Percentage base saturation: Cation exchange capacity (CEC):
= 100 x TEB/CEC7 the sum total of exchangeable cations that a soil can adsorb the CEC determined with 1 M ammonium ethanoate (ammonium acetate) buffered at pH 7.0
Effective cation exchange capacity
the sum of the exchangeable cations (Al3+, H+, Ca2+ and Mg2+) extracted by 1 M potassium chloride the sum of the exchangeable 'basic' cations (Ca2+, Mg2+, K+, Na+ and NH4+) acted with 1 M ammonium ethanoate at pH 7.0.
Total exchangeable bases (TEB):
Discussion. The colloidal clay and humus soil fractions are negatively charged and therefore attract and adsorb positive ions (cations) on to exchange sites. These may be the so-called basic cations defined above, or the acidic cations H+ and Al3+. These cations are not soluble in water when in the adsorbed state, but can exchange with H+ which is present in the acidic vicinity of the plant root system. They are now in solution and able to be absorbed into the plant. The extent to which the exchange sites are saturated with cations, together with the ratios of the cations to each other, indicates the nutrient supplying power of the soil.
The principle behind the determination of the CEC is that ammonium ions will leach the adsorbed metallic cations from the soil (soil ammonium having a ratio small enough to be ignored in this group of calculations) as a solution suitable for analysis by flame emission and atomic absorption techniques. The reagent M ammonium ethanoate is universally adopted for this purpose. The presence of any free basic cations as salts in solution would give an exaggerated TEB value, therefore some workers suggest an initial leaching with aqueous ethanol. This may be 95% ethanol, or more economically for class work, 95% or even 60% industrial methylated spirits (IMS), which is also used to remove excess ammonium ethanoate. The initial leaching is not usually necessary for temperate (UK) soils. In certain cases, ethanol may remove some adsorbed NH4+, and should be replaced with isopropanol. The amount of ammonium ion adsorbed on to all the exchange sites is a measure of the CEC. It may be determined either by leaching with acidified KCl (100 g l-1 KCl + 2.5 ml M HCl) to remove the ammonium ions, then a 25 ml aliquot of this solution is made alkaline to convert NH4+ to NH3 which is steam-distilled over and titrated, or the entire soil sample may be steam-distilled. The latter method has two disadvantages: it is difficult to transfer the entire soil sample to the distillation flask, and any non-exchangeable ammonium in the sample could be liberated to give an inflated CEC value.
The TEB value may be obtained by either the sum of the individually measured cations or by evaporating and igniting a portion of the ammonium ethanoate leachate to convert the metallic cations to oxides and carbonates, followed by addition of excess acid (to convert carbonates to chlorides) and back-titration with alkali. The latter method is difficult if the soil is insufficiently base-rich to provide an adequate amount of bases for the titration. On the other hand, the calcium carbonate in calcareous soils may be partially leached by the ammonium ethanoate at pH 7.0 in addition to the exchangeable bases and thus give an exaggerated TEB value and a percentage base saturation in excess of 100%. The TEB by ignition/titration can serve as a check on the values from the summation method.
If the percentage base saturation as defined above is ^60%, this provides an indication of the need for estimation of exchangeable aluminium and hydrogen, in addition to calcium and magnesium, by the ECEC procedure.
It must be strongly emphasized that the charge on the humus and mineral particles depends not only on the nature of the surface but on the pH, the negative charge, hence CEC, rising with increase in pH. The CEC7 can therefore be far higher than it would be in the field. It is therefore necessary to ascertain the experimental conditions when assessing published data. One way to compensate for this effect is to carry out the leaching with unbuffered 1 M KCl solution (some methods use unbuffered NH4Cl), which will not affect the in situ pH of the soil. Obviously this precludes subsequent analysis of potassium, but this is one of the minor cations. This analysis is termed the effective cation exchange capacity (ECEC). The exchange complex in acidic soils tends to be dominated by Al3+ rather than H+ according to the reaction:
The solubilized aluminium is the main toxic agent to plants in acidic soils, and acid tolerant (calcifuge) plants are usually also aluminium tolerant. The ECEC method determines the levels of Al3+, H+, Ca2+ and Mg2+ extracted by 1 M potassium chloride and is described in Method 5.2.
A detailed discussion of the above topics together with a selection of class projects and test calculations is given in Chapter 7 of Rowell (1994, pp. 131-152), but note his calculation of CEC has an error in that it is based on 250 ml KCl extract, not on 100 ml as per given methodology, and thus requires correction (confirmed by personal communication, 2001). Directions for using the Foss/Perstorp Analytical Tecator Kjeltec Auto 1035/1038 Sample System for CEC determinations are given in USDA, 1996 (pp. 203-210).
