension, were treated with BaCl₂ solution. Because the clays are no longer acid, no H⁺ is released, and the OH-Al polymers are not dissolved. Therefore, the saturating ions play an important role in the dissolution of the OH-Al polymers and cause differences between the CEC and ECEC. We discount organic matter and specifically Al-organo complexes as a source of non-exchangeable Al. Both A and B soils contain very similar pyrophosphate-extractable Al, but show substantial differences in the amount of exchangeable Al.     

Extraction of potassium from some kaolinitic soils of the tropics

Abstract

Three methods for soil potassium extraction (M NH₄OAc pH 7, 0.01 M AgTU and 30 % hot H₂SO₄) were compared for a variety of kaolinitic soils of the tropics. The AgTU‐extractable K was much higher than the M NH₄OAc‐extractable K when vermiculite clay was present in the soil. The correlation between both was given by an R value of 0.937. The amounts of K extracted by 0.01 M AgTU and by hot H₂SO₄ were approximately the same. The R value for these two methods was 0.843.

It is suggested that the AgTU extractant could be used for determination of plant‐available K in soil and for testing for the presence or absence of vermiculite clay in soils.     

A comparison of methods for measuring extractable Ca,Mg, K,Na, Mn,Al, Fe,and P from New England forest soils

Abstract

An experiment was designed to evaluate several of the commonly used extractants and methods for determining “available”; elements in soils. The purpose of the study was to evaluate the suitability of these extraction procedures for use on forest soils typical for New England commercial forests. The extraction procedures selected included NH₄OAc pH 4.8, NH₄OAc pH 7.0, NH₄Cl, Double Acid, Bray, and Mehlich methods. The elements measured varied somewhat by procedure but included the base cations, Al, Fe, Mn, and P. As a bioassay of element availability, a greenhouse study was conducted using six forest soil materials from different horizon types (i.e. O, Ap, B) and three conifer seedling species (red spruce, balsam fir, and white pine). Relatively small differences among extraction procedures were found among the methods used for exchangeable Ca, Mg, K, and Na. Large differences, however, were found among the different horizon types in the amount of exchangeable base cations present. In contrast, significant differences were found among extraction procedures for Al, Fe, Mn, and P depending on the degree of buffering and acidity of the extracting solution. Of the elements measured in this study, only P appeared to be growth limiting with the NH₄OAc pH 4.8 being best correlated with P uptake by seedlings. Further work under field conditions over longer time periods is required to evaluate these methods for measuring P availability in forest soils     

Extraction of silver from soils by chloride solutions

Using chloride solutions as extractants, 6.4–18.4% of Ag were extracted from polluted Annaka and Fuchu soils, whereas only 0.06–0.08% Ag were extractable using NH₄OAc, NH₄NO₃ or (NH₄₎₂SO₄ solution. In the case of unpolluted Konosu and Nagano soils, 28.3–47.0% of Ag were extracted by KCl and 0.7–1.0% of Ag were by NH₄OAc. The silver extracted with chloride solution was assumed to be derived from residual fractions.     

Comparison of extractants for the determination of available phosphorus,potassium, calcium,magnesium and sodium in some Ethiopian and German soils

Abstract

A study was conducted with the purpose of comparing the efficiency of Mehlich 1, Mehlich 3, and calcium acetate lactate (CAL) extractants for the deter‐ mination of available phosphorus (P) and exchangeable cations [potassium (K), calcium (Ca), magnesium (Mg), and sodium (Na)] on 22 Ethiopian and 10 German agricultural soils. The Olsen and NH₄OAc extractants were used as standards against which P and exchangeable cations values were compared. Results showed that, in general, highly significant correlations were found between all of the methods for available P and exchangeable cations determination on the Ethiopian soils. The highest correlation was, however, found with the Mehlich 3 extractant. On the ten soils from Germany, the Olsen method did not give significant cor‐ relation with the CAL method for P determination. The CAL and Mehlich 3 extrac‐ tants were also not good indicators of Na availability when compared with the NH₄OAc method. It can be generalized that the Mehlich 3 is a suitable extractant for P, K, Ca, Mg, and Na in Ethiopian soils, but further study is recommended to confirm these findings under field conditions.     

