Exercise 5: Determination of Soil pH

A pH measurement is the most common (and perhaps most important) chemical test made on soil. Values for pH are relatively quick and easy to determine and are good indicators of the general chemical condition of the soil. Problems such as a need for lime to reduce soil acidity or a need for sulfur to reduce soil alkalinity can be diagnosed directly from pH determinations. Indirectly, the pH of a soil offers clues to plant nutrient deficiencies and how to remedy them, possible toxicities and their remedies, and appropriate selection and use of pesticides.

The definition of pH is the negative logarithm of the hydronium ion activity in solution. The term activity is closely related to concentration, and for our purposes we will assume that the terms are synonymous. A pH value of 7.0 is considered neutral because it indicates equal concentrations of H⁺ and OH^– ions in the solution. Values of pH below 7.0 indicate acidity (a predominance of H⁺ ions over OH^– ions), and values above 7.0 indicate alkalinity (a preponderance of OH^– ions over H⁺ ions). The term pH is derived from mathematical power p (negative logarithm) and H (hydrogen ion activity).

The soil pH indicates the activity of the hydrogen ions in the soil solution. But, the solution pH is an indicator of the proportions of base and acidic exchangeable ions present because the ions in the soil solution are in equilibrium with the exchangeable ions.

Soil pH can be determined in the field or in the laboratory. Pocket pH meters, standard dyes, and pH-indicator paper strips are commonly used in the field. To be reliable, the pocket pH meters must be handled correctly, well maintained, and calibrated often with fresh buffer solutions. Standard dyes are quite reliable when kept clean and fresh. Temperature extremes and prolonged exposure to sunlight can affect the reliability and longevity of dyes. Several commercial kits are available containing dyes but they may use different diluting solutions (such as water or 0.01 M CaCl₂) so pH readings may vary among the kits. pH-indicator paper strips are paper bonded with dyes and can be used in water or salt-solution suspensions of soil. The paper strips are portable, deteriorate slowly, are pre-calibrated, and can give reasonable accuracy in the pH range of the strip. With both the spot plates and paper strips, the liquid solution is separated from the soil for reading the color developed.

The indicator solutions and dyes in paper strips are organic acids or bases that dissolve and partly ionize in water. The organic-acid type usually is used for soil pH measurement because an organic base forms cations that can be absorbed on cation exchange sites. To serve as a pH indicator, the organic compound must produce one color as a molecule and another color as an ion. The degree of ionization (and therefore the color of the solution) depends on the supply of OH^– ions in the soil to react with H⁺ ions from an acid indicator (or on the supply of H⁺ ions in the soil to react with OH^– ions from an organic base indicator). Any one pH indicator solution has a useful working range of about one pH unit in which most of its color change occurs. Outside the working range, the color change becomes too gradual to be read reliably. The user who finds that a pH indicator color is at one end of its range must then use another indicator with a different working range. There are many different indicators available, but a set of three or four indicators will usually cover the pH range of the soils in a state. Such a set of indicators permits measurement of pH with a precision (reproducibility) of about 0.2 pH units, though the absolute accuracy occasionally may be in error by 0.3 to 0.5 pH units. Mixed indicators are sometimes used to avoid the necessity of carrying several bottles. Combining three or four carefully chosen indicators gives a wide working range, but both precision and accuracy are reduced. The reading precision of a mixed indicator is about 0.5 pH units.

The laboratory method is the standard and more accurate method. It uses a potentiometer to measure pH through a combination electrode inserted into the solution or soil suspension being tested. The combination pH electrode consists of a glass electrode and a reference electrode. The reference electrode has a constant electric potential, whereas the electric potential of the glass electrode varies as a logarithmic function of the hydrogen ion activity. The potentiometer measures the difference in electric potential voltage between the two electrodes and converts this to a pH reading. The electric potential difference is very weak and must be amplified considerably. When there is trouble with a pH meter, the difficulty usually is either in the amplifier or a damaged electrode. Some pH meters can be read to 0.0001 pH units; values from soil usually are reported, however, to the nearest 0.1 pH unit.

There are several variations on the procedure used for measuring soil pH with a pH meter. Some of the variations relate to the amount of water mixed with the soil. The soil must be wet to assure good contact between the soil and the electrodes, but adding more water dilutes the concentration of H⁺ in solution. Careful control of the water content therefore is essential. The minimum amount of water that can be used is just enough to fill all of the soil pore spaces. This condition, known as a saturated paste, would seem to deviate least from field conditions but the procedure is time consuming and more difficult than some of the other methods. Also, there is a hazard of scratching or breaking the electrodes with a saturated paste. Another common approach is to use a soil:water ratio such as 1:1, 1:2, or 1:3 by weight or volume (or some other fixed proportion). Allowance should be made for the water already present in the soil when this approach is used. Researchers may avoid the need for careful control of the soil:water ratio by using a solution of 0.01 M CaCl₂ in place of the water. The presence of the calcium chloride stabilizes the pH over a wide range of dilutions and gives a very dependable pH reading. This reading is usually about a one-half unit lower than that of a soil-water suspension. It is probably close to the pH actually encountered by plant roots growing in the field.

