Soil bulk density is defined as the oven-dry weight of soil per unit of its bulk volume. Bulk volume comprises volume of soil solids and pore spaces, and bulk density is expressed as grams/cubic centimetres. Bulk density of soil indicates the degree of compactness and aeration, which is necessary for estimating the weight of soil per unit area, such as per hectare. Bulk density varies with soil texture - soils with fine texture tend to have lower bulk density than coarse-textured soils. - and tends to increase with depth because the lower layers are low in organic matter and microbial activity. Bulk density is considered to have relatively low spatial variability (the coefficient of variation is less than 10%) but its values are required for converting soil organic matter content to tonnes of soil organic carbon per unit area (tC/ha). Bulk density of soils is determined using the following methods (Baruah and Barthakur 1997).
(i) Bulk density using the tube core method for undisturbed soil
The tube core method involves sampling a soil core from a desired depth in its most natural conditions using a soil core sampler and determining the mass of solids and water content of the core; bulk density is calculated from bulk volume and weight of the dried soil.
Material and preparation The equipment needed includes a core sampler, tin sample box, balance, oven for drying samples and scale to measure dimensions of the soil core. Measurements related to bulk density could be made on the same day as that on which the soil is sampled for laboratory estimation of organic carbon. Field and laboratory procedures The following steps could be adopted for estimating the bulk density:
Step 1: Select the same locations as those used for sampling for estimating SOC. Step 2: Measure and record the dimensions (diameter and height) of the soil core sampler. The depth or height of the core could be 15-30 cm. Weigh the core tin box as well.
Step 3: Drive the core sampler vertically into a spot of levelled ground deep enough to fill the soil sampler tin. Step 4: Extract the sampling core without disturbing the soil inside the core;
remove any extra soil adhering to the core and protruding roots, if any. Step 5: Weigh the sample tin along with the soil.
Step 6: Dry the soil in the tin in an oven to constant weight at 105 °C and estimate the weight of the dry soil. Ensure that the soil thus dried is not used for estimating SOC.
Data recording format for bulk density (Location, land-use category, project activity, quadrat number, sampling point number, date and GPS reading also need to be recorded)
Dimensions of the core Weight of the empty tin Weight of the tin with dried soil Above-ground vegetation Location
Latitude and longitude
Calculate the bulk density (g/cc) by dividing the weight of the oven dry soil by the volume of the tin.
Bulk density (g/cc) = weight of dry soil with tin - weight of empty
(ii) Clod method for undisturbed soil
The clod method measures bulk density by taking an undisturbed bulk of soil clod (hence the name). Determine the volume of the clod and dry weight of the soil. Bulk density can then be calculated as the ratio of the weight to the volume. In the clod method, the volume of clod is measured by the volume of water displaced (the clod is coated with paraffin or liquid plastic before immersing it in water). Bulk density can be measured by taking an intact block of soil clod, as follows:
Step 1: Dig the soil and select a clod of soil using a pickaxe and note the depth from which the clod was collected Step 2: Dry the clod in an oven and estimate the weight of the oven-dry clod Step 4: Coat the clod with paraffin wax or liquid plastic
Step 5: Estimate the volume of the clod by using the water displacement method
Step 6: Estimate bulk density using the following equation
Bulk density (g/cc) = weight of the oven dried clod/ volume of the clod
(iii) Bulk density of disturbed soil method This method for disturbed soil involves collecting soil from known depth and filling it into a bottle or a tin and obtaining the weight and volume of the soil in the container. This method has limitations since the original degree of soil compaction cannot be simulated, leading to error. This method can be used only if other methods are not feasible.
Material A small bottle or other container (capacity about 50 ml) and analytical balance.
Procedure Weigh the empty bottle (Wl = bottle) without the stopper. Fill the bottle with soil as explained above. Weigh the bottle again (W2 = bottle + soil). Empty the bottle and fill it with water from a burette and record the observations:
Step 1: Weigh the empty bottle, box or tin.
Step 2: Fill this previously weighed container with soil, adding a small quantity of soil each time and tapping the container after each fill. Once filled to the brim, mark the level of soil in the container. Step 3: Weigh the filled container.
Step 4: Empty the container and refill it with water up to the marked level. Pour the water into a measuring cylinder and note the volume (V) of water.
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