Determine Water Content of Soil by Oven Dry and Pycnometer Methods

What is Water Content of Soil?

The water content of soil is is the ratio of mass of water to mass of soil which is expressed in percentage. Oven Dry Method and Pycnometer Method are commonly used to determine the water content of soil in laboratory.

1. Water Content of Soil by Oven Dry Method

The oven dry method is widely used laboratory method determine the water content or moisture content of given soil sample. It gives very accurate results.

Apparatus Required for Oven Dry Method

To determine water content by oven dry method following equipment is required.

• Hot Air Oven
• Non-corrodible air tight containers ( 3 no’s)
• Digital Weight Machine (accuracy of 0.04% of mass of sample)
• Desiccator
• Tongs

Minimum Soil Sample Quantity

The soil sample collected from the field should be of required quantity to find the water content. The quantity of soil required is depends upon the maximum particle size and gradation of soil sample. Below table shows the soil quantity required for test based on the sieve analysis.

Table 1: Minimum Quantity of Soil Required for Water Content Determination

Size of particles more than 90% of passing	Minimum Quantity ( grams)
425 micron sieve	25
2 mm sieve	50
4.25 mm sieve	200
10 mm sieve	300
20 mm sieve	500
40 mm sieve	1000

Fig 1: Wet Soil Sample

Test Procedure of Oven Dry Method

The test procedure of Oven Dry method to find the moisture content of soil consists following steps.

• In first step, clean and dry the containers and weigh them and note down the mass of each container (M1). Also note down the number of each container along with its weight.
• Collect the soil sample from field. Remove the top layer of soil and collect the wet soil from bottom layers.
• Fill the containers with required quantity of soil sample and weigh the each container and note down its mass (M2).

Fig 2: Placing Containers in Oven

• Place the containers in hot air oven, arrange temperature to 110o ± 5o C and allow them to dry for 24 hours.
• After 24 hours turn off the oven and take out the containers using tongs.
• Cool down the containers in desiccator for one hour.
• After that weigh containers and note down the mass (m3) of each container.

Observations and Calculations of Oven Dry Method

The data collected during the test is noted in below data sheet. From this data the water content of given soil sample is calculated by the below shown formula

Where     M1= Mass of empty container with lid,

M2= Mass of the container with wet soil and lid,

M3= Mass of the container with dry soil and lid.

Table 2: Observations and Calculations of Oven Dry Method

Sl. No.	Observations and Calculations	Determination No.
		1	2	3
Observation
1	Container No.
2	Mass of empty container (M1)
3	Mass of container + soil (M2)
4	Mass of container + dry soil (M3)
Calculations
5	Mass of water Mw= M2 – M3
6	Mass of solids, Ms= M3 – M1
7	Water content= (5)/(6)x100

Result of Oven Dry Method

Water content of the given soil sample = ______%.

2. Water Content of Soil by Pycnometer Method

Pycnometer method is also useful to determine water content. But this is used when the specific gravity of the given soil sample is already known. However, Specific gravity can also be determined using pycnometer.

Apparatus Required for Pycnometer Method

Apparatus used in pycnometer method are

• Pycnometer
• Weighing balance with an accuracy of 1.0g
• Glass rod
• Vacuum pump

Fig 3: Pycnometer Bottle

Test Procedure of Pycnometer Method

Test procedure of pycnometer method is as follows

• Wash, clean and dry the pycnometer and note down its mass (M1) along with brass cap and washer using weighing balance with an accuracy of 1.0 g.
• Now place a sample of wet soil about 200 to 400 g in pycnometer and note down its mass (M2).
• Then add water to the soil in the pycnometer to make it about half full.
• Stir the soil using glass rod to remove air voids of the soil sample. If available connect the vacuum pump to the soil specimen to remove entrapped air.
• Add some more water and after eliminating the entrapped air stop stirring and fix the brass cap. More water is added through hole in brass cap until the water is flush with the hole.
• Now take the mass of pycnometer (M3).
• Now empty and wash the pycnometer. Then fill it with only water and take its mass (M4).

Observations and Calculations of Pycnometer Method

The water content (w) of the soil sample using pycnometer method is calculated from the below formula

Where   M1=mass of empty Pycnometer,

M2= mass of the Pycnometer with wet soil

M3= mass of the Pycnometer and soil, filled with water,

M4 = mass of Pycnometer filled with water only.

G= Specific gravity of solids.

Table 2: Observations and Calculations of Pycnometer Method

Sl. No.	Observations and Calculations	Determination No.
		1	2	3
Observation
1	Mass of empty pycnometer (M1)
2	Mass of pycnometer + wet soil (M2)
3	Mass of Pycnometer soil filled with water (M3)
4	Mass of Pycnometer filled with water only (M4)
Calculations
5	M2 – M1
6	M3 – M4
7	(G – 1) / G
8	w (using above formula)

Result of Pycnometer Method

Water content of the given soil sample = _______%.