Friday, August 19, 2011

Richards pressure plate experiment: developing soil water retention curves in 10 easy steps!

The C-111 project requires the development of soil water retention curves for the dominant soil types at the study site (i.e., Krome, Chekika and Marl). The low pressure (i.e., suction less than 1 Bar) portion of the soil water retention curve is developed using the Tempe cell apparatus and the high pressure (i.e., suction greater than 1 Bar) portion of the graph is determined using the Richards pressure plate. What follows is a brief description of the Richards pressure plate process.



Richards pressure plate: large & in charge


Note: precede operation by reading the user manual's guide on assembling and safely using the Richards plate because it involves very high pressure.


Operating the Richards pressure plate involves the following steps:

1. Place a layer of water on the Richards pressure plate cell and let it stand overnight to ensure the cell is fully saturated.



2. Prepare the soil by sieving with a 2 mm round-hole sieve to make 25 gm replicate samples of the soil under study. The pressure plate cell used in this experiment can fit up to 12 samples (or retaining rings). Note: before placing samples into Richards’s pressure plate connect the outlet tube.




3. Use a spatula to place the samples into the retaining rings to avoid particle size segregation. Pouring out the sample creates a non-representative sample.



4. Level the soil in the retaining rings and raise the depth of water to completely saturate the soil samples from the bottom and let stand overnight.


5. The next day, when samples are fully saturated, remove excess water from the plate using a syringe and plug all unused outlet ports. Then carefully fasten the black o-ring.


6. Seal the chamber by fastening the bolts. Be sure the bolt heads are properly set in the groves.





7. Place the outlet tube into a water holding container (e.g., a graduated cylinder).



8. Gradually increase pressure to the required setting and take precautions not to exceed the maximum pressure of the pressure plate cell inside the chamber.


9. When equilibrium is attained (i.e., no water flows from the outlet tube) turn off the pressure supply to the system. DO NOT open the chamber until all pressure has been released. Immediately record sample weights.






10. Transfer the samples to an oven and dry the samples at 105 C for 24 hours. Record the weight after oven drying.





Success! Now the math! As a final step, calculate the soil water content using the pressure at which the experiment was initiated.


Isaya Kisekka


For more on the C-111 project, see:



2 comments:

  1. Isaya - great job with the Richard's plate; Mike very interesting post. This will be useful - as well as the doc Isaya put together - for many future projects!

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  2. your guide will be useful for your future readers and researchers. keep it up!


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