Fundamentals of Soil Science (1+1)

Exercise: 5

Aim: Determination of Bulk Density of Soils

• The bulk density ( also known as apparent density) is defined as the mass per unit volume which includes volume ( space) occupied by solids as well as pore space and is expressed in g cm-3. It changes with tillage, manuring and cropping. It decreases with loosening and increases with compaction.
• Bulk density tends to increase with depth in the profile due to low organic content in the lower layers.Fine-texture soils tend to have lower bulk densities (and thus higher porosities) than coarse-textured soils due to loose packing of the clay particles.
• Bulk density of a soil is required to determine the degree of compactness, soil pore space, an indicator of aeration status, to convert soil water contents from the gravimetric to the volumetric basis, to provide information about the environment available to the many soil micro-organisms, which live within them.

Methods of determination

• There are different methods adopted for the determination of bulk density in the field and laboratories. The various methods used for determining bulk density are pycnometer, core - tube ,clod coating and gamma density methods.
• The most commonly used core - tube method have been described below:

Principle

• The method involves sampling a soil core, from a desired depth, in its most natural
• condition using core sampler (cylindrical metal sampler) and determining the mass of solids and the water content of the soil core by weighing the wet core, drying it to constant weight in an oven at l05° C, and reweighing after cooling (A core may take 24 hours or more to dry).
• Bulk density is, then, calculated from the measurement of the bulk volume, using the
• core length and the diameter of the cutting edge of the sampler.

Materials required :
(i) Core sampler (ii) Vernier calliper (iii) Moisture boxes (iv) Oven (v) Balance
(vi) Desiccator (vii) Knife (viii) Spatula.

Procedure

• Slice off vegetation, if any, from the site prior to inserting the core sampler into the soil.
• Drive the core sampler vertically into level ground, deep enough, to fill the sampler upto the brim.
• Dig out the sampler by means of a khurpi or spade without disturbing the soil in the core sampler.
• Trim off the protruding soil if any from both ends of the sampler with a sharp knife.
• Take out the soil in a tray and weigh it.
• Take a part of the moist soil (about 50-75 g) in a moisture box and find out water content by drying it in an oven at 105° C and determine the oven dry weight of the soil in whole core (Ms, g).
• Measure the dimensions of the core sampler for computing the core volume (Vt cm3).
• Volume of soil = inside volume of the core sampler

Observations and Calculations:

• Dimensions of core sampler: Radius, r = cm; Length, l = cm
• Volume of core sampler/soil, Vt cm3 = ðr2l
• Weight of moist soil (whole core), g = X
• Weight of empty moisture box, g = A
• Weight of moisture box + moist soil, g = B
• Weight of moisture box + oven dry soil, g = C
• Moisture content in soil, g = (B - C)
• Weight of oven dry soil, g = (C - A)
• Per cent moisture in soil (w) = (B - C) /(C - A) × 100
• Weight of oven dry soil (whole core) Ms, g = X ×100 / w +100

• Bulk density (g cm^-3) =Weight of oven dry soil in the core (Ms, g)/Volume of the core/soil (Vt ,cm³)

Precautions

• The volume of soil is not constant as some clays swell when wet and shrink when they are dried. Hence, bulk density should be measured at field/ natural conditions,preferably at field capacity.
• Always use dent free core.
• Core should be pushed straight into the soil.
• If sampling has to be done for different layers then handling of core is obstructed by the surface soil as the core is not to long. So for this, whole area of sampling is excavated layer wise.