Physical Properties of Soil
Physical Properties of Soil
The soil properties that we can see or feel and are
relatively more permanent than chemical properties are called physical
properties of soil. Such properties are difficult to change. Soil physical
properties include texture, structure, particle and bulk density, porosity, air
and water content, consistency, colour and temperature.Soil Texture and its Significance
It
refers to relative proportions of the three primary soil separates (sand, silt
and clay).
Texture is important because it determines the ease of
tilling the soil, the amount of aeration, water intake rates and water storage
in the soil. It also influences soil fertility. For instance, a fine textured
clayey soil is difficult to till, has poor aeration for good root growth,
difficult to wet and difficult to drain but it can retain more nutrients and
water. A coarse sandy soil is easy to wet, easy to drain and easy to till, has
good aeration but loses more plant nutrients by leaching.
Soil Structure and its Significance
The
term structure relates to the clustering or arrangement of primary soil
particles into secondary particles or units called peds or aggregates .e.g.
Plate like, block like and granular.
Soils with good physical condition are important to good
yield. The formation and maintenance of stable aggregates is essential features
of soil tilth. The rate of water infiltration, water-holding capacity, heat
transfer, aeration, porosity, and root development are greatly influenced by
soil structure. For instance, both granular and single-grain soils have rapid infiltration
rates, prismatic and blocky soils had medium rates and platy and massive soil
conditions have slow infiltration rates.
Aggregated soil is generally the most desirable condition
for plant growth, particularly in the critical early stages of germination and
seedling establishment. The continued existence of large pores in the soil
depends on the stability of the aggregates.
Particle Density
It
refers to the ratio of total mass of the solid particles to their total volume
excluding pore spaces. In technical work, its units are mega grams per cubic
meter (Mg/m3) or grams per cubic centimetre (g/cm3) or kilo grams per
cubic meter (Kg/m3).
Particle density for most mineral soils usually varies between
2.60 and 2.70 Mg/m3. However, the standard value used in
calculations is 2.65 Mg/m3 if actual particle density is not known.
Bulk Density and its Significance
It is defined as the mass of dry
soil solids per unit bulk volume. Bulk volume means the volume of the soil
particles plus pore space. So the density for a volume of soil as it exists
naturally is called bulk density. The value is expressed in Mg/m3 or
g/cm3 or Mg/m3.
The bulk density of soil is used for estimating the mass of
a volume of a soil too large to weigh. For example, the average weight of soil
for a hectare or ace area per unit depth is calculated by multiplying the soil
volume by its bulk density.
It is needed for converting water percentage by weight to
content by volume.
It is also used for calculating porosity when the particle
density is known.
It can indicate the differences in compaction of a given
soil resulting from heavy tillage equipment on wet clayey soil.
Plant roots of field crops are hindered by soil high in bulk
density to varying degree.
Pore Space and its Significance
The
pore space of a soil is the portion of soil bulk volume occupied by soil pores.
It consists of that portion of the soil volume not occupied by solids either
mineral or organic.
The micro pores are generally filled with water in a moist
soil and there is slow movement of air and water into or out of the soil. On
the other hand, the macro pores allow the ready movement of water and air thus
the movement of water and air through a sandy soil is rapid due to the
dominance of the macro pores even though porosity is relatively lower. Clayey
soils, particularly those without good structure allow relatively slow air and
water movements despite having large total pore space. In these soils
dominating micro-pores generally remain full of water. However, improvement in
structure can promote aeration by increasing the proportion of macro-pores.
Therefore, the pore size distribution is more important rather than the volume
of total pore space.
Soil Aeration and its Significance
It
is the process by which air in the soil is replaced by air from the atmosphere.
The rate of aeration depends mainly on the volume and continuity of air-filled
pores within the soil.
In a well-aerated soil, the soil air is very similar in
composition to the atmospheric air above the soil. However, poorly aerated
soils generally contain a much higher percentage of CO2 and a correspondingly
lower percentage of O2 than the atmospheric air above the soil.
Soil air is different from atmospheric air as it has lower
oxygen content (14-20%) and relative humidity (95-99%). Plant roots and
micro-organisms take oxygen from soil air and release carbon dioxide into it.
The oxygen contents are the most important for plant growth
as are needed for respiration. Except rice, most plants need oxygen to be
present in the pores where roots are growing. So the rate at which soil oxygen
exchanges with atmospheric oxygen (Oxygen Diffusion Rate) is important.
Organic matter decomposition by soil microorganisms also
uses oxygen in soil, when there is
little oxygen in soil anaerobes start working and anaerobic microorganisms
produce gases (other than CO2) such as nitrous oxide, methane and hydrogen
sulphide. These gases are harmful and source of pollution.