Determination of CEC and exchangeable cations
Reagents. Note: deionized water and analytical grade chemicals are used throughout unless otherwise stated.
• Ammonium ethanoate, M - dilute approximately 230 ml glacial ethanoic (acetic) acid to 1 l. Dilute approximately 220 ml ammonia solution (ammonium hydroxide) approximately 35% m/m NH3 to 1 l in a fume cupboard. Mix together in a 5-l graduated beaker and adjust the pH to 7.0 using ethanoic acid or ammonia solution added using a disposable polyethylene pasteur pipette. Stir with a glass rod between additions, but allow solution to become still before reading the pH. Dilute to 4 l and transfer to a polythene storage bottle.
• Ethanol, 95% (or industrial methylated spirits, 95%) - dilute ethanol (or IMS) to give 95% v/v ethanol/water.
• Potassium chloride solution - dissolve approximately 100 g KCl in water and make up to 1 l. Add 2.5 ml M HCl, and check the pH is approximately 2.5.
Extraction. Transfer 5 g sieved (^ 2 mm) air-dried soil to a 100-ml glass beaker, add 20 ml M ammonium ethanoate, stir and let stand overnight. Transfer the contents to a filter funnel fitted with a 125 mm Whatman No. 44 filter paper and held in a 250 ml volumetric (graduated) flask. Wash the beaker with ammonium ethanoate reagent from a wash (squeeze) bottle to remove all the sample, then add successive 25 ml volumes of reagent to leach the soil in the funnel, allowing it to drain between additions. With the collected leachate volume approaching 250 ml, remove the funnel to a rack or place in a 250 or 500 ml conical flask, and make up the volume to the 250 ml mark with reagent and retain for analysis of exchangeable bases.
The soil in the funnel is washed free of excess reagent by five successive additions of 95% ethanol, allowing to drain between washings. A wash bottle containing ethanol enables the interior surface of the funnel, the outside of the stem, the exposed surface of the paper and the soil to be thoroughly washed. Any remaining ammonium ethanoate will elevate the final CEC value. The washings, which are flammable, should be collected in a waste solvents bottle for safe disposal.
The funnel is now placed in a 100-ml volumetric flask and leached with successive 25-ml portions of potassium chloride solution, allowing draining between additions, until nearly 100 ml has been collected. Make up to the mark and retain for determination of CEC.
This is achieved by using an atomic absorption spectrophotometer (or less accurately with a flame photometer). Some details could be instrument specific, so refer to the manufacturer's handbook, application data sheets, and obtain technical support if you lack experience in this area. Some general guidelines will be noted here.
The use of a nitrous oxide-acetylene flame obviates the need for releasing agents to be added to samples and standards, but may be hazardous to use. It also requires addition of a reagent of an easily ionized compound, such as potassium, to be added to suppress ionization. It is suggested that an air-acetylene flame is more appropriate for routine use. Releasing agents are chemicals which protect the analyte atoms in the flame from forming compounds with other molecular or ionic species, which will depress the absorption in an erratic manner. Either strontium or lanthanum salts are used for this purpose. It is essential that all standard solutions are made up in the same reagent as the samples. This ensures that they not only have any impurities introduced by the reagent solution, but that they have the same viscosity (which can greatly affect the rate of aspiration by the nebulizer) and exert the same interference effect in the flame. A blank solution should always be included, and a control obtained from a bulk sample is good practice for any analysis, and enables one to detect if a systematic error or instrument malfunction should arise. The sample solutions will often require dilution to suit the sensitivity of the particular instrument, however, the sensitivity may be able to be reduced either electronically or by rotation of the burner, and so avoid this extra step. If the standard curve begins to level out towards the horizontal, the flame is probably becoming saturated with the analyte, and dilution is essential.
Wavelengths for AAS. Calcium is measured at 422.7 nm and magnesium at 285.21 nm.
• Calcium stock solution, 1000 pg Ca2+ ml-1 - stock solutions of many elements for determination by AAS are available commercially. Details for in-house preparation will also be given. Anhydrous calcium nitrate, Ca(NO3)2, is dried for 1 h at 105°C, then cooled in a desiccator. Transfer 2.05 g to a 100-ml beaker containing water and stir to dissolve. Immediately add 1 ml HCl (36% m/m) to prevent hydrolysis, add with washings to a 500 ml volumetric flask, make up to the mark with water, and mix by shaking.