A comparison of buffered and unbuffered ammonium salts to determine exchangeable base cations in acid soils

Abstract

In soil samples from two study sites in southern Norway, exchangeable cations were determined using two different ammonium (NH₄)‐salts as extractant. As expected, the cation exchange capacity (CEC) determined in 1M ammonium acetate (NH₄OAc), buffered at pH 7.0 was higher than the CEC measured in ammonium nitrate (NH₄NO₃). By contrast, the amount of exchangeable calcium (Ca), magnesium (Mg), and barium (Ba) was lowest in the NH₄OAc extract, in particular in the upper soil horizons high in organic matter (O‐ and E‐horizon). This suggests that NH₄ in 1M NH₄OAc does not compete effectively with multivalent base cations. The relatively high levels of exchangeable base cations in NH₄NO₃ could not be explained by increased weathering. An increase in selectivity of especially divalent cations may explain the relatively low amount of exchangeable base cations extracted by NH₄OAc, as this involves increased deprotonation and thus a higher negative charge.     

Use of Single Extraction Methods to Predict Bioavailability of Heavy Metals in Polluted Soils to Rice

Human exposure to toxic heavy metals via dietary intake is of increasing concern. Heavy-metal pollution of a rice production system can pose a threat to human health. Thus, it was necessary to develop a suitable extraction procedure that would represent the content of metal available to rice plants (Oryza sativa L.). The aim of this study was to predict, on the basis of single extraction procedures of soil heavy metals, the accumulation of heavy metals (cadium, lead, copper, and zinc) in rice plants. Six extracting agents [Mehlich 1, Mehlich 3, EDTA (ethylenediaminetetraacetic acid), DTPA–TEA (diethylenetriaminepentaacetic acid–triethanolamine), ammonium acetate (NH₄OAc), and calcium chloride (CaCl₂)] were tested to evaluate the bioavailability of heavy metals from paddy soils contaminated with lead–zinc mine tailings to rice. The extraction capacity of the metals was found to be of the order EDTA > Mehlich 3 > Mehlich 1 > DTPA–TEA > NH₄OAc > CaCl₂. The correlation analysis between metals extracted with different extractants and concentrations of the metals in the grain and stalk of the plant showed positive correlations with all metals. The greatest values of correlation coefficients were determined between the NH₄OAc- and CaCl₂-soluble fractions of soil and contents in plants in all four metals studied. Therefore, NH₄OAc and CaCl₂ were the most suitable extractants for predicting bioavailability of heavy metals in the polluted soils to rice. The results suggested that uptake of heavy metals by rice was mostly from exchangeable and water-soluble fractions of the metals in the soils. Soil-extractable metals were more significantly correlated with metal accumulation in the stalk than in the grain. The pH had more significant influence on availability of heavy metals in the soils than total content of metals and other soil properties. The bioavailability of metals for rice plants would be high in acidic soils.     

Problems in CEC determination of calcareous clayey sediments using the ammonium acetate method

Cation‐exchange–capacity (CEC) results of calcareous soils and clays can be erroneous if the ammonium acetate method is used. In this study, a model is proposed to explain the process for systematic underestimation of the CEC. Seven clayey sediments from Germany with varying calcite and low organic‐C content were studied. After several exchange treatments with concentrated ammonium acetate (NH₄Ac) solutions, the exchange population is assumed to be in homoionic ammonium form. Throughout the cation‐exchange experiment, calcite reacts with the NH₄Ac exchange solution generating Ca²⁺ cations. During the necessary washing steps to remove excess salt, calcite dissolution is lower but still occurs. The permanently added Ca²⁺ ions compete successfully with NH , especially during the washing steps. This leads to a more or less partial biionic exchange population resulting in an underestimation of the CEC which is calculated based on NH concentration of the clay by Kjeldahl analysis. The biionic exchange population was proven using the new silver thiourea technique with presaturation of calcite, AgTU calcite . The clay with 148 g kg^–1 calcite had a fraction of 16.4 cmol₊ kg^–1 exchangeable Ca²⁺. This is ca. 50% of the CEC of this clay being 31.8 cmol₊ kg^–1_. For clays with similar mineralogical composition, this trend is proportional to the calcite content.     

