1.7: pH and Electrical Conductivity

Purpose:

The purpose of this exercise is to introduce students to a variety of laboratory techniques to measure soil pH and electrical conductivity.

Learning Outcomes

– Upon completion of this exercise you should be able to:

• determine soil pH using hydrion paper and a meter
• measure electrical conductivity using an EC meter

Part 1. Soil pH

Background: 

From Brady and Weil, The Nature and Properties of Soils, 13 th Ed. The degree of soil acidity or alkalinity, expressed as soil pH, is a master variable that affects a wide range of soil properties. Soil pH greatly influences the availability for plant roots to take up nutrients and toxins. Soil pH also affects soil microbial activity and influences what plant species can grow at a particular site. Soil pH affects the mobility of many pollutants in soil by influencing the rate of biochemical breakdown, solubility, and adsorption to colloids. Acids can build-up to such high concentrations in soils, typically associated with mine tailings, that the acid itself is considered a pollutant. Soil pH also influences soil structure by dispersing or stabilizing clays. At low pH fungi dominate, fungi cause soils to form large soil aggregates. At high pH clays are dispersed and the soil has little or no structure. Remember that structure in part controls the movement of air and water into and through the soil profile.

Soil pH is measured in pH units. Soil pH is defined as the negative logarithm of the hydrogen ion concentration. The pH scale ranges from 0 to 14 with pH 7 as the neutral point. As the amount of hydrogen ions in the soil increases, the soil pH decreases thus becoming more acidic. From pH 7 to 0 the soil is increasingly more acidic and from pH 7 to 14 the soil is increasingly more alkaline or basic. A change of 1 pH unit is actually a factor of 10; for example, pH 4 is ten times more acidic than pH 5 and 100 times more acidic than pH 6.

Descriptive terms commonly associated with certain ranges in soil pH are:

• Extremely acid: < than 4.5; lemon=2.5; vinegar=3.0; stomach acid=2.0; soda=2–4
• Very strongly acid: 4.5–5.0; beer=4.5–5.0; tomatoes=4.5
• Strongly acid: 5.1–5.5; carrots=5.0; asparagus=5.5; boric acid=5.2; cabbage=5.3
• Moderately acid: 5.6–6.0; potatoes=5.6
• Slightly acid: 6.1–6.5; salmon=6.2; cow's milk=6.5
• Neutral: 6.6–7.3; saliva=6.6–7.3; blood=7.3; shrimp=7.0
• Slightly alkaline: 7.4–7.8; eggs=7.6–7.8
• Moderately alkaline: 7.9–8.4; sea water=8.2; sodium bicarbonate=8.4
• Strongly alkaline: 8.5–9.0; borax=9.0
• Very strongly alkaline: > than 9.1; milk of magnesia=10.5, ammonia=11.1; lime=12 From: College of Environmental Science and Forestry Syracuse University, Syracuse, N.Y.

Equipment required:

• Soil samples
• Hydrion paper (wide-range and narrow-range)
• pH meters (3)
• pH meter buffer solutions
• 50 mL beakers
• Stirring rod
• Deionized water bottle
• Small graduated cylinder
• Balance
• Mud bucket
• Paper towels

Exercise:

You will measure pH for your samples using two types of hydrion paper and a low-cost penstyle pH meter. You need 20 g each from the three soil samples you dried, ground, and sieved in the sample preparation exercise.

Hydrion paper

1. Add 20 g of dried and ground soil sample to a 50 mL beaker.
2. Add 20 mL of water to create a soil solution.
3. Use a stirring rod to thoroughly mix the sample for at least 5 minutes. 
4. Use the stirring rod to drip a small amount of soil solution onto wide-range hydrion paper (i.e., 0 – 13 pH).
5. Hydrion paper will change color – compare to color chart on the case to determine pH.
6. Record wide-range hydrion paper pH.
7. Narrow-range hydrion paper provides a more accurate pH measure, so re-test your sample using the appropriate narrow-range paper (i.e., 0 – 6 pH or 5 – 9 pH).
8. Record the pH of the narrow-range hydrion paper.

pH meter (eco Testr pH2)

1. Before using the pH meter to test soil pH, the meter must be calibrated using three pH buffer solutions (pH 4, 7, and 10).
2. To calibrate, pour about 1 inch of each buffer solution into 50 mL beakers.
3. Turn on the pH meter.
4. Dip the sensor into the calibration solution starting with pH 7 buffer solution, stir gently, and wait for the reading to stabilize. Press the “cal” button, then press the “hold/ent” button. If calibrated correctly, the display with read “Ent”. Rinse the sensor clean with water.
5. Repeat step 4 to calibrate with the pH 4 buffer solution then the pH 10 buffer solution.
6. Rinse the pH meter electrodes with water before using each buffer solution and upon completing calibration procedures. Used buffer solution can be dumped down the sink drain.
7. Submerse calibrated pH meter into the soil solution and gently stir with pH meter for about 1 minute.
8. Allow the reading to stabilize and record soil pH.
9. Rinse and clean the pH meter with water, replace cap, and store.