• Calcium standards, 50 and 0-5.0 pg Ca2+ ml-1 - pipette 25 ml stock solution into a 500-ml volumetric flask, make up to the mark with M ammonium ethanoate reagent and mix to give a solution of 50 pg Ca2+ ml-1. Pipette 0, 0.5, 1, 2.5, 5 and 10 ml of this solution into 100 ml volumetric flasks and make up to the mark with ammonium ethanoate reagent. Standard values are 0, 0.25, 0.5, 1.0, 2.5 and 5.0 pg Ca2+ ml-1.
• Releasing agent - dissolve 2.68 g lanthanum chloride heptahydrate (LaCl3.7H2O) in water and make up to 100 ml.
• Magnesium stock solution, 1000 pg Mg2+ ml-1 - dissolve 1.6581 g magnesium oxide (previously dried at 105°C overnight and cooled in a desiccator) in the minimum of hydrochloric acid (approximately 5 M). Dilute with water to 1 l in a volumetric flask to obtain a solution of 1000 pg Mg2+ ml-1.
• Magnesium standards, 10 and 0-1 pg Mg2+ ml-1 - pipette 5 ml stock solution into a 500-ml volumetric flask and dilute to the mark with M ammonium ethanoate reagent to obtain a stock solution of 10 pg Mg2+ ml-1. Pipette 0, 2, 4, 6, 8 and 10 ml of the 10 pg Mg2+ ml-1 stock solution into 100 volumetric flasks and make up to the mark with M ammonium ethanoate reagent and mix. This will give solutions containing 0, 0.2, 0.4, 0.6, 0.8 and 1.0 pg Mg2+ ml-1.
Analysis of solutions. Pipette 20 ml of sample and standard solutions into 50-ml beakers, then pipette 1 ml releasing agent solution into each beaker and mix. If readings are off-scale, pipette 5 ml extract plus 15 ml M ammonium ethanoate and the 1 ml releasing agent and retest. Whatever dilution is necessary, ensure the sample plus M ammonium ethanoate solution add up to 20 ml before addition of the 1 ml releasing agent.
Measurement of potassium and sodium by flame photometry
These elements are best determined using flame photometry, as their high atomic emission energy in the flame exceeds their absorption of energy, which results in a higher sensitivity than with atomic absorption spectrophotometry. Check that the appropriate filter is in place for the element being determined, ignite the air-propane flame and ensure an adequate warm-up time. Aspirate the blank solution and adjust the reading to zero. Aspirate the highest standard to allow sensitivity adjustment to give an emission of about 90% maximum reading, and then re-check the zero with the blank. Ensure the standard curve is reasonably linear, then proceed to analyse the samples. Repeat the standards at about 10-min intervals to permit correction for any changes in sensitivity. A quality control sample may be analysed at intervals of about 48 samples.
• Potassium stock solution, 1000 pg K+ ml-1 - weigh 1.293 g potassium nitrate (previously dried for 1 h at 105°C and cooled in a desiccator) into a 100-ml beaker. Dissolve in water, add 1 ml hydrochloric acid (approximately 36% m/m HCl) and 1 drop of toluene, then transfer with washings to a 500-ml volumetric flask, make up to the mark and mix well by shaking.
• Potassium standard solutions, 100 and 0-10 pg K+ ml-1 - pipette 10 ml of the stock solution into a 100-ml volumetric flask and dilute with M ammonium ethanoate reagent to the mark and mix to give a solution of 100 pg K+ ml-1. Pipette 0, 2, 4, 6, 8 and 10 ml of this solution into 100-ml volumetric flasks and dilute to the mark with M ammonium ethanoate reagent and mix. These will contain 0, 2, 4, 6, 8 and 10 pg K+ ml-1.
• Sodium stock solution, 1000 pg Na+ ml-1 - weigh 2.542 g sodium chloride (previously dried for 1 h at 105°C and cooled in a desiccator) into a 100-ml beaker. Dissolve in water, add 1 ml hydrochloric acid (approximately 36% m/m HCl) and 1 drop of toluene, then transfer with washings to a 1000-ml volumetric flask, make up to the mark and mix well by shaking.
• Sodium standard solutions, 100 and 0-10 pg Na+ ml-1 - pipette 10 ml of the stock solution into a 100-ml volumetric flask and dilute with M ammonium ethanoate reagent to the mark and mix to give a solution of 100 pg Na+ ml-1. Pipette 0, 1, 2, 3, 4 and 5 ml of this solution into 100-ml volumetric flasks and dilute to the mark with water and mix. These will contain 0, 1, 2, 3, 4 and 5 pg Na+ ml-1.