Evaluation of universal extractants for determining plant available potassium in intensively cultivated soils

Abstract

Eighteen soils from northwestern Switzerland were used to study the value of seven universal extractants (CaCl₂; DB‐DTPA; Mehlich 1, 2, and 3; Morgan‐Wolf; and NH₄OAc‐EDTA) for predicting plant available potassium (K) as compared to a bioassay (a modified Neubauer test with winter rye). These extractants were evaluated on the basis of K uptake by the bioassay test and the soil K status. In order to create the sufficiency level of exchangeable K for plant growth, soils were treated with 0, 20, 40, 80, and 160 mg K/kg of soil. The range of K uptake by the bioassay tests was between 89.2 and 403.0 mg/kg of soil for the control pots, and 136.6 to 495.8 for the K treatments with optimal conditions for plant growth. The average amounts of K extracted by the seven universal extractants, in ascending order, were: CaCl₂ < Morgan‐Wolf < Mehlich 1 < Mehlich 2 < NH₄OAc‐EDTA < Mehlich 3 < DB‐DTPA. The highest simple correlation with K uptake versus the bioassay test was obtained with the DB‐DTPA (r = 0.89) extractant and the lowest with the Mehlich 1 (r = 0.53) extractant. The DP‐DTPA, NH₄OAc‐EDTA and Mehlich 3‐K procedures showed an advantage over K procedures based on water soluble and exchangeable K pools in the investigated soils in order to predict the amount of plant‐available K. A simple regression and the Cate‐Nelson graphic method offer the possibility of assessing the soil‐K status using K values obtained by these universal extractants and to calibrate them against K forms as follows: exchangeable, water soluble, and non‐exchangeable.     

Testing for calcium,magnesium, and potassium in soilless potting media: Differences and similarities between extractants

Abstract

Interpretation guidelines for the availability of calcium (Ca), magnesium (Mg), and potassium (K) in soilless media have been developed through plant growth studies and comparisons amongst extractants. The extractants used were 70% ethanol (EtOH), water, DTPA, ammonium acetate (NH₄OAc) and silver thiourea (AgTu). Ethanol, which removes ions in pore water, extracted only 4.5, 13, and 26% of the Ca, Mg, and K, respectively, that could be extracted by AgTu from Pinus radiata bark of pH 5.66 and CEC of 11.2 cmol⁺/L. Acidification to pH 4.62 increased these proportions to 22, 40, and 38%. Correlations between water and DTPA for 39 media were excellent for both individual elements and the ratios Ca/Mg, K/Mg, and Ca, Mg, and K/(the sums of their concentrations in the extractants) (r² = 0.88–0.98). Correlations between these extractants and AgTu and NH₄OAc were poor for individual elements (r² = 0.37–0.75) but high for ratios (r² = 0.71–0.96). For Petunia ’Celebrity Salmon’ growing in peat media of similar pH but widely different Ca, Mg, and K proportions, the ratios of these elements in the shoots were highly correlated with their ratios in 2 mM DTPA extracts of the media. Similarly high correlations were obtained between the Ca/Mg ratios of the shoots of three Asplenium species growing in pinebark media and this ratio for DTPA, NH₄OAc, and AgTu extracts of the media. The results indicate that the ratios of Ca, Mg, and K to one another in water and DTPA extracts of soilless media are good indicators of the availability of these elements to plants. Healthy specimens of the test plants grew in media whose DTPA extracts had a minimum Ca/Mg mole ratio of 1 or 2, depending on the Ca requirements of the species. The upper limit for good growth was deduced to be about 6. Limits for NH₄OAc and AgTu extracts were similar to those found for crop plants in soils, at 1.6 or 3 to about 9. Minimum amounts of Ca, Mg, and K extracted by water and DTPA that were adequate for short‐term growth in the absence of further inputs were about 9,2.5, and 5 cmol⁺/L medium, respectively, at pH 6.0. The effect of pH on cations removed by water and DTPA raises the minima to about 19, 5, and 7.5 cmol⁺/L at pH 5.0.     

