Exercise 3: Determination of Exchangeable Base Cations

Major amounts of exchangeable Ca²⁺ and Mg²⁺, plus smaller amounts of Fe²⁺, Cu²⁺, Mn²⁺, and Zn²⁺ constitute most of the exchangeable divalent cations in soils. These cations are essential plant nutrients, and the cation exchange system is the principal storehouse of nutrient forms available for plant growth. The macronutrients Ca and Mg are base (also called basic or nonacid) cations that represent, on average, about 80 and 15%, respectively, of all exchangeable base cations in Iowa soils. The remaining 4 or 5% of the base cations are mostly the monovalent cations, K⁺ and Na⁺. The micronutrients usually total only a few tenths of one percent of the exchangeable divalent cations.

Sodium occurs in most soils in small amounts. The main exception to this generality is the sodic soils of arid regions. A determination of exchangeable sodium should be made to evaluate this condition on soils that have a pH of 8.5 or above.

The soil pH is controlled by the kinds and amount of cations on cation exchange sites, the origin of the cation exchange sites, and the soil water content. The origin of cation exchange sites is controlled by the types and amounts of clay and organic matter present in soil, and these can be considered constant for any specific soil. With constant or known water content, the soil pH then is dependent only on the proportions of the various exchangeable cations associated with the exchange sites. The particular proportion that is most important in determining soil pH is the percentage of exchangeable base cations as a fraction of the cation exchange capacity. This percentage is called the percentage base saturation. Because the sum of exchangeable base cations and exchangeable acidic cations equals cation exchange capacity, the percentage base saturation can be determined by knowing any two of the three values (units are meq/100 g soil):

Cation exchange capacity (CEC) = exchangeable base cations + exchangeable acidic cations

Percentage base saturation = exchangeable base cations / CEC × 100

Any particular soil may be considered to have a specific relationship between soil pH and the exchangeable ions present. Also, other soils containing similar types and proportions of mineral and organic matter behave similarly in their relationship of soil pH to percentage base saturation. Curves showing typical relationships between pH and percentage base saturation are given in the exercise on lime requirement.

One common method of determining CEC is to saturate all CEC sites with a common cation, such as ammonium, and then displace the ammonium with another cation such as calcium, and measure the quantity of displaced ammonium. This is cation saturation by a 1 N ammonium acetate (NH₄OAc) solution. The pH of the solution may or may not be buffered. If buffered at pH 7.0, the procedure may overestimate CEC of acid soils because some CEC is pH dependent. The base ions determined here plus the amount of acidic cations (Exercise 4) when summed together will equal the soil CEC by the summation method.

Iowa tests indicate that Mehlich-3 and 1 N ammonium acetate extract similar amounts of base cations on soils with pH <7.3. Because we will also use Mehlich-3 for P determination later in the course, we will then use the same solution here to determine exchangeable base cations.

After extraction from soil, the quantity of base cations in solution will be determined by atomic absorption spectroscopy. Atomic absorption spectrometry determines the concentration of an element in a sample because each element can absorb light energy at a specific wavelength. The method requires standards with known concentrations to establish the relationship between the measured absorbance and the concentration. The outer-shell electrons of the atoms in the flame are excited to higher energy states (excited state) for a short period of time (nanoseconds) by absorbing a defined quantity of energy of a specific wavelength. This amount of energy is specific to a specific element. The difference without a sample and with a sample in the flame is measured by a detector and the ratio between the two values (the absorbance) is converted to concentration.

The milliequivalents of base cations determined by this procedure may exceed the cation exchange capacity if the procedure is used on calcareous or saline soils. The reason for this anomaly is that the leaching solution will dissolve divalent cations from calcium carbonate and other salts and these cations will be measured along with the divalent cations from exchange sites. Calcareous soils and most saline soils can be identified by the bubbling action of CO₂ that is released when a drop or two of 1 N HCl is applied to the soil. Such soils have 100% base saturation and the actual amount of exchangeable base cations is equal to the cation exchange capacity of the soil.

Parts Per Million (ppm). Many soil testing laboratories and popular publications use ppm in expressing concentrations, so students need to understand it. This is a convenient way of expressing small concentrations. Think of ppm similarly as you think of parts per hundred (pph), a term we often use with percentages. Thus, ppm is just a part in a million parts rather than a part in one hundred parts. In a year’s time, a pph equals 3 days, 15 hr, and 2160 sec; a ppm equals 32 sec.

Most scientific journals do not allow ppm because it can be ambiguous. In scientific writing, for example, one uses µg/mL or mg/kg; both are ppm. The first, however, is based on weight-to-volume and the second is based on weight-to-weight. The units of ppm may be volume-to-volume, volume-to-weight, weight-to-volume, or weight-to-weight. With ppm, one needs to carefully understand the units involved!

 

Convert % (wt/vol) to ppm in solution

Formula is [latex]0.0001% \times 10^{6} / 100 = 1ppm[/latex]

%	ppm
0.0001	1
0.0002	2
0.0005	5
0.001	10
0.0025	25
0.005	50
0.01	100
0.02	200
0.025	250
0.05	500
0.1	1000
1.0	10000

 

Procedure

1. Weigh 3.00 ± 0.05 g < 2.0 mm air-dried soil into each of two 50-mL centrifuge tubes. Record the exact weight to the correct number of significant figures.

A 2-mm sieve is standard for sieving soil. Break soil peds and clods into small enough fragments to pass through the sieve so that the sample will be representative of the whole soil. The sample size may be reduced to 0.5 g for soils that are high in clay and/or organic matter. Check with your instructor if in doubt.

2. Add 30.0 ± 0.1 mL of Mehlich-3 solution to each tube.
3. Shake the soil solution briefly and set on the shaker bed horizontally. Equilibrate for 5 min at high speed.

Intimate contact between the solution and the soil colloids is needed to make the reaction quantitative. Sieved soil was used to assure accessibility of the soil particles to the solution. The exchangeable base cations in the soil are displaced by mass action by ammonium and hydrogen ions from the solution. After this step, the exchangeable base cations will be in solution.

4. Place the tubes in an upright position and load onto the centrifuge @ 3000 rpm for 10 min. Pass the supernatant through a Whatman No. 42 filter paper and collect the filtrate in a 50-mL Erlenmeyer flask.

This step separates the soil particles from the solution. Refilter if filtrate is unclear.

5. Dilute the extract by pipetting a 1.0-mL aliquot into a 15-mL test tube and adding 9.0 mL of Mehlich-3. Thus, the original extract is diluted 10-fold. Cover the tube with parafilm and shake. Transfer the remainder of the extract into 50-mL screw cap polypropylene tube and store in the refrigerator for later P determination (Exercise 13).
6. Determine Ca²⁺, Mg²⁺, and Na⁺ contents by atomic absorption. Determine K⁺ by measuring the intensity of light emitted when K in the sample is excited by the flame of the instrument. The instrument measures the amount of absorption or emission relative to the standards used. For each cation, determine the regression line using Microsoft Excel and then calculate the mg/L of the cation in solution.

 

7. Calculate the exchangeable base cations in meq/100 g of soil.

Ask a classmate or the instructor for help if you don’t understand the calculations.

 

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Date_____________________

Section__________________

Soil number ________

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