Calculation (1). Results have traditionally been expressed as milliequivalents per 100 g soil. An alternative more recent expression is centimole charge per kilogram soil (cmolc kg-1), but both expressions give the same numbers. The concentrations of cations using the above methods may be obtained by multiplying the concentration of cation (pg ml-1) in the sample extract solution (obtained by comparing sample readings with the standard curve) by the following factors (plus any dilution factors to bring readings on scale):
Calcium, 0.249; magnesium, 0.412; potassium, 0.128; sodium, 0.2175 Explanation: If a reading of X pg K ml-1 is obtained for a solution of 5 g soil in 250 ml extractant (1 in 50 dilution), this amounts to X/(39.098 x 103) milliequivalents K ml-1, or 250X/(39.098 x 103) milliequivalents K in 250 ml extractant. This is derived from 5 g soil, thus 100 g soil would contain (20 x 250 x X)/(39.098 x 103) = 0.128 milliequivalents K.
Thus if 2 g soil were taken instead of 5 g, an additional factor of x 5/2 should be used. If the sample solution for calcium determination was diluted 5 ml solution plus 15 ml M ammonium ethanoate reagent before addition of 1 ml releasing agent, then an additional factor of x 4 will be necessary.
Determination of cation exchange capacity (CEC)
The ammonium extracted by the potassium chloride reagent is analysed by steam distillation. This may be carried out using an automatic instrument such as the Kjeltec Auto 1035 Analyzer (USDA, 1996, pp. 203-210), or a micro (or semi-micro) steam distillation unit such as that described by Bremner and Keeney (1965), or the readily available Markham still. We will describe the manual procedure.
• Ammonium-N standard solution, 140 pg ml-1 nitrogen - weigh 0.661 g ammonium sulphate (dried at 105°C for 1 h and cooled in a desiccator) into a 100-ml beaker and dissolve in ammonia-free water (distil deionized water acidified with sulphuric acid), transfer with washings to a 1-l volumetric flask and make up to the mark with the ammonia-free water and mix. This should be stored in a refrigerator, but a quantity allowed to warm to room temperature in a stoppered container before use.
• Boric acid solution, approximately 2% m/v - prepare fresh weekly.
• Mixed indicator - dissolve 0.3 g methyl red and 0.2 g methylene blue in 250 ml ethanol.
• Magnesium hydroxide suspension - heat magnesium oxide (heavy) for 2 h at 800°C. After cooling in a desiccator, make a suspension of 17 g in 100 ml water.
• Octan-2-ol - antifoam agent: use 1 drop when flasks <150-250 ml capacity are used.
Procedure. Steam is passed through the steam distillation apparatus for 20-30 min. Check the performance by pipetting 5 ml ammonium-N standard solution into the distillation unit, add 1 drop octan-2-ol, 6 ml magnesium hydroxide suspension and steam distil the released ammonia into 5 ml boric acid solution in a 100-ml conical flask. After approximately 40 ml distillate has been collected over a 5-min period, wash the tip of the condenser into the distillate, add 2-3 drops mixed indicator solution and titrate with 0.005 M H2SO4 until the colour changes from green to purple. A blank distillation/titra-tion is carried out using 5 ml ammonia-free water and subtracted from the standard titre to give a result which should be 5.00 ml.
Pipette a 25-ml (or y ml, where y ^ 50 ml) aliquot of the soil extract in KCl into the distillation apparatus and proceed as above. If the titre (s) lies outside of the range 0.2-7.0 ml, adjust the volume of extract accordingly. Repeat using a similar aliquot of KCl extractant solution to give a blank titre (b).
Determine the percentage moisture content (z%) of the air-dry soil by oven drying overnight at 105°C. Then calculate the above as CECc x 100/(100 - z)
cmolc kg-1 oven-dry soil. If the moisture content is relative to the oven-dry soil (z% of oven-dry soil), the calculation becomes:
Notes. Depending on the apparatus, wearing insulating gloves and eye protection, remove the steam lead and then the distillation flask while still hot (or remove the flask, then turn off the steam supply). This is to prevent suckback of flask contents into the steam generator, also to prevent seizure of ground glass joints because of the effect of magnesium oxide.
Other variations include the use of bromocresol green-methyl red indicator, titrating from green through colourless to a pale pink end-point. Magnesium oxide may be used straight from the bottle or replaced by 10 ml 50% m/v NaOH solution. The acid may be 0.01 M HCl (equivalent to 0.005 M H2SO4).
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