Soils, like bodies, are made up of many parts. How these parts
are put together and in what quantities dictate how a soil behaves.
The ‘bones’ of soil are clay, silt, sand and organic
matter that are joined together in aggregates. The structure of
a soil depends on the size, shape and arrangement of these aggregates.
Structure also refers to the size, number and arrangement of pores
within and between these aggregates. The network of pores in the
soil, like veins in our bodies, are essential as they store and
transport water and air to plants
Some history
During the 1930s, the Great Plains of America lost so much soil
due to wind erosion that dust clouds reached from Canada to Mexico,
blocking out the sun. The area became known as the ‘dust bowl’.
The main cause was continuous cultivation of soil to grow wheat,
resulting in a breakdown of soil structure. Farmers were producing
high yields as the combine harvester rolled out into the fields
to join the humble tractor after WW1, so they saw no reason to alter
their farming practices. However, when a period of prolonged drought
hit the region the wind picked up the soil and simply carried it
away.
Though smaller in scale, a similar process had already taken place
in Canterbury in the 1870 and 1880s, during the decades of bonanza
wheat cropping. Large areas of land were being cultivated year after
year to grow wheat. As a result there was not enough organic matter
in the soil and the soil structure suffered. Farmers then watched
enormous amounts of precious topsoil blow out to sea.
Events such as these forced people to pay more attention to soil
structure. During the 1950s and 1960s, an Australian scientist,
W W Emerson, showed how clay particles and organic matter build
up soil structure. He then developed the Emerson stability test
that categorised soil according to their structural stability. The
development of this test sparked off a lot more research, which
improved our understanding of factors that affect soil structure.
Hopefully we can avoid situations that create events such as ‘dust
bowls’ in the future.
Soil is made up of several distinct layers or horizons, these layers
form what is known as the soil profile.
The top layer of soil or topsoil is the richest, having the most
amount of humus (partially decomposed organic soil material). This
phenomenon is largely due to the presence of decomposers (predominantly
bacteria, fungi, and earthworms) that recycle dead organic matter
(plants and animals) into humus. The subsoil is below the topsoil
and is low in humus. However, this is where most soil nutrients
are found. Below the subsoil is weathered parent material, which
is full of rock particles and minerals with no humus. Parent material
is the initial state of soil and can be bedrock, organic material
or deposits from water, glaciers, volcanoes, or wind. Physical weathering
over millions of years has broken them down into fine particles,
and soil inherits physical and chemical properties from this parent
material. Bedrock sits underneath the weathered parent material
and is made up of solid rock. This solid rock will stay hidden and
undisturbed until an earthquake or erosion exposes it to the surface,
where some of it will be weathered to make way for the next batch
of parent material, which starts the soil-forming process all over
again. However, soil is not the end product of weathering rock,
it is simply a stage in the mineral cycle – the process by
which nutrients such as carbon, nitrogen, and calcium cycle between
living things, and the atmosphere and soils.
The amount of clay and organic matter in a soil play an important
role in determining soil structure. Clays are made up of minute
‘plates’ that slide across each other, giving them a
very large surface area. A single gram of clay powder can have a
total surface area larger than a football field! Like organic matter,
clays carry a negative electric charge and can attract positively
charged cations and water molecules, forming small aggregates. Sand
and silt do not have any charges, but they are also combined into
these aggregates when their surfaces are coated with clay or organic
matter. These small aggregates can then form larger aggregates with
the help of fungal hyphae, cementing substances from organic matter
and plant roots.
Organisms of a range of sizes live in the soil and can have a considerable
influence on soil structure. For example, micro-organisms produce
substances that act like glue, helping to bind soil aggregates together,
whilst earthworm burrows are important in allowing air and water
to penetrate deep into the soil.
In your garden building up good soil structure is like laying the
foundations for a house because it is the base from which all else
follows. A ‘well structured’ soil will hold large amounts
of water and dissolved nutrients, the aggregates will withstand
cultivation and will not ‘puddle’ when wet or become
dusty or set hard when dry. The network of pores will ensure adequate
drainage and aeration, essential for healthy growth of plant roots.
And of course a good structure will provide an excellent medium
from which seedlings can emerge and through which roots can explore
in their search for moisture and nutrients. Soils with poor structure,
however, have ‘unstable’ aggregates and in a heavy textured
soil the aggregates may form large dense clods with few pores restricting
root growth. Soils with poor structure are also more susceptible
to erosion (both by wind or water).
Good soil structure depends on many variables but the main one
in practically all cases is the presence of organic matter and humus.
The word ‘organic’ comes from the Greek word ‘organon’
meaning ‘tool’ or ‘instrument’, and when
it comes to the soil structure of your garden, organic matter is
indeed one of the most efficient and necessary tools.
Humus is one of those amazing substances that can improve soil
condition in most situations, e.g. it can help a porous sandy soil
to hold more water and a heavy soil to hold less water. It also
has a greater capacity than clay to hold plant nutrients, preventing
them from being leached away in the drainage water. In a clay soil,
adding humus helps to bind the particles together into aggregates,
whilst in a sandy soil humus coats the soil particles, helping them
to hold moisture and nutrients.
It may take at least three to four years to improve a poorly structured
soil as the build up of soil organic matter levels is a slow process.
This is why it is so important to try to prevent the reduction of
organic matter in the first place instead of finding that you need
to restore it later once problems occur.
Ensuring a constant supply of decomposable organic materials, adding
composts or using green manures can all help to maintain and improve
a soil’s structure. Mulching may also protect the structure
of the soil surface from rainfall damage. The presence of calcium
in the soil may improve structural stability in a few situations
(which is why adding lime can improve structure as it contains calcium).
The extent to which a soil is aggregated can be assessed based
on the following observations:
Stuctureless soil: no aggregates have formed and
the soil consists of either individual separate grains, as in sand,
or a densely packed mass of particles without many pore spaces.
Weakly developed soil structure: poorly formed
aggregates that are hard to distinguish from the rest of the soil.
Moderately developed soil structure: mainly well
formed aggregates that, when disturbed, will break down to whole
and broken aggregates and only a little unaggregated soil.
Strongly developed soil: almost all of the soil
particles are in clearly identifiable aggregates.
You can also assess the structural stability of soil aggregates
by gently lowering a few dry aggregates into a saucer of water and
leaving them to sit for half an hour.
The aggregates of an unstable soil will disperse into primary
particles of soil (the clay particles will make the water look
cloudy).
If the structure is semi-stable, the aggregates will break down
into a flattened pile of very small aggregates but the water will
remain fairly clear.