Modelling cation exchange in columns of disturbed and undisturbed subsoil

Cation exchange is often studied with disturbed and dried soils, but the applicability of the results to undisturbed soils is not straightforward. We investigated the value of exchange coefficients obtained from standard procedures for predicting cation exchange in soil. Columns of undisturbed and disturbed subsoil of a Luvisol (SBt horizon) were leached under saturated conditions with 0.4, 4, 20, 41, 102 and 205 mm BaCl₂ at a Darcy velocity of 1400 mm day^?1. The model PHREEQC was used to calculate one‐dimensional transport, inorganic complexation and multiple cation exchange. Two model variants were tested: m1 (exchangeable cations obtained by percolation with NH₄Cl) and m2 (exchangeable cations obtained by shaking the soil with BaCl₂). The exchange coefficients (Gaines–Thomas formalism) were calculated from the ion activities in solution and exchangeable cations obtained by NH₄Cl percolation (m1) or shaking with BaCl₂ (m2). Variant m1 predicted cation exchange of the disturbed (homogenized) soil for the entire BaCl₂ concentration range, whereas variant m2 resulted in a two‐fold overestimation of desorbed K for all experiments, which was related to large amounts of K released from the soil by shaking with BaCl₂. In experiments with undisturbed soil, variant m1 predicted the concentrations of Mg, Ca, K, and Na in the solution phase and the sum of cations released from exchange sites. However, variant m2 predicted changes in ion concentrations and exchangeable cations somewhat less well. This study suggests that the amounts of exchangeable cations and exchange coefficients obtained from experiments with homogenized soil by percolation are useful to predict cation concentrations in column experiments with undisturbed soils.     

Ammonium Chloride Solution as an Alternative Laboratory Procedure for Exchangeable Cations in Southern Brazilian Soils

Most Brazilian soil-testing laboratories use Mehlich 1 and 1.0 M potassium chloride (KCl) solutions as extractants for the determination of phosphorus (P), potassium (K), and sodium (Na) and for exchangeable calcium (Ca), magnesium (Mg), manganese (Mn), and aluminum (Al) in agricultural soil samples. Other laboratories use a combination of exchangeable ionic resin and KCl procedures. With recent adoption of the inductively coupled plasma (ICP-OES) in routine soil-testing laboratories, soil extraction with 1.0 M ammonium chloride (NH₄Cl) became an alternative due to the possibility of determining all exchangeable elements in one run (Ca, Mg, K, Mn, Na, and Al), leaving determination of phosphorus (P) with Mehlich 1 or exchangeable ionic resin. To evaluate the performance of the NH₄Cl solution, an experiment was carried out with thirty-seven samples of soils representative of the southernmost state of Brazil, Rio Grande do Sul. Four extraction solutions [Mehlich 1 at soil/solution ratio of 1:10 and 1.0 M ammonium acetate (NH₄OAc), 1.0 M KCl, and 1.0 M NH₄Cl at soil/solution ratio 1:20] were used with three different shaking times (5, 30, and 60 min). Correlation coefficients among all methods were high. Mehlich 1 did not perform well against NH₄OAc and NH₄Cl, despite the high correlation coefficients, with values consistently lower for K, even when the time of extraction was increased from 5 to 30 or 60 min. However, for concentrations less than 0.30 cmol kg^?1 (i.e., in the range of K deficiency), both solutions performed similarly. Calcium and Mg increased with time of shaking. Comparable values of exchangeable Ca, Mg, and K, as well as of Al and Mn, were obtained with 1.0 M NH₄Cl with 60 min shaking and the standard procedures of 1.0 M NH₄OAc and 1.0 M KCl. The determination of Al by traditional titration/back-titration of the 1.0 M KCl solution gave slightly greater results compared to ICP-OES obtained using extraction with 1.0 M NH₄Cl. The results indicate that for Ca, Mg, Mn, and Al, it is possible to replace the traditional 1.0 M KCl extraction with 1.0 M NH₄Cl solution, with 60 min shaking time and a soil/solution ratio of 1:20.     

Evaluation of Extractants and Quantity–Intensity Relationship for Estimation of Available Potassium in Some Calcareous Soils of Western Iran

