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Delamere forest, Cheshire, a remnant of a royal hunting forest, probably originally deciduous
woodland dominated by oak. But today a carnivorous plantation with scots pine dominant and we're
going to look at the way in which soils can vary even within a comparatively small area of the
forest. This profile, it is of course a well-developed podsell, has been cleared in a bankside near a
path and it clearly displays several characteristic layers or horizons. At the top leaf litter and
then a dark almost black horizon. Below this a very pale grey layer much broader than the first.
Another dark horizon possibly made up of a number of thinner layers which eventually merges into the
orange yellow sands upon which the soil has been formed. If we are to examine this profile in
detail samples must be taken from each horizon preferably from the centre taking care that
sufficient soil is taken for several tests to be carried out and ensuring that each bag is
clearly numbered for identification in the laboratory. Any study must begin in the field
however. On the surface there is a layer of leaf litter, in this case pine needles and bracken.
The dark horizon below contains this material in various stages of decomposition. This is
the humus horizon and the amount of organic material in this and other horizons can be
measured in the laboratory. All the samples are allowed to air dry for about two days by
which time they're dry enough to be sieved. Each sample is then lightly ground to break
up any particles which have aggregated. It's then sieved through a two millimetre sieve.
This isolates the fine fraction which will be used for further analysis. Each sample is then
divided into representative sub samples using a riffle box. Each time the soil is poured into
the riffle box it's automatically divided into two equal portions. To establish the organic content
a weighed sample of soil is placed in an oven which has been preheated at least to 750 degrees
C. At this temperature the organic material in the soil will ignite producing weight loss
which is proportional to the organic content of the sample. After an hour it is taken out
and allowed to cool in a desiccator. This keeps it dry. And then the sample its organic content
now reduced to ash is reweighed so that loss of weight can be established. The pattern of
weight loss in the profile is interesting. As expected the percentage weight loss is very high
in the litter and the surface horizon. But there is a second zone of concentration at a depth of
about 30 centimetres. This is reflected in the dark appearance of these horizons. Both are dark
brown or black. The upper layer contains plant remains and the humus is comparatively coarse.
Whereas in lower horizon the material is more amorphous indeed it is even oily in texture.
On the surface the litter which you can see contains many pine needles and pine cones
decomposes at a slow rate. One reason for this is the nature of the litter itself. Pine needles
are very waxy and this slows down the rate at which they decompose. A second factor which
can slow down the formation of humus is the acidity produced by the decomposition of these
waxy needles. Soil acidity is a reflection of the amount of hydrogen available to enter into
reactions in the soil and it can be measured by calculating the concentration of hydrogen.
Some of this hydrogen is freely available in the form of hydrogen ions. The pH is the
concentration of hydrogen ions in the soil. A larger proportion of the hydrogen is already
bound to other molecules but even this can ionize and enter into reactions. This bonded
hydrogen together with the hydrogen ions make up the total acidity of the soil. Acidity is
important because it controls the reactions which take place within the soil. Reactions
which not only break down organic material and minerals in the upper layers here but
also allow them to be washed down through the profile and to be redeposited at a lower
level here. This is known as leaching. The most widely used measure is that of pH. The
measurement of free hydrogen ions in the soil. For this a pH meter is used. This has two
electrodes. The test electrode here which is sensitive to hydrogen ions and the standard
electrode here. When immersed the potential difference between the electrodes depends
on the concentration of hydrogen ions in the liquid and the measurement of this difference
allows the concentration of hydrogen ions to be calculated. First the meter is standardized
using the solution of known pH. In this case the reading should be 7, the point of neutrality
on the pH scale and the display is adjusted accordingly. A soil sample is mixed with water.
A reading is taken. 3.6. This is a low pH indicating that the concentration of hydrogen
ions is high and suggesting that soil acidity is also high. This is confirmed for the profile
as a whole. Figures are generally low but particularly so in the horizons with a high
organic content. How does this compare with the picture for total acidity? To measure
this the samples must be carefully prepared. An extracting agent is added to the soil and
water mixture. This will release all the available hydrogen into solution so that it can be measured
by titration. The samples are agitated for at least one hour. Each extract is then filtered
and measured. The subsequent analysis will be performed on this filtrate. Total acidity
can be measured by titration against a standard alkali. We're using in this case sodium hydroxide
solution. The amount of alkali used to neutralize the acids in the soil gives the total acidity.
If total acidity is plotted for each horizon it is clear that there is in this case a strong
relationship between total acidity and soil pH. You will see that high total acidity figures
correspond to low pH values and both are highest in the horizons where organic content is high.
That is in the surface layers and in a zone of redeposition below. But if organic material
is being leached through the soil by the rainfall what about the minerals in the profile? Simply
by continuing the titrations we've just seen the concentration of one mineral, aluminium,
can be measured. If sodium fluoride is added to the neutralized solution it reacts with
the available aluminium in the solution and releases sodium hydroxide. This is of course
an alkali and it can be neutralized by titrating against a standardized acid, in this case
hydrochloric acid. From the volume of acid used it's now possible to calculate the amount
of available aluminium in the solution. Once again the results for the profile as a whole
are interesting. The effects of leaching are again apparent and redeposition is obviously
taking place at depth. It is significant however that the aluminium is not redeposited in the
same zone as the organic material. The main concentration is slightly lower down the profile
indicating that the zone of redeposition is much wider than seemed at first sight. This
is the pattern for aluminium. Are other important minerals distributed in a similar way? To
measure the concentration of some other minerals in the soil a flame photometer can be used.
