Estimation of the soil water content Introduction
Since the concentrations of chemical components (nutrients, pollutants etc) are invariable expressed as concentrations based on dry weight, we will have to determine the moisture content of our soil samples. The moisture content of a soil sample is estimated by drying at 105°C for 24 hours.
The loss of weight is a measure for the absorbed water content and is calculated as a percentage of the dry mass.
Sample
Air-dry well homogenized sample < 2 mm (fine earth fraction).
Procedure
Weigh ca. 5 gram of soil accurately (on a four-decimal balance) in a numbered
aluminum evaporating dish. Place in an oven at 105°C for 36 to 48 h. Cover threequarters of the dish with a lid. After drying, remove the dish, place in a desiccator for
cooling to room temperature and weigh (use the same balance). The weight lost is the amount of absorbed water in the soil sample. Express the loss of water as a percentage of the dry soil.
Making extraction Introduction
Extraction with water or an aqueous salt solution is generally used to measure the extractable nutrients in soil as these are considered to best represent the nutrients available for uptake by plants.
Sample
Air-dry sample, < 2 mm and well homogenized. Determine the moisture content separately according the procedure for the determination of the soil water content.
Procedure
Weigh 5.0 grams of the soil accurately (on a two-decimal balance) and transfer into a 200 mL polyethylene bottle. Add 50.0 mL demi water (also on a twodecimal
balance). Assume 1 mL demi water is 1 gram. Shake during two hours at room temperature.
Centrifuge the suspensions for 15 minutes at 2000 rpm. Take a few mls of the clear supernatant for the determination of the pH. Filter the remaining part over a 0.2 �m membrane filter. Collect the filtrate in a dry polyethylene 100 mL-bottle.
Measure in the filtrate the ammonium, nitrate, phosphorous, potassium, copper, zinc and TOC.
Analysis of nitrate, ammonium and phosphate content Nitrate, 4.4 - 110.7 mg NO3- /L
Method
In sulphuric and phosphoric acid solution nitrate ions react with 2,6-dimethylphenol (DMP) to form 4-nitro-2,6-dimethylphenol that is determined photometrically Procedure
Procedure
Reagent NO3-1 4.0 mL Pipette into a dry test tube
Pretreated sample (5 - 25 °C) 0.50 mL Add with pipette, do not mix!
Reagent NO3-2 0.50 mL Add with pipette (Wear eye
protection! The mixture becomes hot!) and mix, holding only the upper
part of the tube!
Leave the hot reaction solution to stand for 10 min (reaction time).
Do not cool with cold water!
Ammonium, 6 - 193 mg NH4+/L 1.
Method
Ammonium nitrogen (NH4-N) occurs partly in the form of ammonium ions and partly as ammonia. A pH-dependent equilibrium exists between the two forms.
In strongly alkaline solution ammonium nitrogen is present almost entirely as ammonia, which reacts with hypochlorite ions to form monochloramine. This in turn reacts with a substituted phenol to form a blue indophenol derivative that is
determined photometrically.
Procedure
Reagent NH4-1 (20 - 30 °C) 5.0 mL Pipette into a test tube
Pretreated sample (20 – 30°C) 0.10 mL Add with pipette and mix
Leave to stand for 15 min (reaction time), then fill the sample into a 10-mm cell, and measure in the photometer
Phosphate, 2 - 229 mg P2O5 / L Method
In sulphuric solution orthophosphate ions react with molybdate ions to form molybdophosphoric acid. Ascorbic acid reduces this to phosphomolybdenum blue (PMB) that is determined photometrically.
Procedure
Distilled water (10 - 35 °C) 8.0 mL Pipette into a test tube Pretreated sample (10 –– 35 °C) 0.50 mL Add with pipette and mix.
Reagent PO4-1 0.50 mL Add with pipette and mix.
Reagent PO4-2 1 dose Add and shake vigorously until the reagent
is completely dissolved.
Leave to stand for 5 min (reaction time), then fill the sample into a 10-mm cell, and
Analysis of Potassium content in CaCl2 extract Introduction
Potassium should be measured in the present of an excess of sodium ions. This prevents the ionization of potassium in the flame. CaCl2 is added to the standard solutions to match the matrix of the sample solutions.
Reagents
1) Potassium standard solution, c = 1000 mg K/L
2) Diluted potassium standard solution, 100 mg K/L: pipette 10.00 mL of the standard solution (sol. 1) in a 100 mL volumetric flask. Add 18 M water to the mark and mix.
3) Sodium chloride solution, 40 g Na/L 0.2 M HNO3. Dissolve 100 grams NaCl in 800 mL demi-water (ELGA). Add 14 mL HNO3 (65%) and dilute with 18 M water to 1000 mL in a volumetric flask and mix.
4) Sodium chloride solution, 4 g Na/L in 0.02 M HNO3. Dissolve 10 grams NaCl in 800 mL 18 M water (ELGA). Add 20 mL HNO3 (1 mol/L) and dilute with ELGA-water to 1000 mL in a volumetric flask.
5) Calcium chloride solution (1 M). Dissolve 147 grams CaCl2.2H2O in 200 mL 18 M water (ELGA) and dilute to 1000 mL in a volumetric flask.
Procedure
Pipette 5.00 mL or less* (v ml) of the extract into a dry tube of 15 mL. Add, if necessary, with a pipette 0.01 M calcium chloride solution to a volume of 5.00 mL and with a 5.0 mL pipette solution 4. Close the tube and mix. If a precipitate forms, centrifuge for 10 minutes at 2000 rpm.
* The expected highest concentration should be diluted at least 10 times. Pipette 1.0 ml in a dry tube and add 4.0 ml demiwater . Continue as described before.
To calibrate the AES (Atomic Emission Spectrophotometer) prepare 4 solutions in 100 mL volumetric flasks: Pipette 0.– 5.0 .–10.0 .– 20 mL from solution 2 (diluted standard solution); add 5.0 mL NaCl solution (solution 3) and 5.0 mL solution 5. Dilute to the mark and mix. The concentrations are 0.0 - 5.0 - 10.0 .– 20.0 mg K/L. Measure the emission of the standard and sample solutions with the AES at 766.5 nm Use an air-acetylene flame and the 5 cm burner head.
Calculation
Express the concentration of potassium as mmol K/L and finally convert to mmol K per kg soil.