Frequently asked questions Laboratory equipment
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How do I take samples for the different laboratory analysis equipment............................................ 1
What is pF ?................................................................................................................................. 1
Why would I want to determine the pF curve in a laboratory. ......................................................... 1
How many samples must be taken to analyze soil in the lab............................................................ 2
What kind of equipment do I use to take the samples ? ................................................................. 2
When do I use the sand and / or kaolin box and when the pF plates................................................ 2
What is the measuring ranges and accuracy of extractors and gauges ............................................. 2
How many samples can be placed on one plate ?........................................................................... 3
Is it possible to work with undisturbed samples on a plate ? ........................................................... 3
Which sieves do I need to comply with what standards ?................................................................ 3
There are open and closed permeameters. What is the difference? ................................................. 3
What is the difference between the hydrometer and the pipette method. ....................................... 3
Q: How do I take samples for the different laboratory analysis equipment
A: Depends on what type of research you need to do.
a) Volumetric soil moisture content (any method) gr. water/ml soil undisturbed sample.
b) Determining porosity with the pycnometer undisturbed
c) Permeability for water or air undisturbed
d) pF determination by suction (sand and kaolin) undisturbed
e) Weight-weight moisture content gr. water/gr. soil disturbed
f) Sieving disturbed
g) Lutum content determination (hydrometer or pipette apparatus disturbed
h) Soil structure stability analysis (by wet sieving method) disturbed
Q: What is pF ?
A: The logarithmic expression of the force with which soil holds water. The pF may vary between 0
and higher than 4.2. Where 0 is no suction (soil saturated with water) and 4.2 is the 10log of 16.000
cm water suction. This is the so called wilting point or the point where most plants start having
severe growing problems since they cannot get water out of the pores anymore. For some plant this
wilting point may be much lower than 4.2. For instance a (sugar) beet plant will have considerable
growth problems when pF=2.0.
A sand soil may have a lot smaller “space” (porosity) to hold water than clay (e.g. 38%) but the pores
itself are so large that almost all this 38 vol% of water is readily available for the plants. Only a few
percent of the initial moisture will stay behind in micro pores. A clay soil however with a volumetric
soil moisture content that may go well above 60% only has small pores. This means that adhesion
forces can hold the water tightly once the roots have used the water from some macro pores. Clay
soils may appear bone dry when still holding 25vol% of water ! This means that measuring suction
(expressed in Pascal or pF) gives direct information on plant growth stress whereas volumetric soil
moisture content (obtained by classical gravimetric or modern FDR or TDR techniques) first must be
‘translated’ by means of a pF curve of that very soil type. Only then a good irrigation advice can be
given.
Q: Why would I want to determine the pF curve in a laboratory.
A: One reason may be that you have a volumetric soil moisture measuring device of which you want
to translate the values to irrigation advice.
Another reason would be to determine the very agronomic qualities (price) of a very plot of soil.
Imagine two farmers that want (or must) swap or sell land to each other or the state for a new
highway. A soil with a pF curve that facilitates plant growth and has good drainage and tilling
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properties (which is simply said a soil between a sand and a clay soil like loess), is more worth than a
soil where plants suffer easily from drought or that bears difficulties when draining or irrigating. Then
a pF curve determination is a valuable and objective way to determine that price together of course
with (other) soil profile characteristics, soil chemistry, slope, permeability, susceptibility for erosion
and so on.
Q: How many samples must be taken to analyze soil in the lab.
A: A tricky question on which we will give a tactical answer: Depends on the variability of your soil
and the accuracy with which you want to do the final measurements. To say it short: One
measurement is NO measurement. Three samples measured can already be of value but is still tricky
and 16 on a hectare should normally be sufficient. Please consult sampling standards for this.
Q: What kind of equipment do I use to take the samples ?
A: To take undisturbed samples short so called Kopecki rings must be preferred. The reason is simple:
The shorter the ring the smaller the chance that the sample will be compressed when sliding in the
coring tube which of course alters the physical characteristics. Generally you will use set 07.53 SC
equipment for undisturbed soil sampling. Disturbed soil sampling can be done more or less with any
auger, gouge or coring tube.
