Soilaeration, porosity and permeability

1. Keaboka Mate & Nicole Ntombifikile Moyos
Presentation on Soil porosity, aeration and Permeability.

2. Soil Particles
 Soil as the skin of the earth consists of a mixture of different
particle types such as sandy clay or silt sand.
 Soils are classified according to their particle size, probably
one might have a good idea of what gravel and sand particles
look like but maybe not silt or clay. Silt particles are about as
big as the thickness of your hair and clay particles are much
smaller than that.

4. Porosity: Is the fraction of the total soil volume that is taken
up by pore space thus it is a single value qualifications of the
amount of space available to fluid
within a specific body of soil.
 In simple terms it is a measure of open space and depends on the size and
shape of the particles of a media(soil).
 It tells us how much water a rock or soil can retain.
 It is a measure of how much of a rock or soil is open space , this space can be
between grains or within cracks of the rock.
 Although clay and shale are porous and can hold a lot of water, the pores
in these fine grained materials are so small that water flows very slowly
through them hence clay has high porosity and low permeability.

5. HOW DOES ONE MEASURE SOIL
POROSITY This can be done by using the Archimedes method.
 Use oven dry soil cores which have been used to determine
bulk density(which is the weight of soil in given volume),then
place the soil core in a pan of water and allow it to saturate or
satiate, then reweigh to obtain the data needed to calculate
soil porosity.
 Satiated soil is one whose pores is filled with water except for
a small amount of entrapped air. The same volume of water in
satiated soil core approximates the amount of pore space and
is used to calculate the soil’s approximate soil porosity.

6. Example: A 500 cubic cm oven dry core has a bulk
density of 1.1 grams per cubic centimeters. The soil core is
placed in a pan of water and becomes water saturated.
Both the oven dry soil and water at saturation point weigh
825grams. …>Calculate the total soil porosity.
SOLUTION: Weight of oven dry soil = 500cubic cm x
1.1grams per cubic centimeters
=550grams.
Weight of water in saturated core =
825g -550g
=275g
%Porosity = cubic centimeters pore
space x 100/ cubic centimeter soil volume
=275 cm3 x 100/ 500
= 55%.

7. FACTORS AFFECTING SOIL POROSITY
Soil type
 Compaction
 Soil organisms
 Soil texture
 Anthropogenic activities
Soil Structure

8. SOIL PERMEABILITY.
 It is the ease with which plant roots, water or air penetrate or pass
through soil or it is the property of a porous material which permits
the passage or seepage of water through its interconnecting voids.
 In simple terms it is a measure of how easily a fluid like water can
pass through a porous media for example soil.
 High permeability occurs on loose soil thus fluids easily flow whilst
compact soil such as clay promotes low permeability.

9. FACTORS AFFECTING PERMEABILITY.
 Degree of saturation.
 Adsorbed water.
 Entrapped air & organic impurities.
 Impurities in water.
 Particle size.
 Human activities

10. PERMEABLE PAVEMENT
A soil sample with a high permeability allows liquids to flow through easily, whereas it is
harder for liquids to flow through soil with low permeability. For instance, pavements can be
constructed with a material with a high permeability so that water does not accumulate in
puddles on the surface.

11. SOIL AERATION
 Is a phenomena of rapid exchange of oxygen and carbon dioxide between the soil pore
space and atmosphere, in order to prevent the deficiency of oxygen or toxicity of carbon
dioxide in the soil air.
 The well aerated soil contains enough oxygen for the respiration process of roots and
aerobic microbes and for oxidation reaction to proceed at optimum rate.
 In simple terms it is a process that involves the ventilation of the soil with gases moving into
and out of the soil.
 The two most important gases in soil aeration are oxygen and carbon dioxide.
 Ventilated soils allow gases to be exchanged within the atmosphere thus two processes are
promoted and these are mass flow which involves air forced in by winds or pressure and
diffusion which involves the movement of gases between the soil and the atmosphere
according to pressure.

12. The diagram below illustrates the aeration
of soil.

13. FACTORS AFFECTING SOIL AERATION.
Drainage.
Type of soil.
Particle size.
Soil structure.
Vegetation cover.
Soil living organisms.
Anthropogenic activities.
SOIL TEXTURE

14. Human Activities
Grain size