Quantitative interrelationships of solid, liquid and air components of soil (Fig 4.1) are defined in terms of many useful parameters of soil.
Density of individual soil particles and of bulk is reported in relation to density of water, which is 1.0 gram per cubic centimeter (1.0 g cm-3). Solid rock particles that go to form the soil, normally, weigh about 2650 kg m-3. Weight of water being 1000 kg m-3, specific gravity of soil is thus equals 2650/1000 = 2.65.
The particle density (ρs), also called density of soil solids, is the ratio of mass of soil solids (Ms) to volume of soil solids (Vs) and is expressed in g cm-3 or Mg m-3.
Density of soil solids or particle density of 2.65 g cm-3 means that its weight (mass) is 2.65 times that of water. In most mineral soils, the mean density of particles varies from 2.6 to 2.7 g cm-3. Presence of soil organic matter lowers its value.
Dry bulk density:
A given bulk of soil is not all solids. On volume basis, it may contain about 50 per cent pore space occupied by water and air. The dry bulk density or simply bulk density (ρb) is the ratio of mass of oven dry soil solid particles (Ms) to the total volume of the soil (Vt). This volume includes the volume of soil solids (Vs), soil water (Vw) and soil air (Va). The volume of water and volume of air constitute the total volume of pores (Vf).
Therefore, bulk density is expressed as:
Soil texture, structure, organic matter content and soil management practices influence bulk density of soil. For a soil in which the pores constitute half of the volume, bulk density is half of particle density, namely 1.3 to 1.35 g cm-3. In sandy soils, bulk density can be as high as 1.6 g cm-3. In extremely compacted soils, bulk density might approach, but never reach, the particle density. Ideal bulk density for optimum crop growth varies from 1.2 g cm-3 for a clay soil to about 1.4 g cm-3 for a sandy soil.
Total or wet bulk density:
It is the ratio of total mass of soil (Mt) to the total volume of soil (Vt).
The wet bulk density depends more on the soil wetness (moisture content) of the soil.
Soil pores are those parts of soil bulk, not occupied by solid soil particles. In a field, soil pores are filled with water, air and other gases. Porosity (f) is the ratio of total volume of pore spaces (Vf) to the total volume of soil (Vt) and is expressed as a fraction or as percentage.
Porosity is an index of relative pore volume in the soil. Its value, generally, lies in the range of 0.3 to 0.6 (30 to 60%). Coarse textured soils tend to be less porous than fine textured soils, though the mean size of individual pore is greater in the former than the later. In clayey soils, porosity is highly variable as the soil alternately swells, shrinks, aggregates, disperses, compacts and cracks. The total porosity reveals nothing about the pore size distribution.
Porosity is related to bulk density as indicated below:
ρb = (1 – f) ρs
Types of pore spaces:
Macro and micro pores occur in soils. Usually, pores larger than about 0.06 mm in dia are considered as macropores and those smaller than this as micropores. Macropores allow ready movement of water and air. Micropores are mostly filled with water in moist soil and do not permit air movement. Soil texture, organic matter and tillage influence the soil porosity.
Void ratio (e):
It also an index of fractional volume of soil pores, but it relates that volume to the volume of solids rather than to the total volume of soil.
Advantage of this index over the previous one (f) is that a change in pore volume changes the numerator alone, where as a change of pore volume in terms of porosity will change both the numerator and denominator of the defining equation. Void ratio is the, generally, preferred index in soil engineering and mechanics, where as porosity is the more frequently used index in agricultural soil physics. Generally, e varies between 0.3 and 2.0.
Relation between porosity and void ratio:
Gravimetric water content or mass wetness (w):
It is the mass (weight) of water (Mw) relative to mass of oven dry soil particles (Ms) and is expressed as a fraction or percentage.
Volumetric water content or volume wetness (θ):
It is the ratio of total volume of water occupied in the pore spaces at a specific time to the total volume of soil and is expressed as a fraction or percentage.
Volumetric water content can be computed from gravimetric water content by multiplying the value with bulk density.
θ = w x ρb
The volumetric water content expressed as fraction, when multiplied by depth of soil gives the amount of water in terms of depth.
Relationship between mass wetness and volume wetness:
Degree of saturation (θS):
It is the ratio of volume of water present in the soil at a particular time to the volume of pores.
Relationship between volume wetness (soil water content) and degree of saturation:
θ = s f
Air-filled porosity (fa):
It is the ratio of volume of air space to the total volume of soil.
This index is a measure of relative air content of the soil. It is a useful criterion of soil aeration.