Sand or clay, get to know your soil

Like much of Australia, The Garden of St Erth’s natural soil is challenging and difficult to work with. Calling it soil at all would actually be misleading.

Gold mining here in the 19th century meant any naturally occurring topsoil was completely destroyed, leaving a heavy lifeless mix of white clay and broken up stone. It would be hard to find ground less suitable for creating a garden or growing healthy plants.

Human-made soil

Yet after decades of organic horticulture, compost and care, St Erth is a thriving garden producing an abundance of flowers, fruit, vegetables and, perhaps most importantly, soil. Areas like our vegetable garden and herbaceous border now have topsoil several feet deep; human made soil. The garden produces a huge amount of material to make compost and mulch to return to the soil, further building it and feeding the soil life.

Large parts of Australia have poor impoverished soil, ranging from the heavy clays we have in central Victoria to the inert sands of Western Australia. Regardless of which soil you start with, a fertile abundant garden is possible and topsoil grows with the garden if you nurture it.

Often a good place to start is ensuring your garden has sufficient nutrients and minerals to support both the plant growth and soil life that will keep your soil healthy. Sandy soils are particularly devoid of nutrients and using a mineral fertiliser like rock dust to bolster the reserve of minerals is advisable. Clay-based soil on the other hand usually has a good reserve of minerals, which will become available once soil structure and biological activity are established. The addition of agricultural lime is important on acidic clays to both counter the acidity and help break up the clay.

Water soluble fertilisers – whether artificial or organic – can be used to boost plant growth and add necessary macronutrients like nitrogen, potassium and phosphorus. They will not, however, create a fertile rich soil by themselves and care should be taken not to over apply them to make up for poor soil structure and a lack of soil biology. Excessive fertilisation can deplete organic matter in the soil, making things worse in the long run.

Organic matter matters

Perhaps the most important aspect of creating a healthy soil is the addition of organic matter. Whether starting with sand or clay, organic matter will break down in the soil, binding sand particles together and breaking fine clay particles apart, combining them into a complex living organic structure. Organic acids created by the breakdown dissolve minerals from the clay or base material bringing them into the biological realm and making them available to plants and microbes.

Homemade compost, even in small quantities, is invaluable in building soil fertility and topsoil. The controlled, constantly moist environment of a compost heap or bin is the perfect way for organic matter to break down into a stable, nutrient-rich humus compound.

A handful of well-made compost contains billions of soil building microbes, which act as an inoculant in the soil, encouraging further decomposition of organic matter. Compost also acts as a sponge for both water and nutrients, holding them until accessed by the feeder roots of plants.

Adding large amounts of undecomposed organic material to the soil, either in the form of hay, lucerne, purchased compost or from a green manure crop grown in situ, can be a good way to kick-start the soil and get biological activity going. As with the compost heap though, the area needs to be kept moist until the material has properly decomposed.

Plants should not be planted until the material has broken down somewhat as the decomposition can temporarily tie up nutrients in the soil, leaving them unavailable for plant growth.

Breathing life into the soil

Fertile soil is created by a plethora of microbes, worms and other soil biology, which work day and night to digest its contents and make them available to our plants. Creating the right environment for this soil life is the key to improving soils and growing healthy plants. Moisture, organic matter, nutrients and even air play a role.

Keeping the soil as aerated as possible is essential. All our beneficial soil life breathe air and thrive in a high-oxygen environment. Care should be taken when working the soil to do as little damage as possible to the soil structure and the many air pockets and channels the soil biota have created.

Using a tyned garden tool like a strong garden fork is preferable to a spade. Lifting the soil and taking care not to pulverise it is important.

Wherever possible rotary hoes and mechanical cultivators should be avoided as they turn the soil into powder and destroy its structure.

In a similar way, smothering the soil with heavy mulches of straw (or even worse, plastic matting) excludes air from the process and can be detrimental to soil development.

In a similar way, smothering the soil with heavy mulches of straw (or even worse, plastic matting) excludes air from the process and can be detrimental to soil development.

Get to know your own soil

All of these processes will help to address deficiencies and improve your garden soil, but it is important to remember that garden soils are as individual as those who garden them – each with its own challenges and opportunities depending on location, aspect, history and geology.

By understanding your soil’s pH (acidity or alkalinity), its structure and water holding capacity, you will soon learn what direction is needed to improve the characteristics of your soil. Choosing plants that have adapted to thrive in your particular soil type, however, is the best place to start to ensure a successful and bountiful garden.