EA2007 - Soil Chemical Properties - Soil Properties and Processes - B. pH and Electrical Conductivity - Science Assignment Help

EA2007: Soil Chemical Properties, Soil Properties and Processes Assignment Help

Assignment Task:

Practical 3. Soil chemical properties

Introduction
In this practical, spread over three weeks, you will examine several soil samples, measuring some key chemical properties. You will use soil from various places that we have collected previously. However, you are welcome to bring your own soil samples and analyse them instead if you wish.

 

By the end of this practical you will be able to:

 Measure the following characteristics of a soil sample:

o field texture (ie. perfect your technique)
o organic matter content
o pH
o electrical conductivity
o cation exchange capacity
 

Understand how these parameters may be related to each other and to other characteristics of the soil and environment.

•  Write a scientific report

 

 

 

B. pH and electrical conductivity (EC)

Background
Soil pH can tell you a lot about the chemistry of a soil. Soil pH is a measure of the concentration of the hydrogen ion (H + ). It is determined by the net effect of all the acidic and alkali materials in soil. Because it is the net effect of the many reactive materials in soil, pH gives a good indication of many soil properties, especially the availability of various nutrients and toxic materials to plants. Soil pH influences the availability of nutrients directly by influencing the solubility of materials in soil, and indirectly by influencing biological activity. You have measured pH in the field using the indicator method. Now you will measure pH in the lab using a 1:5 soil:water suspension and a pH electrode (Rayment and Lyons 2011).

Soil electrical conductivity (EC), is related primarily to the amount of salt in the soil. Salts originate from rainfall, irrigation water, the sea, groundwater, weathering of minerals and mineralization of organic matter. In wet climates they are easily lost by leaching, but in places where evaporation exceeds precipitation they can accumulate. High levels of salinity limit plant growth, mostly due to osmotic effects. There are many salts in soil and water, having different composition and solubility. Sodium chloride is the most common salt in most Australian landscapes, and is also the most soluble. As most salt solutions have a similar relationship between concentration EC, and as EC is closely related to osmotic potential, EC is a useful way of measuring salinity. That is straightforward in water, but in soil we need to add pure water (drastically dilute the soil) to obtain enough solution to measure the EC. When measuring EC of soil it is critical to record the ratio of soil: water used. Common ratios are 1:5 soil:water (by weight), or a ‘saturated paste’. We will use a 1:5 ratio (the same suspension we use to measure pH). As a general rule of thumb, soils are considered saline (saline enough to adversely affect plant growth) if EC 1:5 is & 0.3 dS/m. Materials

•  Sieved soil samples (2 per group)
•  Balances for weighing out soil
•  Specimen jars (50 mL, 2x per group)
•  Beaker containing deionised water (500 mL, 1x per group)
•  Beaker for waste rinsing water
•  Syringe or pipette for measuring out 25 mL (eg. 60 mL syringe)
•  pH and EC meters and calibration buffer solutions
•  Wash bottle containing deionised water

Method
1. Weigh 5.0 g (±0.2 g) of each soil into each specimen jar.
2. Add 25 mL of deionised water to each and shake by hand for a few minutes.
3. Calibrate pH meter and EC meter and dip into suspension to measure pH and EC.
4. Record results in Table 3. The EC meters are labeled ?S (micro siemens), but the actual values are

S/cm (see label on calibration solution). Other commonly used units are mS/cm or dS/m (which are equal to each other). Divide ?S/cm by 1000 to obtain mS/cm or dS/m, which is the unit you should use for recording your results.

 

C. Soil organic matter content

Soil organic matter has many important functions: it is the main source of N, P and S for plants (via mineralization by microorganisms), has high cation exchange capacity, provides energy and nutrients for microorganisms and fauna, is an important pool of elements in global cycles, buffers against changes in
pH, alters the mobility and reactivity of pollutants, and influences soil physical properties. You will use the ‘Loss on ignition’ method to measure soil organic matter content. In this method the soil is heated to 400ºC, which burns off the organic matter (converted to gases), but leaves the mineral component remaining. The application of this technique to Australian soils has been discussed by Spain et al. (1982). Materials

•  Sieved soil samples (2 per group)
•  Balance accurate to 0.01 g
•  Aluminium dishes or crucibles (2x)
•  Oven set at 105ºC
•  Muffle furnace set at 400ºC

Method (First week)
1. Record the engraved number on your 2 aluminium dishes in Table 2, then weigh them and record their masses.

