Sulfate system (digestion-distillation apparatus and washing solution) with H2S




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a. Weigh 10 g of soil into a centrifuge tube which can be stoppered with glass or plastic.

b. Add 20 ml of the 1N KCl extracting solution and shake for one hour. The shaking action should be sufficiently vigorous to keep the soil suspended in solution.

c. Filter through 42 Whatman filter paper (or equivalent) into sulfur-free filter tubes.


a. Lubricate all spherical joints with a minimal amount of the treated lubricant. Place 10 ml of the pyrogallol-sodium phosphate solution in the gas washing column of the digestion-distillation apparatus. The washing column should be filled with N2 gas before the washing solution is added to reduce the possibility of the oxidation of the pyrogallol. Saturate the system (digestion-distillation apparatus and washing solution) with H2S by using one of the standard solutions and following the procedure described below, with the following exception: H2S-N2 is vented into the atmosphere when the system is being saturated. Note:This step is necessary only after the system has been washed and/or when new washing solution has been introduced.

b. Place 35 ml of the dilute zinc acetate-sodium acetate solution in a 50-ml volumetric flask. Connect the glass delivery tube to the side arm of the gas washing column and clamp the receiving flask in place. The delivery tube should be near the bottom of the receiving flask.

c. Place 4 ml of the reducing reagent (hydroiodic, hypophosphorus, formic acid mixture) into a boiling flask. Transfer

2 ml of a standard solution or of a soil extract to the boiling flask containing the reducing reagent.

d. Attach the boiling flask to the condenser and connect the tube from the nitrogen supply. Adjust the nitrogen flow rate so that about two bubbles per second issue from the receiving flask. The rate is not extremely critical. Make certain cool water is flowing through the condenser. After about five minutes of nitrogen flow, light the microburner; with nitrogen continuing to flow, maintain the contents of the boiling flask at a low boil for one hour.

e. Remove the receiving flask, leaving the glass delivery tube in the zinc acetate solution. Using a rapid delivery pipette, add 5 mls of the p-amino dimethylaniline solution. Quickly stopper the volumetric flask and mix thoroughly, then add one ml of the ferric ammonium sulfate solution; mix again, remove the glass delivery tubes, then make to volume with glass-distilled water and mix thoroughly. The methylene blue color is stable and may be read after 10 minutes but within 24 hours after development at a wavelength of 670 mu. If the color is too intense to read, make the appropriate dilution with a solution containing 10 ml of the p-amenodimethyl-aniline solution and 2 mls of the ferric ammonium solution in 100 ml of glass-distilled water. Dilution with water alone causes changes in color intensity which render the readings useless.















Soil pH


This is the activity of the negative log of the hydrogen ion in a suspension of 1:5 soil : water. It is the de facto standard pH measurement for most soil test interpretations. The pH measured in 1:5 soil : water suspension is sensitive to seasonal variations in the pH of soil solutions .





 1. Prepare a 1:5 soil:water suspension. Weigh 10 g air-dry soil  into a bottle and add 50 mL deionised water

2. Calibrate the pH meter according to manufacturer’s instructions using the buffer at pH 6.86 and either the 4.0 or 9.18 buffer depending on the expected values for the soils. Stir these solutions with a mechanical stirrer during measurement. Thoroughly wash the electrode between measurements with deionised water.

3. Immerse the electrode into the soil suspension. Record the pH value obtained when the equilibrium is reached while stirring with a mechanical stirrer.












                  Soil EC

Soil pH can be measured at either 1:1 or  2:1 (water:soil) ratio. Soil EC can be measured in the field, and lab testing is inherently inaccurate. It is a best practice to measure EC using a probe consistently, at the same time as pH. A 2:1 water : soil ratio is ideal for clayey or heavily organic soils, and if lime requirement is not also being calculated. A 1:1 water : soil ratio is ideal if lime requirement is also being calculated, and is reasonable for soils with a lower clay content. 


·         Measure 10 grams of soil into the beaker or Falcon tube.

·         Add 20 ml of DI water.

·         Stir frequently or place on a shaker for 15 minutes.

·         Calibrate EC meter


                        Soil Eh

Reduction potential (Eh){displaystyle E_{h}} is a measure of the tendency of a chemical species  to acquire electrons and thereby be reduced. Reduction potential is measured in volts (V), or millivolts (mV). Each species has its own intrinsic reduction potential; the more positive the potential, the greater the species’ affinity for electrons and tendency to be reduced


·         Measure 1 grams of soil into the beaker or Falcon tube.

·         Add 10 ml of DI water.

·         Soil Eh can be measured by an automatic ORP meter (FJA-5



                              Soil Resistivity


The soil acts as a reservoir for water and soluble salts, the content of this reservoir being easily assessed by measuring the electrical resistance of the soil. Clay soils have a greater ability to hold water and retain soluble salts so their resistivity is low, usually less than 3000 ohm cm. Sandy soils drain more readily. The soluble salts get washed away and so their resistivity is greater, usually in excess of 5000 ohm cm. Chalky soils often have resistivity between the two.

 The low resistivity clay soils are therefore the most corrosive. There are exceptions to this rule of course, for example, a sandy soil in a low lying, marshy area will be wet and show a low resistivity. It may therefore be more corrosive than many clay soils.


Wenner Method:

The Wenner four-pin method, as shown in figure above, is the most commonly used technique for soil resistivity measurements.


Schlumberger Method :


In the Schlumberger method, the distance between the voltages probe is a and the distances from voltages probe and currents probe are c .Using the Schlumberger method, if b is small compared to a and c, and c>2a.

ho _{E}={pi }{frac {ccdot (c+a)}{a}}R_{S},}