Other variations are concerned with whether the suspension should be stirred or allowed to settle. To simulate stirring we will swirl the sample. Most soil fertility laboratories test the pH of a stirred sample, so that procedure will be used in this exercise. The best theoretical procedure is to let the soil settle and to read the pH with the gass electrode in the settled soil and the reference electrode in the liquid above the soil. In a stirred sample, the pH will continue to change slightly after placing the electrodes in the soil:water suspension. Some laboratories will allow 30 sec to settle and then take the reading.

Procedure (pH Test Strips Method)

1. Obtain test strip paper from a supplier (garden store or scientific and laboratory supply store) such as Hydrion^TM pH test paper. Match the range of the paper with the expected pH of the soil.

The paper is impregnated with a dye that changes color based on the pH of the solution. This color is compared with a color chart that comes with the package. This procedure is more of a home procedure rather than a field procedure because of the steps involved. Alternatively, litmus paper can be used and the color depends only on whether the sample solution is acidic or basic, turning red or blue, respectively.

2. In duplicate, add a small quantity of soil (tablespoon) to a 50-mL beaker (any clean container will work).

The soil should be dry enough and broken up sufficiently to expose a large surface area to the solution.

3. Add enough distilled water to create a mixture the consistency of a milkshake (approx. 1 part soil to 1 part water).

The distilled water may adsorb CO₂ from the air, which could lower the measured soil pH. It is best to boil the distilled water and store the cooled water in a tight container. Using tap water may unduly affect the resulting pH.

4. Agitate the solution for about 1 min and then leave the mixture alone for 30 min.

This step allows the soil to settle to the bottom of the container. Another procedure calls for placing a test strip in the bottom of a cup, inserting a coffee filter in the cup and above the test strip, adding soil:water slurry that has been mixed in a separate container. The coffee filter will filter the soil and allow the solution to come into contact with the test paper.

5. Place a drop of the solution from above the settled soil on a test strip and wait 1 min.

One can use a pipette, eye dropper, or wet spoon to transfer the drop. Alternatively, you may try to place the test strip directly into the solution, leave 1 min, take out, and rise with distilled water. Determine which procedure works best on your soil.

6. Rinse the strip with distilled water.

This step will wash away soil particles that might have been retained in the drop.

7. Match the test strip color with the chart and record the indicated pH to the nearest 0.1 pH unit.

Based on the range of test strip paper used, one may only be able to make an estimate of the pH to the nearest 0.5 pH units.

Procedure (Indicator Method)

1. Be sure that the spot plate or other sample holder to be used is clean and free of contamination.

A porcelain spot plate with several small depressions is usually used for the test but a glass container or a piece of waxed paper can be used. Some of the most troublesome contaminants are laboratory chemicals and fingerprints. These can be removed by wiping with tissue or paper towel or, in the field, with some of the soil to be tested.

2. Place small amounts of soil (about ¼ the volume of a depression) in each of two or three spot-plate depressions.

The soil should be broken up sufficiently to expose a large surface area to the solutions but not pulverized enough to float.

3. Add enough of one indicator to saturate the soil in one depression, plus about one drop more.

Begin with the indicator most likely to include the soil pH in its range. The color will be read in the excess solution around the ring outside the soil but there must be enough soil to control the pH of the solution.

4. Agitate the suspension for about 1 min.

Hold the spot plate in one hand and bump it gently against your other hand. Sometimes it is necessary to stir the soil solution mixture with a clean stirring rod. If stirring makes the suspension muddy and masks its color, let the suspension stand until some of the indicator near the edge of the depression is clear.

5. Compare the color of the solution outside the soil to the color chart for the indicator used.

Daylight is best for the comparison because it provides the full spectrum of light. The comparison is made on the basis of shades between primary colors such as, for example, yellow, yellowish green, green, bluish green, to blue. Darkness and intensity of color are not used because these characteristics depend on the amount of soil in suspension and the strength of the solution rather than on the pH.

6. Record the pH to the nearest 0.1 unit if it is within the range of the indicator; otherwise, retest the sample by choosing a different indicator and repeating steps 2 to 6.

The color change of the indicator becomes too gradual if the pH is outside its working range but it will still serve as a guide for choosing another indicator to use next. For example, if the apparent pH is at the lower end of the working range for an individual indicator, choose a second indicator where this pH would be in the mid-range. Remember, indicators are usually most sensitive in the middle of their working ranges.

Procedure (pH Meter Method)

1. Place 15.0 ± 0.1 g of soil and 15.0 ± 0.1 mL of deionized water into 1 50-mL beaker and 15.0 ± 0.1 g of soil and 15.0 ± 0.1 mL of 0.01 M CaCl₂ into another 50-mL beaker. Stir vigorously for 5 sec and let stand for 10 min.

A standard ratio of soil-to-water is used to obtain consistent results. Determinations made on dry soil give erratic results because the contact between the soil and the electrodes is poor. Too much water also should be avoided because the pH rises with dilution. An adjustment in the water added should be made for the water already in the soil when wet or moist samples are tested. The sample weight then should be increased to still contain approximately 5 g of dry soil and the amount of water should be 15 mL, including that contained in the soil.