A solution is drawn into the instrument along a thin tube. It sprays into a flame. Metals
in the solution cause the flame to change colour. Coloured light is directed through
the flame and the intensity of the emerging light is measured by a photoelectric cell
and a reading is given on the galvanometer. This reading can be used to determine the
concentration of certain minerals in solution. Let's look at the distribution of some of
these important minerals. Potassium tends to be concentrated in the upper part of the
profile and in the zone of redeposition. But calcium is mainly found in the humus horizon
suggesting that it is being taken up by roots and is returned to the surface in the leaf
litter. Both minerals are poorly represented in the pale horizon below the humus layer
suggesting that the effects of leaching are greatest here. The dominant process at work
in this area is leaching and the movement of both organic material and minerals down
the profile and their redeposition has produced the horizons which are so characteristic of
pod cells and the texture of the original sand has also clearly been changed. In places
the soil is still obviously very coarse. Here for example in the leached horizon. In other
places the material is fine and the texture is almost oily as we see here in the zone
of redeposition. The scale of these differences is reflected in the particle size distribution
which can be measured using simple equipment. First the organic material must be removed
by adding hydrogen peroxide. This digests the organic material in the soil and to speed
up the process we place it on a water bath. A dispersing agent is added and the solution
is agitated. The soil is then dispersed in water and the density of the mixture can be
measured using a hydrometer. This looks like a standard hydrometer with a glass bulb weighted
at the end with lead shot but the stem is specially calibrated to show the percentage
of material in suspension. It's shaken and two readings will be taken. The first after
about five minutes when the coarse material or sand has settled out. A second reading
is taken two hours later when the silt has settled. From these two readings it's possible
to calculate the percentage of sand, silt and clay in the sample. It is the distribution
of fine material in the profile which is most interesting. This confirms that leaching is
the dominant process at work. Since the organic material in the samples was digested before
analysis there are no figures for the humus horizons but in the horizon below there is
little fine material, less in fact than in the parent material itself and this suggests
that it has been washed out of this horizon and redeposition of fine material has obviously
taken place below this. The soils which have developed here reflect the parent material
in this case sand. The vegetation cover which is here scots pine with bracken and grass
and the climate and in particular the rainfall which gives a dominant downward movement of
water through the profile produce both the leaching and redeposition of organic material
and minerals. This is the situation under scots pine. What happens to the soils of the
forest edge where the vegetation changes dramatically? First to deciduous trees and then to arable
farmland. Here for example the vegetation is very different. It is obviously broad leafed
and deciduous, predominantly sycamore in fact. There is less litter suggesting that it has
decomposed more rapidly and this is confirmed by the profile itself. The humus layer and
leached horizon are much less distinct. The root network is obviously denser and deeper
and there is more worm activity. This indicates that while leaching is still taking place
it is less important and there is no clear zone of redeposition. This in turn suggests
that this soil is less acid. Let's test this by measuring the pH. A sample of soil is placed
in a test tube containing barium sulphate which will clear the suspended solids. Some
distilled water is added. The test tube is then shaken. Next an indicator is added which
changes colour and this colour can then be compared with a standard colour chart. In
the previous profile the pH was about 4. The comparable figure here is 5. Less than 50
metres away the land use changes yet again. This is a field laid down to spring barley.
If we dig a pit in this field the profile reflects the change and in particular the
effect of ploughing. The upper horizons have been destroyed and there are now only two
distinct layers separated by a plough line at a depth of about 25 centimetres. Roots
and wormholes are plentiful in this upper layer and a pH test gives much higher values
suggesting that soil acidity has been reduced by application of fertiliser and lime. These
changes can be related to changes in vegetation but here on the slope leading down to a small
pond only a few metres from the first profile the vegetation is still predominantly scotts
pine and if we look at the soils which have developed on this slope it isn't a catena.
The differences are striking. At the top of the slope the profile bears a strong resemblance
to the first profile. The humus horizon is well defined, a thick leached horizon and
a zone of re-deposition. A few metres down the slope and the profile is very different
a confused mixture of organic layers and light coloured sands scattered throughout the profile.
At the bottom of the slope near the pond there's a thick layer of organic material. It's
fibrous and not like the humus seen in the other profiles. It is in fact peat and below
this is a layer of sand. Both layers are waterlogged. Why do these changes occur? The same sands
underlie the three profiles, the vegetation cover is similar if not identical and there
can obviously be very little difference in the climate. Why then are the soils so different?
The changes we have seen can be understood not in terms of soil type but in terms of
the processes at work. Processes which have caused the original podsole to be modified
in so many different ways.
Thank you for watching.
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