2. Place approximately 8 teaspoons in the dishes and place in oven set at 105ºC, with lid off, to remove the water.
Method (Second week)

3. Replace the lids on the dishes, allow them to cool, and weigh them.

4. Place the dishes in a muffle furnace set at 400ºC for 16 hours (with lid off).

Method (Third week)

5. Remove the dishes from the oven, replace the lids, allow them to cool, and weigh them.

6. Calculate organic matter content of the soil as a % of oven-dried soil mass. % organic matter = 100 x (B-C)/(B-A).

D. Cation exchange capacity (CEC)

Cation exchange capacity (CEC) is a measure of the negative charge of soil, which is the same as its ability to retain cations in ‘exchangeable’ form. Exchangeable cations are held electrostatically, are in equilibrium with ions in solution, and are available to plants but are not easily lost by leaching. Many of the elements that are essential for plants, including calcium, potassium and magnesium, behave as cations in soil, so CEC is important for soil fertility. In this practical you will measure the

• CEC of soil using a relatively simple method (Soon 1988). A solution of methylene blue is shaken with the soil and the amount of methylene blue that becomes adsorbed to the soil (which is proportional to CEC) is determined by measuring the amount of methylene blue remaining in solution after contact with the soil. Reagents (prepared for you)
•  50 mM sodium acetate: Dissolve 4.1g sodium acetate in deionized water, making up to 1000 mL (MW of sodium acetate anhydrous = 82.03).
•  5 mM methylene blue (MB) solution (buffered at pH 6.8 in 50mM sodium acetate): Dissolve 1.779g MB in 1000 mL of the 50 mM sodium acetate solution (MW of MB =
• 355.89). This provides more than enough, as each group needs 40 mL, plus another 30 mL required for making the primary standards.
•  Primary standards: 0, 1, 2, 3, 4, and 5 mM MB (10 mL of each). Pipette 0, 2, 4, 6, 8, and 10 mL of MB into test tube and add then 10, 8, 6, 4, 2 and 0 ml deionized water to each tube respectively.

Materials

•  Sieved soils
•  Balance (accurate to 0.01 g), teaspoon
•  50-mL centrifuge tubes (2x per group)
•  20-mL Test tubes (8x per group)
•  Auto pipettes: 1 mL (set to 0.2 mL) and 10-mL (for pipetting 9.8 mL or 20 mL)
•  Tips for 1-mL and 10-mL autopipettes
•  Shaker and centrifuge
•  Spectrophotometer, cuvettes
•  Facial tissues (for cleaning cuvettes)
•  Disposable teat pipettes (for filling cuvettes)

Method
1. weigh 0.2 g (±0.01 g) soil into a 50-mL tube, add 20 mL of 5 mM MB solution using the 10-mL pipette

2. shake on end-over-end shaker for 15 minutes, then centrifuge at 3000 rpm for 5 minutes

3. Using a pipette, remove 0.2 mL of supernatant and place into a 20 mL test tube. Then add 9.8 mL deionized water (final volume of 10 mL).

4. Prepare samples for a ‘standard curve’ by pipetting 0.2 mL of each of the 6 primary standards into a 20 mL test tube and adding 9.8 mL deionized water to each (final volume of 10 mL).

5. Pour the prepared standard samples into spectrophotometer cuvettes, filling them to about 5 mm below the top. Take care never to touch the clear sides of the cuvettes. Before measuring absorbance, wide the clear sides with a tissue to ensure they’re spotless.

