LaMotte MULTI-RANGE CONDUCTIVITY METER DA-1 1929 - Manual (Page 6)

DETERMINATION OF SOIL CONDUCTIVITY &
TOTAL DISSOLVED SALTS
An approximation of the Total Dissolved Salts in soil is obtained by
making a soil extraction with distilled water and measuring the
conductivity of the soil extract. For purposes of comparison, all
conductivity values are recorded at 25°C.
NOTE:
For determination of Soluble Salts in greenhouse soils, see page 7.
PRO CE DURE
1.
Fill a 50 mL beaker with the soil to be tested. Tap it lightly to
eliminate any trapped air. Strike the excess soil off the surface.
2. Empty the contents of the beaker into a 250 mL widemouth flask and
add 100 mL of distilled water.
3. Stopper and shake vigorously. Allow to stand for thirty minutes.
During the thirty minutes waiting period the sample should be
shaken vigorously three or four times.
4. Filter the contents of the flask and collect filtrate in suitable
conductivity chamber.
5. Take conductivity reading according to method given for General
Operating Procedure.
6. To convert to Total Dissolved Salts use the following formula:
micromhos/cm @ 25°C x 0.7 = ppm Total Dissolved Salts
If the Soluble Salts as determined above are greater than 1000 ppm, the
chlorides and sulfates should be determined in order to learn whether the
Soluble Salts are chlorides or sulfates.
Usually in calcareous soils, the sulfates represent gypsum and cause little
injury to plants, while high concentrations of chlorides might result in
damage to the plants. Greenhouse and sensitive plants may be damaged if
the Souble Salts are over 500 ppm of chlorides. Most other plants will grow
well in soils containing less than 1000 ppm chlorides.
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DETERMINATION OF SOLUBLE SALTS
FOR GREENHOUSE SOILS
The meter is specially designed to read conductivity in the limited volume
of soil extract obtained through the soil extraction procedure employed in
the LaMotte Model GS-01 Greenhouse Outfit. Use the following formula
to convert the conductivity reading to parts per million Soluble Salts:
Conductivity Reading (micromhos/cm) x 0.7 = ppm Soluble Salts
PRO CE DURE
1.
With control knob in the “OFF” position, meter pointer must
balance at zero. To adjust the zero position, use meter-zero adjusting
screw (“X” slot screw on front of meter face). The mirror-faced scale
insures accurate readings by eliminating errors due to parallax
problems which would otherwise occur when the indicating needle
and the scale are not properly aligned by the reader’s eye.
2. Switch the control knob to the “BAT CK” position. Meter should
indicate greater than 80 micromhos/cm, if not then replace the
battery. (See instructions for battery replacement.)
3. Set the “TEMP” knob at 25°C, the Range Selector at “X100,” and
switch the control knob to the “CAL” position. The meter should
indicate exactly 100 micromhos/cm, if not then carefully adjust by
using the “STD” knob.
4. Measure the temperature of the sample solution, then set the
“TEMP” knob at the mark indicating the temperature. (Note the
markings are in centigrade.) Temperature has a pronounced effect
upon conductivity, and must be monitored accurately.
5. Immerse the probe into soil extract (in special reading chamber) to at
least cover the slots at bottom of the probe. Make sure all bubbles are
removed from the probe.
6. Switch the control knob to the “ON” position and the range selector
to either “X1,” “X10,” “X100,” or “X1000.” Read the conductivity
directly from the meter face. For the best accuracy, select the range
which gives readings as close to mid-range of the scale as possible.
Multiply the reading from the meter face by the appropriate factor.
The probe should not be in contact with any surface other than the
solution under test.
7. After completing the conductivity measurement, switch control knob
to the “OFF” position. Rinse probe thoroughly with water, preferably
distilled water, and drain.
8. Do not discard soil extract if it is to be used for other chemical tests.
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