2. Set the pH meter control switch at the neutral or standby position, if the meter has this feature, and clean the electrodes as demonstrated by your instructor.

The pH meter needs a 30-min warm-up period before use. The electrodes should be rinsed with deionized water from a wash bottle and blotted dry with soft absorbent tissue just before use. NOTE: Glass electrodes are thin and fragile so extreme care should be taken to avoid striking the glass electrode against any solid object.

3. Your instructor will demonstrate use of the pH meter and calibration of the meter using buffers of known pH (pH 4.0 and pH 7.0). Note that the switch is placed on neutral or standby position whenever the electrodes are out of solution. Rinse the electrodes into the beaker provided and blot dry with tissue.

The calibration should be checked before any samples are tested and again periodically after several samples have been tested. Use calibrants with pH near the expected sample value.

4. Place the electrodes into the slurry, swirl carefully, and read the pH as soon as the meter reading stabilizes (30 sec). Report pH to the nearest 0.1 unit.

This is the method used in many soil testing laboratories. Other laboratories use the theoretically better method of testing the pH with the soil settled to the bottom, the glass electrode in the soil, and the reference electrode in the liquid above the soil. Some meters have two reading positions and two scales for pH. The proper scale must be read to correspond with the switch position.

5. Set the switch to standby and clean the electrodes again. Lower them into a container of deionized water for storage until time for their next use.

New electrodes or electrodes that have been allowed to dry out must be soaked in a buffered solution or deionized water for a day or so before they will give reliable readings.

6. If your pH is below 6.9, continue on by determining the buffer pH of your soil. Procedures are outlined in Exercise 6. Samples with pH 6.9 or higher may be discarded.

Lime requirements will be determined in the laboratory next week. The standard determination by buffer pH is a step that normally follows determination of soil pH. Because the materials are available this week, we will determine soil-buffer pH and discuss it next week. If your soil has a pH equal or greater than 6.9, lime application would not be recommended.

Record the soil-buffer pH of your two replicates in the appropriate locations in both Exercises 5 and 6.

Procedure (Sikora Method)

1. Add 15 ± 0.1 mL of Sikora buffer solution to each beaker containing 15.0 ± 0.1 g of soil and 15 mL of deionized water. Stir thoroughly for 10 min.

The buffer solution used for many years at Iowa State and elsewhere was the SMP buffer named for three individuals (Shoemaker, McLean, and Pratt) from Ohio State. Because the SMP buffer contained para-nitrophenol and chromate (both hazardous), the ISU laboratory switched to the Sikora buffer in 2012. The lime calibration curves (Table 1) developed for the SMP procedure also work with the Sikora buffer. To prepare a liter of the Sikora buffer, first dissolve 149 g of KCl in about 750 mL water. When KCl has dissolved completely, add the following in this order: 5.11 mL of glacial acetic acid, 6.70 g MES (2-(N-morpholino) ethanesulfonic acid monohydrate), 0.936 g of imidazole, 9.23 mL triethanolamine, and 5 mL 40% (wt/vol) NaOH. Bring to final volume and mix well before adjusting the pH to 7.70 ± 0.01 with drop-wise addition of either 40% (wt/vol) NaOH or 50% (vol/vol) HCl. To further verify the solution pH, add 50 mL of deionized water to 50 mL of buffer, stir, and measure pH. The pH should be 7.53 ± 0.03. Add 5 mL of the 0.5 M HCl to the 1:1 diluted buffer, stir, and measure pH. The pH should be 5.68 ± 0.06.

This method measures both active and reserve (potential) acidity. The potassium ions replace the acidity ions on the cation exchange sites. The acidic ions are then in solution where they can be measured by the pH glass electrode. With the known buffering capacity of the Sikora solution, the depression of pH relates to lime requirement.

2. Let stand for 10 min and determine the soil-buffer pH with the pH meter immediately after stirring using the procedure outlined for pH determination.

The lime requirement of the soil is indicated by the extent of pH drop below the original pH 7.7 of the buffer solution. The greater the total (active plus reserve) acidity of the soil, the greater the lowering of the buffer pH, and the greater the quantities of lime required to neutralize the acidity.

3. Find the soil-buffer pH and the corresponding lime requirement for a 6-inch plow layer in Table 1. Enter the values on the answer sheet for the amount of ECCE needed to reach pH 6.5 and 6.9. If adding to a 7-inch depth, increase this amount proportionately or decrease proportionately if adding only to a 5-inch depth.

Each 0.1 unit of pH change in the buffer represents approximately 0.83 meq of exchangeable acidity per 100 g of soil. How does the acidity determined by the drop in buffer pH in this exercise compare with your earlier measured value for total acidity?

4. Discard the soil-buffer solution in the container designated by the instructor.

*If a tillage depth other than 6 inches is desired, the lime requirement is adjusted proportionately for depth. For example, if the lime for 6 inches is 2800 lb ECCE but it is to be worked into 8 inches of soil, more lime is needed. The amount needed can be determined by:

6/8 = 2800/X;

solving for X = 3733 lb ECCE

This says 6 is to 2800 lb as 8 is to 3733 lb.

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