Part 2. Electrical Conductivity

Background:

From Brady and Weil, The Nature and Properties of Soils, 13 th Ed. Salt-affected soils, particularly common in semi-arid and arid regions under irrigation, limit plant growth and reduce agricultural productivity. Salts are detrimental to plant growth for two primary reasons: 1) salts lower the osmotic potential of soil water so roots have a harder time extracting water from the soils, and 2) several salt ions are toxic to plants or nutrient exchange sites are blocked by salt ions. Salts, primarily chlorides and sulfates of calcium, magnesium, sodium, and potassium, typically accumulate in soils in regions where the precipitation: evaporation ratio is less than 1 – water evaporates but salts remain in the soil. Keep in mind that salinity refers to all salts while sodicity refers specifically to sodium. Salt accumulation is not typically a problem in humid regions because sufficient precipitation flushes salts from the soil.

Salinity is commonly estimated by measuring soil electrical conductivity (EC) or total dissolved solids concentration (TDS). Pure water is a poor conductor of electricity, but as salts are added the ability to conduct an electrical charge increases. So, measuring EC of a soil solution gives an indirect measurement of a soil’s salt content. Electrical conductivity is also influenced by soil texture, with coarser-grained soils typically having lower EC values than silt- and clay-rich soils. The most common method for measuring EC, and the method used in this exercise, is the saturation paste extract method. A soil sample is saturated with deionized water and mixed to the consistency of a paste – the deionized water dissolves the salts from the soil, which can then be measured with an EC meter. Measuring TDS is a more complex process in which a soil sample is saturated with deionized water to dissolve all the salts from the soil, the water is then extracted into a beaker, weighed, completely evaporated, and the weight of the remaining residue is weighed to determine TDS. TDS concentration and EC are directly related, so some EC meters also provide information on TDS concentration.

Class of salt-affected soils:

• Saline: EC > 4 ds/m but have an Exchangeable Sodium Percentage (ESP) < 15 (i.e. lots of salts, but most salts are not sodium)
• Saline-Sodic – EC > 4 ds/m and ESP > 15 (i.e. high levels of salts, with a high proportion of sodium)
• Sodic – EC < 4 ds/m and ESP >15 (i.e. low level of salts, but relatively high levels of sodium)

Equipment required:

• Soil samples
• EC meters (low and high)
• EC calibration solution
• 50 mL beakers
• Stirring rod
• Deionized water bottle
• Top load balance
• Mud bucket
• Paper towels

Exercise:

25 g of soil sample must be prepared following Exercise 5 methods (pick roots, dry, grind, and sieve) before proceeding.

1. Add 20 grams of soil sample to 50 mL beaker.
2. Add R-O water to soil sample until you form a gooey paste (a consistency between pudding and a milkshake). If you make it too runny, you can add more soil.
3. Use a stirring rod to stir the mixture for at least 10 minutes. Cover/seal the sample and allow sample to sit for 24 hours.
4. Before using the EC meters to test soil EC, the meters must be calibrated using a single EC buffer solution.
5. To calibrate, pour about 1 inch of buffer solution into a 50 mL beaker.
6. Turn on the meter, dip the sensor into the calibration solution, and wait for the value to stabilize. Press the “cal” button. Press “hold/ent” until the blinking value matches the value of the calibration standard. Release the “hold/ent” button to accept the calibration value. If calibrated correctly, the display will read “Ent”.
7. Rinse the EC meter electrodes with R-O water upon completing calibration procedures. Used buffer solution can be dumped down the sink drain. 
8. After 24 hours has elapsed since the sample was prepared, thoroughly mix the sample, submerse calibrated EC meter into solution and gently stir mixture with EC meter for about 1 minute.
9. Allow reading to stabilize and record soil electrical conductivity. 
10. Rinse and clean EC meter in R-O water, replace cap, and store.
11. Steps 4 through 10 must be completed for both the eco Testr EC low and EC high meters.