Measuring infiltration:
30
20
10
0
After 30 secs
measure new
level
The water rapidly leaks away into the pore spaces
in the soil. Rate of infiltration will depend on
previous weather. If the soil is already wet less
infiltration will take place. Sandy or well
structured soils (eg crumb structures) will
encourage rapid infiltration.
Mm decline in level
Place a sawn off
tube in the
ground and fill
to a marked line
with water.
During first half
minute....
0
1
Minutes
2
3
4
5
Measuring infiltration:
End of first half minute..
Record new
water level and
top up again
The new water level is recorded and the water
level rapidly refilled to the original marker line.
The timer continues.
30
20
10
0
Mm decline in level
The first point can be plotted on the graph.
0
1
Minutes
2
3
4
5
Measuring infiltration:
After first full minute..
Record new
water level and
top up again
In the second half minute the water level dropped
less rapidly because there are fewer “unfilled soil
pores” near the infiltration ring.
30
20
10
0
Mm decline in level
Capillary action in dry soil is effective at “sucking
in” water. Once the soil is wet, capillary forces
become less important and infiltration slows.
0
1
Minutes
2
3
4
5
Measuring infiltration:
Second minute
Record new
water level and
top up again
Depending on the difference between the first and
second readings you may wish to either (i)
continue recording at 30 second intervals or
(ii) record every minute only.
30
20
10
0
Mm decline in level
In this example we will next measure at the end of
the second minute as shown below.
0
1
Minutes
2
3
4
5
Measuring infiltration:
Subsequent minutes
30
As the soil beneath the infiltration ring becomes
saturated the main control on rate of infiltration is
gravity pulling water through the matrix of pore
spaces. This represents the true infiltration rate of
the soil.
This is normally given in mm/minute and
can be read off the graph as the final horizontal
line.
10
20
Final infiltration
rate = 9mm/min
0
Mm decline in level
Record new
water level and
top up again
0
1
Minutes
2
3
4
5
20
30
Factors affecting
infiltration rates - 1
Infiltration curves can give information about
10
1) differing weather conditions for the same soil
2) differing soil types for the same weather.
0
Mm decline in level
Measuring infiltration:
In the top graph, infiltration rates are plotted for
the same soil in different weather conditions yellow = drought; red = normal; blue = wet.
Steepness of the curve reflects how long it takes
to fill soil pore spaces - this depends on amount
of water already there. The final flat part of the
curve represents the rate of flow through full
pore spaces - ie an indication of soil porosity.
0
1
2
3
4
5
30
20
10
0
Mm decline in level
Minutes
0
1
Minutes
2
3
4
5
In the lower graph, infiltration rates have been
plotted for different soils in similar weather
conditions. Steepness reflects how long it takes
to fill the soil pore spaces - dependent on soil
porosity (because weather was the same for
each). The final level represents rate of flow
through full pore spaces - also dependent on soil
porosity. In these examples green is the soil with
highest infiltration rate and yellow the lowest.
Measuring infiltration:
Factors affecting
infiltration rates - 2
Management of soil influences infiltration rates.
Management can influence positively or negatively.
The diagram show a soil ped - a collection of
textural particles (clay, sand etc) glued together
by organic material such as humus (shown
brown) and capillary forces of water beads
between soil grains (shown blue).
After heavy rainfall the soil pores fill with water
and instead of capillary action holding grains
together, excess water pressure pushes them
apart (“positive pore water pressure”). This
makes the peds very fragile.
Tractors on wet soil
destroy soil structure
Good infiltration
Poor infiltration
If the soil is stressed in these conditions - by heavy
machinery (eg tractors) or trampling by people or
livestock - the peds break down entirely, resulting in
a structureless mass described as a “poached soil”.
The destruction of peds means that the large pores
between peds disappear, vastly reducing infiltration
rates
Managing infiltration:
Managing infiltration structure
positively.
Good infiltration is a key to a good soil, allowing
drainage (to avoid waterlogging) and aeration (to
allow micro-organisms to breed and breathe!).
Good infiltration depends to a large extent on good
soil structure.
Higher infiltration rates result
in lower erosion rates as
water infiltrates
underground, leaving little
for surface erosion.
Soil structure is currently declining in many parts of
the developed world because artificial fertilizer has
been used instead of organic manure. Whilst
artificial fertilizer puts the right nutrients in the
ground it does not provide the raw organic matter
for glueing textural particles into peds.
Structure can be improved by:
1) adding organic fertilizers to the soil
2) maintaining an effective crop rotation
3) ploughing the soil to break up poached layers
4) avoid cultivating the soil during wet conditions
This is the final slide - press backarrow to go back or any other key to
end the show.