To CLASS PAGE:
Soil Fractionation and Characterization
|I. Percent Moisture|
|a.) Determine moisture content on a subsample (5.0g) of the air dry <2mm soil sample.|
|b.) Place sample in the oven at 105 oC and leave overnight.|
|c.) Cool the sample to room temperature in a dessicator (about 20 minutes) and weigh container and sample.|
|d.) All handling of the sample should be by tongs or paper towels.|
|e.) All future calculations will be on an oven-dried weight basis.|
|II. Soil Fractionation|
|To characterize the minerology in soil samples, sixe fractionations will be performed. Some minerals tend to occure in certain size fractions. Samples of most soils are best dispersed by raising the pH to about 10 using a monovalent cation (Na+)|
|1.) Weigh 20.0 g of soil into one 250 mL centrifuge bottles.|
200 mL of Na2CO3 solution to each bottle.
Shake for 1 h to achieve dispersion.
Pass contents through a 50 μm sieve and cature the < 50 μm
fraction (the silt + clay) in a beaker
placed under a funnel attatched to a ring stand.
Wash the sand thoroughly with Na2CO3
solution from a wash bottle to push particles through
the sieve (Note: Do not use any force stronger than a stream of solution from a wash botle
to push particles through the sieve.)
Transfer the sand fraction from the sieve into a bottle to avoid
prolonged contact with the sieve.
sieve and place on drying rack.
Place sand fraction in dryer (~40o) and then obtain an
weight (105o ) of the sand
Pour the silt + clay fractions back into centrifuge bottles and
200 mL of Na2CO3 solution
and mix well.
Centrifuge at 750 rpm for 3.5 min.
Decant suspended clay into a large, labeled beaker. The
will not be clear and care
should be taken not to pour silt into the clay fraction.
After pouring clay into the beaker, add more pH 10 Na2CO3
solution into the centrifuge bottle
containing the silt and redisperse the sample.
Repeat centrifuge and washing procedure unti almost a clear supernatant
silt from the centrifuge bottle into a labeled container andplace in
dryer to remove free
water. Obtain an oven-dry weight of the silt fraction.
||15.) At this
point, there will be a large volume of clay contained in the
beaker. To reduce the
volume of clay, let your samples sit overnight (or longer).
|16.) Place thge clay fraction in the dryer to remove excess
water. Obtain an over-dry weight of
the clay fraction.
|17.) Calculate percent
recovery of your fractionation and % sand, silt, and clay.