Campbell Scientific Sentek TriSCAN® Manual v 1.2a - Manual (Page 42)

TriSCAN Manual Version 1.2a
Copyright © 1991 – 2004 Sentek Pty Ltd All rights reserved Page 35
Step 6
Analyse soil EC for each sample using one of the methods below.
Laboratory Methods
Saturated Paste Extract
The standard procedure for measuring soil EC consists of mixing a soil sample with sufficient water to
produce a saturated paste, and then extracting the solution by suction for measurement. Measuring the
electrical conductivity (EC
e
) of a saturation extract has an advantage in that saturation percentage is directly
related to field available moisture range. The soluble salt concentration in the saturation extract tends to be
about one half of the concentration of the soil solution at the upper end of the field available moisture range
and about one quarter the concentration that the soil solution will have at the dry end of the available
moisture range.
This method relies heavily on the ability of the operator to add water to the soil until it is just glistening. With
practice, this technique is surprisingly reproducible, despite this subjective consideration on the part of the
technician.
1:5 Determination
A more practical method requiring less specialized equipment consists of measuring the EC
1:5
of a water
extract of soil. This result can then be converted to the more widely recognized EC
e
value by a simple
multiplication factor. Relationships exist between EC
1:5
and EC
e
for different ranges of soil textures (Shaw et
al, 1987).
Steps to follow in determining a 1:5 EC are:
1. Air dry an aliquot of each soil sample for a few days to a week (spread out on a tray).
2. Gently crush the dry clods and pass the sample through a 2 mm sieve.
3. Mix the sieved soil thoroughly
4. Weigh three 20g ( ±0.1g) sub samples of dry soil into separate 100ml extraction containers.
5. Add 100ml deionized water (EC
(H2O)
< 30
µ
S/cm) to each sub sample
6. Extract samples in an end-over-end rotational mixer for 1 hour
7. Allow samples to stand for 1 hour
8. Carefully measure the EC of the liquid above the settled soil using a calibrated EC meter
9. Check the EC values again after 1 hour to ensure no significant change
10. Calculate the average EC value of the assay triplicates. There should not be a variation greater than
5%
11. Calculate the average EC value of the depth triplicates. It is quite possible that the variation here
may be greater than 5% due to the variability of the soil and infiltration pattern
12. Make a correction for EC
e
if required using the equations:
ECe = EC
1:5
x WCF
WCF = -2.21 x %CLAY
0.5
+ 23.78
Ref: Shaw, 1987