1.5: Sample Preparation and Gravimetric Water Content

Purpose: 

The purpose of this exercise is to learn proper laboratory techniques to prepare a soil sample for laboratory analysis and measure gravimetric water content. 

Learning Outcomes: 

Upon completion of this exercise you should be able to:

• prepare a sample for soil moisture analysis

• calculate soil moisture content using the gravimetric method

• measure infiltration rates of different soils in the lab

Background: 

From Brady and Weil, The Nature and Properties of Soils, 13^th Ed. Water is vital to the ecological functioning of soils. The presence of water in soils is essential to support plants and soil organisms. Soil moisture regime, a result of climate, is a major determinant of ecosystem productivity. Also, soil water contains many dissolved constituents that can greatly impact local and regional water resources.

Water causes soil particles to swell and shrink, to adhere to each other, and to form structural aggregates. Water is involved in most chemical reactions that release or bind nutrients, create acids, and dissolve minerals. In addition, soil moisture affects micronutrient availability, soil temperature, and engineering properties of soils such as compaction, consistency limits and stability. Soil moisture also affects the performance of septic systems, drain fields and contaminant leach fields.

It is important to keep in mind how soil samples are collected and stored before laboratory analysis. You can imagine a soil sample collected and stored in a cardboard box in your car will have a much different soil moisture content compared to a sample collected in an air tight container and stored in a refrigerator. Core tubes, sealable jars, and Ziploc baggies are usually the preferred vessels for soil collection because they minimize exposure to elements such as heat and prevent moisture from entering or leaving the sample. Samples should be kept on ice in a cooler or in a refrigerator until lab work is ready to begin in order to yield the most accurate results possible. Try to minimize time between sample collection and analysis for best results.

Equipment Required

• Convection oven
• Containers w/lid
• Sharpie
• Insulated oven mitt
• Tweezers
• Soil spatula
• Top load balance
• Soil samples
• Calculator
• Specimen cups and lids
• Mortar and pestle
• #10 sieve and sieve pan
• Masking tape

Exercise:  

1. Heat VWR convection oven to 80^oC (~175^oF).
2. Thoroughly wash three containers and lids with soap and hot water.
3. With masking tape and a Sharpie label each container with your initials and sample # (DO NOT WRITE DIRECTLY ON THE CONTAINER!) and place containers in oven upside down for 3-4 hours to allow to thoroughly dry.
4. Remove containers from oven to allow to cool and turn on balance, tare balance (zero the balance), and allow balance to stabilize at 0.0 g (may require repeated taring)
5. Place container on tared balance, allow reading to stabilize, and record container weight (including unit of measurement).
6. Prepare samples by breaking up peds into smallest aggregates possible and picking out as many roots and rootlets as possible using tweezers.
7. Add approximately 300 g of prepared soil sample into container.
8. Place container with soil sample on same tared balance, allow reading to stabilize, and record container + wet soil weight.
9. Place container with soil sample into oven at 80oC and allow sample to dry for at least 48 hours, longer is fine.
10. After 48 or more hours, remove container with soil sample from oven and place on tared balance. Record container + dry soil weight.
11. Compute the following calculations:

    	SAMPLE 1	SAMPLE 2	SAMPLE 3
    A. Container Weight
    B. Container Weight + wet soil sample
    C. Container weight + dry soil sample
    D. Wet sample weight (B – A)
    E. Dry soil weight (C – A)
    F. Moisture weight (D – E)
    G. Soil Moisture (%) = 100 x (F)/(E)

     

    Example:	SAMPLE
    A. Container weight	115.27 g
    B. Container weight + wet soil sample	177.57 g
    C. Container weight + dry soil sample	168.10 g
    D. Wet sample weight (B – A)	62.30 g
    E. Dry soil weight (C – A)	52.83 g
    F. Moisture weight (D – E)	9.47 g
    G. Soil Moisture (%) = 100 x (F)/(E)	17.93%

12. After you have weighed and recorded container + dry soil sample, dump contents of sample 1 container into a #10 sieve with sieve pan underneath and gently shake. Remove any rocks or other non-soil debris that does not pass through the sieve and put into a mud bucket.
13. Dump the soil that does not pass through the sieve into a mortar and grind with pestle.
14. Dump contents of mortar into #10 sieve with sieve pan underneath and gently shake. Remove any rock or other non-soil debris that does not pass through the sieve and put into a mud bucket.
15. Repeat steps 13 and 14 until all soil passes through the #10 sieve and all rocks and other non-soil debris are removed.
16. Dump contents of sieve pan (i.e., soil sample that passed through the sieve) back into the container and put a lid on it.
17. Repeat steps 12 – 16 for your other samples. You do not need to thoroughly clean the sieve, sieve pan, mortar, or pestle between samples; just wipe them out with a dry towel.
18. Clean mortar, pestle, #10 sieve, sieve pan, and any other equipment using soap, water, and a sponge. When cleaning the sieve, make sure there is no material trapped within the sieve screen – if material is trapped, remove by gently pushing through from the bottom using your fingernail, a pencil, or other pointed object – BE VERY CAREFUL NOT TO DAMAGE THE SIEVE SCREEN!