Accurate estimation of the available potassium (K⁺) supplied by calcareous soils in arid and semi‐arid regions is becoming more important. Exchangeable K⁺, determined by ammonium acetate (NH₄OAc), might not be the best predictor of the soil K⁺ available to crops in soils containing micaceous minerals. The effectiveness of different extraction methods for the prediction of K‐supplying capacities and quantity–intensity relationships was studied in 10 calcareous soils in western Iran. Total K⁺ uptake by wheat grown in the greenhouse was used to measure plant‐available soil K⁺. The following methods extracted increasingly higher average amounts of soil K⁺: 0.025 M H₂SO₄ (45 mg K⁺ kg^?1), 1 M NaCl (92 mg K⁺ kg^?1), 0.01 M CaCl₂ (104 mg K⁺ kg^?1), 0.1 M BaCl₂ (126 mg K⁺ kg^?1), and 1 M NH₄OAc (312 mg K⁺ kg^?1). Potassium extracted by 0.01 M CaCl₂, 1 M NaCl, 0.1 M BaCl₂, and 0.025 M H₂SO₄ showed higher correlation with K⁺ uptake by the crop (P < 0.01) than did NH₄OAc (P < 0.05), which is used to extract K⁺ in the soils of the studied area. There were significant correlations among exchangeable K⁺ adsorbed on the planar surfaces of soils (labile K⁺) and K⁺ plant uptake and K⁺ extracted by all extractants. It would appear that both 0.01 M CaCl₂ and 1 M NaCl extractants and labile K⁺ may provide the most useful prediction of K⁺ uptake by plants in these calcareous soils containing micaceous minerals.     

Solubility and uptake of heavy metals from a Swedish soil

Samples of the surface layer (Ap) and of grass, collected from: (1) grass ley fertilized in the normal way; (2) permanent pasture fertilized in the normal way; and (3) permanent pasture treated with large amounts of sewage sludge five years earlier, were analysed for Mn, Zn, Cu, Cr, Co, Ni, Pb, and Cd.The soil samples were extracted with: (1) distilled water saturated with CO₂; (2) 1 M neutral ammonium acetate; (3) ammonium acetate + acetic acid, pH 4.75; and (4) 2 M nitric acid on a waterbath. The efficiency of these extractants differed greatly and, in relative values, was: 1 for H₂O + CO₂, 3.4 for NH₄OAc, 20.7 for NH₄OAc + HOAc, and 343 for 2 M HNO_3- The dissolving effects of the extractants differed markedly with the kind of element.Grass from the field treated with sewage sludge showed much higher contents of Mn and Zn and somewhat higher contents of Cu and Pb than grass from the untreated field. The levels of Cr, Co, Ni, and Cd were practically uninfluenced by the treatment. Grass from a field close to a highway accumulated large amounts of air-borne Pb and Cd during the summer.It is concluded that the total contents of heavy metals in soils have only limited importance for the uptake by plants. Weak extractants therefore give better information about the plant-available amounts in soils.     

The effect of biochar loading rates on soil fertility,soil biomass,potential nitrification,and soil community metabolic profiles in three different soils

Purpose

Biochar is increasingly being used as a soil amendment to both increase soil carbon storage and improve soil chemical and biological properties. To better understand the shorter-term (10 months) impacts of biochar on selected soil parameters and biological process in three different textured soils, a wide range of loading rates was applied.

Materials and methods

Biochar derived from eucalypt green waste was mixed at 0, 2.5, 5, 10 % (wt/wt) with a reactive black clay loam (BCL), a non-reactive red loam (RL) and a brown sandy loam (BSL) and placed in pots exposed to the natural elements. After 10 months of incubation, analysis was performed to determine the impacts of the biochar rates on the different soil types. Also, microbial biomass was estimated by the total viable counts (TVC) and DNA extraction. Moreover, potential nitrification rate and community metabolic profiles were assayed to evaluate microbial function and biological process in biochar-amended soils.

Results and discussion

The results showed that biochar additions had a significant impact on NH₄ and NO₃, total C and N, pH, EC, and soil moisture content in both a soil type and loading-dependent manner. In the heavier and reactive BCL, no significant impact was observed on the available P and K levels, or the total exchangeable base cations (TEB) and CEC. However, in the other lighter soils, biochar addition had a significant effect on the exchangeable Al, Ca, Mg, and Na levels and CEC. There was a relatively limited effect on microbial biomass in amended soils; however, biochar additions and its interactions with different soils reduced the potential nitrification at the higher biochar rate in the two lighter soils. Community metabolic profile results showed that the effect of biochar on carbon substrate utilization was both soil type and loading dependent. The BCL and BSL showed reduced rates of substrate utilization as biochar loading levels increased while the opposite occurred for the RL.

Conclusions

This research shows that biochar can improve soil carbon levels and raise pH but varies with soil type. High biochar loading rates may also influence nitrification and the function and activity of microbial community in lighter soils.