Q: When do I use the sand and / or kaolin box and when the pF plates.
A: To keep it simple: To get the complete pF curve you will need the sand box (for very low pF values
in fact up till 1.0 meter of water suction = pF 2.0), the kaolin box for somewhat higher values (up till
5 meters of water suction or pF 2.7). Above that, nature does not allow suction to be used as the
force to drive out the water from the pores (the rings are weighed after having increased the suction
to see the loss in water). With the pF plates pressure must be used. This only can be done with
disturbed soil which you press in very low rings. These rings are placed on a porous water saturated
ceramic plate. With overpressure (going up to 16 bars) the system will try to press the water out of
the sample. The ceramic plate is fitted with a drain tube and the drained water is collected and
measured. All the equipment together will result in a pF curve ranging from saturation to wilting
point. Keeping out the pressure plates will result in a pF curve of which the drier section is missing.
Keeping out the sand and kaolin boxes will result in pF curves of which the wetter part is missing
which is, specially for sandy soils essential.
Q: What is the measuring ranges and accuracy of extractors and gauges
A: 1500F1
15 BAR CER. PLATE EXTRACTOR ONLY, 4 cell capacity
Measuring ranges:
5 PSI
0.344 BAR
pF 2.536
1600
217.5 PSI
14.99 BAR
pF 4.175
MAX PRESS (over range limit)
MAX PRESS (over range limit)
MAX PRESS (over range limit)
250 PSI
17.23 BAR
pF 4.236
5 BAR PRESSURE PLATE EXTRACTOR ONLY, 4 cell capacity
Measuring ranges:
1 PSI
0.06 BAR
72.5 PSI
4.99 BAR
MAX PRESS (over range limit)
MAX PRESS (over range limit)
2
80 PSI
5.51 BAR
pF 1.778
pF 3.698
MAX PRESS (over range limit)
pF 3.741
Accuracy of the Gauges:
0780P0300
Test Gauge 0 – 300 PSI / 0 – 20 BAR
Subdivision 2.0 PSI / 0.1 BAR
0.25% FULL SCALE ACCURACY
0780P0100
Test Gauge 0 – 100 PSI / 0 – 6.5 BAR
subdivision 0.5 PSI / 0.02 BAR
0.25% FULL SCALE ACCURACY
Q: How many samples can be placed on one plate ?
A: One can place easily 10 sampling rings on the ceramic plates, however 12 rings is a maximum.
Q: Is it possible to work with undisturbed samples on a plate ?
A: It is possible to use undisturbed samples on the ceramic plates, but one need to use silica flower
and filter cloth on the plate to ensure full contact of the sample with the plate.
Q: Please give me the reference numbers of the different ceramic plates:
A:
0675B0.5M2
1/2 BAR HIGH FLOW PRESSURE PLATE CELL
0675B01M1
1 BAR PRESSURE PLATE CELL
0675B01M3
1 BAR HIGH FLOW PRESSURE PLATE CELL
0675B03M1
3 BAR PRESSURE PLATE CELL
0675B05M1
5 BAR PRESSURE PLATE CELL
0675B15M1
15 BAR PRESSURE PLATE CELL
Q: Which sieves do I need to comply with what standards ?
A: In fact you should consult the standard for that. With sieves 63, 90, 125, 180, 250, 355, 500, 710,
1000, 1400 (1400 not in standard set) and 2000 microns the set complies to ISO 565, B(ritish)
Standard 410, DIN 4188, NEN 2560 and A(merican)STM-11 (soil science standards)
If the number of sieves is reduced to: 63, 105, 150, 210, 300, 420 en 2000 microns then it will still
comply to ISO 5104 (which is a common geotechnical standard).
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a certain level after a certain sinking time. That sample is dried and the weight increase of the
container is determined resulting in the quantity of fines in the soil sample. The pipette method asks
for more skills to operate but can be more accurate and versatile. It is generally preferred and
standardized in Europe. In the US the hydrometer method is more popular. Check local standards (or
habits) to make your selection.
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