Good soil is the basis of a good garden. By understanding, managing and improving your soil you will be able to use less water, while maintaining an attractive looking garden, even in the toughest times. Understanding your soil will also help you choose the right plants for your conditions.

Understanding soil

The water holding capacity of a soil directly relates to the soil type, especially its structure and texture.

Soil structure and soil texture

A good soil consists of peds (soil particles held together by humus) with spaces between them called ‘soil pore space’.

These spaces can accommodate air, water, micro-organisms and roots of plants. The amount and type of pore space varies with soil texture and structure. A good soil has up
to 50 per cent pore space by volume

.
Soil structure relates to the arrangement of soil particles and their pore space.

Texture relates to the soil particle sizes from gravels through to coarse sands, fine sands through to silts, and finally clays which have the smallest of soil particles.

The infiltration rate of the soil relates to the rate at which water will soak into soil. A good textured and structured soil should have a good infiltration rate.

A simple test to determine your soil
type

Take a handful of your soil and make it just
damp. Try forming it into a ball or sausage in
your palm. If it forms a ball that readily stays
in shape then your soil is clay. If it tends to
hold its shape, but is a bit crumbly, then it is
loam. A sandy soil will not hold shape but fall
apart very quickly.

Loam

A loamy textured soil with a crumbly
structure and with good pore space will allow
water to move through it by capillary action
similar to how water moves when it soaks
through blotting paper.
Well-structured clays and sandy loams with
lots of organic matter are the best soils for
holding the largest amounts of water for plant
growth.

Clay soil

If your soil is clay you may want to add
gypsum to help break it up. This helps water
get in more easily, and also breaks up the
crust that often forms and makes water run
off the surface, rather than soak in. Gypsum
is available in both a powder and spray-on
formula.

Clay soils may need deep-ripping or breaking
up when almost dry to help with initial
aeration, but it is bad practice to cultivate
wet or very dry clay soils.

Sandy soil

Sandy soils lose their moisture very quickly.
Many are also hard to wet again once
they have dried out. With sandy soils it is
important to constantly add old organic
matter.

Indigenous Plants and soil

Our local flora has evolved over many
thousands of years to thrive in our local
soils. One of the advantages of growing
indigenous plants is that you don’t need to
import foreign soil and apply large amounts
of fertiliser. This also reduces the risk of
introducing weeds and other soil-borne pests
and diseases. Some local native plants will
adapt to soils which have been altered or
improved in some way, for example by the
addition of organic matter or clean clay or
sand.

In an established garden where the soil
has been dramatically altered, careful plant
selection is required. While many indigenous
plants will adapt to changed soil some may
struggle in the altered conditions. Potential
problems include too many nutrients, poor
drainage leading to water-logged soil,
competition from weeds, and changes to the
natural make-up of micro-organisms in the
soil.

In new development areas always work with
the existing soil. If you need additional soil
– for example, you want to create some
mounds for interest and improve drainage
– then determine your soil type and try
to purchase the same type of soil from a
location close by. An adjoining site cut for a
new house is often a good option.

Improve your soil

Always continually improve your soil by
adding lots of composted organic matter.
This is best done in winter and spring.
Organic matter in the soil helps to feed the
plants, keeping them healthy and healthy
plants cope much better with less water. It
also encourages earthworms, whose little
tunnels form great conduits to allow water
to penetrate into the soil. The organic matter
also holds the moisture in the root zone of
plants where they can use it.

Humus (organic matter broken down by
micro-organisms and other soil life) acts
as a binding agent in the formation of soil
peds (crumbs), ensuring good water holding
capacity as well as good drainage. Humus
can hold several times its own weight in
water and helps prevent plant nutrients from
leaching away. Humus gradually breaks
down, releasing nutrients to plants. A humusrich
soil will also help keep a more constant
temperature throughout the year.

Melaleuca decussata - Totem Poles.

Water and soils

We need to ensure that water will soak into
the soil when we water the garden or when
it rains.

Improve water penetration with
terracing

Creating flatter ground by terracing slopes,
as well as adding organic matter, allows
much better water penetration as the water
gets a chance to infiltrate the soil, rather
than just run off. Make the ground as level as
possible, but ensure there is a gentle slope
so that in very wet periods, or times of heavy
downpours, excess water can get away.

Improving hydrophobic soils

If you find your soil does not take the water
in easily and it tends to sit on the surface
then the soil is referred to as ‘hydrophobic’.
In this case it may pay to add one of the
soil-wetting agents. There are a number
available and they can be bought in powder
or liquid form. They are often a special type
of detergent that acts by breaking up surface
tension, allowing water to spread through
the soil particles. Wetting agents are best
applied at the start of the dry season, as they
are biodegradable and only last for around
six months. Don’t use these types of wetting
agents near waterways as they have been
known to affect aquatic creatures such as
frogs.

Other soil additives to hold water

Another example of a soil additive that helps
to hold water are water crystals. These swell
up to a jelly-like consistency, and while they
do not overcome hydrophobia in soils, they
are useful for holding moisture in the root
zone of plants. This is particularly useful for
reducing transplant shock in newly-planted
plants and seedlings, and for keeping
moisture in potting mixes that dry-out rapidly.
Coco peat or copra peat is an organic
material made from the husks of coconuts.
Incorporated into the soil, it is also able to
hold large amounts of water. It can be used
in the same situations as water crystals,
and as a general aid to moisture retention in
garden beds.

Photo courtesy PepperGreen Farm.