6. Using the spectrophotometer, measure and record (in Table 4) the absorbance of the diluted standards (at 550 nm). Draw a graph of absorbance against known MB
concentrations, on the next page or in Excel. On the graph, label the concentration axis using the primary standard concentrations, because the sample will be diluted by the same amount. Draw a line of best fit, which is called the ‘standard curve’ or ‘calibration curve’. The line of best fit may be drawn by eye and hand or using regression (‘trend line’) in Excel. The graph must have an explanatory caption

7. Using the spectrophotometer, measure and record the absorbance of the diluted supernatant and determine the concentration of the MB (mM MB) in the supernatant
solution by reading the value off the standard curve. Record results in Table 5.

8. Calculate CEC, using the formula CEC = [(5 – c)/10] * [extractant volume/mass soil], where CEC is in cmol(+)/kg, c is the concentration of MB in the supernatant solution (mM), extractant volume is 20 (mL), and mass of soil is 0.2 (g).

 

8
6. Using the spectrophotometer, measure and record (in Table 4) the absorbance of the
diluted standards (at 550 nm). Draw a graph of absorbance against known MB
concentrations, on the next page or in Excel. On the graph, label the concentration axis
using the primary standard concentrations, because the sample will be diluted by the
same amount. Draw a line of best fit, which is called the ‘standard curve’ or ‘calibration
curve’. The line of best fit may be drawn by eye and hand or using regression (‘trend
line’) in Excel. The graph must have an explanatory caption
7. Using the spectrophotometer, measure and record the absorbance of the diluted
supernatant and determine the concentration of the MB (mM MB) in the supernatant
solution by reading the value off the standard curve. Record results in Table 5.
8. Calculate CEC, using the formula CEC = [(5 – c)/10] * [extractant volume/mass soil],
where CEC is in cmol(+)/kg, c is the concentration of MB in the supernatant solution
(mM), extractant volume is 20 (mL), and mass of soil is 0.2 (g).
Table 4. Data for standard curve.
MB conc. (mM) 0 1 2 3 4 5
Absorbance at 550 nm

Table 5. Cation exchange capacity results
Soil:
Absorbance at 550 nm
MB conc. (mM)
CEC (cmol(+)/kg)

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E. Acid sulfate soils

Acid sulfate soils are very important in coastal areas and also in waterlogged saline areas inland. They need to be identified and managed carefully (Dear et al. 2014). We will demonstrate the field test for acid sulfate soils, in which the pH of a soil sample is measured before (pH F ) and after oxidation using hydrogen peroxide (pH FOX ), according to the method of Ahern et al. (2004). The hydrogen peroxide oxidation is a rapid simulation of the oxidation that occurs when potential acid sulfate soils (PASS) are exposed to air and become actual acid sulfate soils (AASS). PASS may have neutral pH, whereas AASS have pH<4. Laboratory analysis is required
for confirmation and for estimation of the amount of potential acidity and internal neutralizing capacity (calcium carbonate).

PASS contains iron sulfides and high concentrations of potentially toxic elements in sulphide form (e.g. Fe, Al, As, Co, Cu, Ni, and Pb). Oxidation of the PASS releases sulphuric acid and high concentrations of the potentially toxic elements, which are soluble in acid conditions. Environmental impacts of oxidised ASS occur in the disturbed soils and downstream. In addition to toxic concentrations of metals and metalloids, when the water’s pH is raised (eg. due to mixing with seawater), oxidation of the Fe 2+ to Fe 3+ consumes all the oxygen in the water, leading to asphyxiation of animals.

This activity is not included in your report. Materials (for lecturer/tutor)

•  PASS sample from mangrove area (~200 g), sampled less than a week before the practical, using a 1-m Gouge auger and hammer and kept in a sealed plastic bag in the fridge
•  pH probe and 4x plastic or glass jars or beakers (eg. 100 mL beaker)
•  Plastic spoon
•  Deionised water
•  Hydrogen peroxide (H 2 O 2 , 30 % at pH 4.5-5.5), >200 mL
•  Safety goggles and gloves for handling H 2 O 2

Method (for lecturer/tutor)
 Put 6-10 g soil in each of 4 tubes. Add water to two of them and H 2 O 2 to the other two. Be careful, as fumes, heat and bubbles may be generated. Allow to react and then measure pH of all.

 

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