To get the bulk density of soil, you need to determine the dry weight of the soil and divide it by the soil's volume. Here's a detailed breakdown:
1. Understanding Bulk Density:
Bulk density is a crucial soil property representing the mass of dry soil per unit volume. It reflects the soil's compaction and porosity, impacting root growth, water infiltration, and aeration.
2. Materials Needed:
- Soil corer or sampling auger of known volume
- Oven
- Scale (accurate to 0.01g)
- Drying tins or containers
3. Procedure:
- Sample Collection:
- Carefully collect a soil core or sample using the auger or corer. Ensure minimal disturbance to the soil structure. The volume of the corer must be accurately known.
- Weighing the Soil:
- Weigh the moist soil sample immediately after collection (Wet weight). Record this weight. This step is optional if you immediately dry the sample.
- Oven Drying:
- Place the soil sample in a drying tin and put it in an oven preheated to 105°C (221°F).
- Dry the soil until it reaches a constant weight (usually 24-48 hours). This ensures all moisture is removed.
- Weighing the Dry Soil:
- Remove the dried soil from the oven and allow it to cool to room temperature in a desiccator (to prevent moisture absorption).
- Weigh the dry soil sample. Record this weight (Dry weight).
4. Calculation:
The bulk density (ρb) is calculated using the following formula:
ρb = Wd / V
Where:
- ρb = Bulk density (typically expressed in g/cm³ or Mg/m³)
- Wd = Dry weight of the soil (g or Mg)
- V = Total volume of the soil sample (cm³ or m³) - This is the inner volume of the corer or auger used.
Example:
Let's say you collected a soil sample with a volume of 100 cm³. After oven drying, the dry weight of the soil was 120 g.
ρb = 120 g / 100 cm³ = 1.2 g/cm³
5. Factors Affecting Bulk Density:
- Soil Texture: Sandy soils generally have higher bulk densities than clayey soils due to larger particle sizes and less pore space.
- Organic Matter Content: High organic matter content decreases bulk density because organic matter is less dense than mineral particles.
- Compaction: Compaction increases bulk density by reducing pore space.
- Tillage: Tillage can initially decrease bulk density but may lead to compaction in the long run.
6. Importance of Bulk Density:
- Plant Growth: High bulk density can restrict root growth due to increased soil strength and reduced aeration.
- Water Infiltration: Bulk density affects the rate at which water infiltrates into the soil. High bulk density reduces infiltration.
- Soil Aeration: Adequate pore space is crucial for gas exchange in the soil. High bulk density reduces aeration.
- Soil Health: Bulk density is an indicator of overall soil health and its ability to support plant growth and other ecological functions.
