What is Water Content of Soil?
Soil water content is a crucial parameter in soil science, which is measured as the percentage ratio of the mass of water to the mass of soil. In laboratory settings, two commonly used methods to determine the water content of soil are the Oven Dry Method and Pycnometer Method.
The Oven Dry Method involves taking a sample of soil and weighing it. The sample is then dried in an oven at a temperature of 105-110°C until a constant mass is achieved. The mass of the dried soil is then measured and recorded. The difference between the initial weight of the soil and the weight of the dried soil gives the weight of water present in the soil. The water content is then calculated as a percentage of the total mass of the soil.
The Pycnometer Method, on the other hand, involves taking a sample of soil and placing it in a container known as a pycnometer, which is filled with water. The weight of the pycnometer filled with water is recorded, and the sample is allowed to settle to remove any trapped air. The weight of the pycnometer filled with soil and water is then recorded. The water content is calculated as the difference between the weight of the pycnometer filled with soil and water and the weight of the pycnometer filled with water alone, expressed as a percentage of the total mass of the soil.
Both the Oven Dry Method and Pycnometer Method are commonly used in laboratory settings to determine the water content of soil, an important parameter in soil science.
1. Water Content of Soil by Oven Dry Method
The laboratory method known as the oven dry method is commonly utilized to determine the moisture content or water content of a given soil sample. This method is highly regarded for its accuracy in producing precise results.
Apparatus Required for Oven Dry Method
The oven dry method for determining water content requires several pieces of equipment. Firstly, a hot air oven is necessary to dry the sample. Additionally, three non-corrodible air tight containers are required to hold the sample before and after drying. A digital weight machine with an accuracy of 0.04% of the mass of the sample is needed to measure the weight of the sample before and after drying. Finally, a desiccator and tongs are necessary to cool and handle the sample after drying. All of these equipment pieces are essential for accurately determining the water content in a sample using the oven dry method.
Minimum Soil Sample Quantity
To determine the water content of a soil sample collected from the field, it is important to have the right amount of soil for testing. The required quantity of soil depends on the maximum particle size and gradation of the soil sample. The table below provides information on the amount of soil required for testing 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 for the Oven Dry method to determine the moisture content of soil involves several steps. First, the containers to be used in the test should be cleaned and dried. Their mass should be weighed and recorded as M1, along with the container numbers and weights.
Next, soil samples are collected from the field. The top layer of soil should be removed, and the wet soil from the bottom layers should be collected.
The containers are then filled with the required quantity of soil sample. Each container is weighed, and its mass is recorded as M2.
Fig 2: Placing Containers in Oven
The containers should be placed in a hot air oven and the temperature should be set to 110°C ± 5°C. They should be allowed to dry for a period of 24 hours. Once the 24-hour drying period is over, the oven should be turned off and the containers should be carefully taken out using tongs. Subsequently, the containers should be cooled down in a desiccator for one hour. After the cooling process, the containers should be weighed and the mass (m3) of each container should be noted down for further reference.
Observations and Calculations of Oven Dry Method
During a soil testing process, data is collected and recorded in a data sheet. This data is used to calculate the water content of the soil sample. The formula used for this calculation involves three different values: M1, M2, and M3. M1 represents the mass of the empty container along with its lid. M2 represents the mass of the container with wet soil and lid, while M3 represents the mass of the container with dry soil and lid.
Using these values, the water content of the soil sample can be calculated using the provided formula. It is important to accurately measure and record these values in order to obtain an accurate calculation of the water content. This information can be valuable in determining the quality and properties of the soil, which can be important in various industries such as agriculture or construction.
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
The Pycnometer method is a commonly used technique to determine the water content of soil samples. However, this method requires that the specific gravity of the soil sample is already known. In cases where the specific gravity is not known, the Pycnometer method cannot be used.
It is worth noting that the Pycnometer method can also be used to determine the specific gravity of a soil sample. This involves weighing the empty Pycnometer and then filling it with distilled water at a known temperature and weighing it again. The Pycnometer is then emptied, dried and filled with the soil sample at the same temperature and weighed again. From these measurements, the specific gravity of the soil sample can be calculated.
Overall, the Pycnometer method is a versatile technique that can be used for determining both water content and specific gravity of soil samples, provided that the necessary information is available.
Apparatus Required for Pycnometer Method
The pycnometer method requires the use of several apparatus. These include a pycnometer, a weighing balance with an accuracy of 1.0g, a glass rod, and a vacuum pump. The pycnometer is a specialized container designed to measure the density of liquids. The weighing balance is used to accurately measure the weight of the pycnometer before and after filling it with liquid. The glass rod is used to remove any air bubbles that may be present in the liquid being tested. Finally, the vacuum pump is used to remove any trapped air from the pycnometer and liquid, ensuring accurate density measurements. Together, these apparatus are essential for conducting accurate and precise density measurements using the pycnometer method.
Fig 3: Pycnometer Bottle
Test Procedure of Pycnometer Method
The test procedure of the pycnometer method involves several steps. First, the pycnometer needs to be washed, cleaned, and dried, and its mass (M1) needs to be noted down, along with the brass cap and washer, using a weighing balance with an accuracy of 1.0 g.
Afterwards, a sample of wet soil, weighing between 200 to 400 g, needs to be placed inside the pycnometer and its mass (M2) needs to be noted down. Then, water is added to the soil to make it about half full, and the soil is stirred using a glass rod to remove air voids. If available, a vacuum pump can be connected to the soil specimen to remove any entrapped air.
Next, some more water is added, and after eliminating any entrapped air, the brass cap is fixed onto the pycnometer. More water is added through the hole in the brass cap until the water is flush with the hole. The mass of the pycnometer (M3) is then taken.
Finally, the pycnometer is emptied and washed. It is then filled with only water, and its mass (M4) is taken.
Observations and Calculations of Pycnometer Method
The pycnometer method is a technique used to determine the specific gravity of soil samples. To calculate this value, a formula is used which takes into account several different measurements. The first measurement is the mass of the empty pycnometer, denoted as M1. The second measurement is the mass of the pycnometer when it is filled with wet soil, denoted as M2. The third measurement is the mass of the pycnometer and soil when it is filled with water, denoted as M3. Finally, the fourth measurement is the mass of the pycnometer when it is filled with water only, denoted as M4.
Using these measurements, the specific gravity of solids in the soil sample can be calculated. This value is denoted as G in the formula. By rearranging the formula and plugging in the appropriate values, it is possible to determine the specific gravity of the soil sample. This information can be useful for a variety of purposes, such as assessing the strength and stability of soil in construction projects.
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 = _______%.