Understanding your soil is vital to provide the right foundation for a healthy and successful organic garden.

Soil is the top part of the earth’s surface in which plants can grow. It consists of layers made up of minerals, sand and organic material. The top surface layer, the topsoil, contains organic matter so is usually dark in colour. Subsoil resides under the topsoil and is lighter in colour with a heavier texture. The base layer is derived from underlying rock. Depending on your local earth structure, the depth of each soil layer can vary.
Most Australian soil is classified according to its clay, sand or silt content. Clay soils are dense and sticky, have poor drainage and compact easily. When they dry out they set like stone. Sandy soil doesn’t hold nutrients or moisture well but some plants, particularly natives, love it. And there’s loam — somewhere in between clay and sand — which is widely considered ideal.

Healthy soil is the foundation to a vigorous and successful garden and is particularly important in the edible garden where good soil provides the medium in which the most nutritious food can be grown. It’s crucial that you prepare soil properly before you begin planting, nurture it well and maintain it year after year. Even if your soil appears feeble or poor, there are lots of ways you can improve it in order to sustain strong, thriving plant life.

• 

• 

Clay soil

Clay soil is the heaviest soil classification. It’s composed of closely packed particles that allow less water and air movement through the soil. Clay soils cool slowly and warm slowly, but can hold large volumes of nutrients. Never cultivate clay soil when it’s wet as this will cause compaction. Adding coarse organic material — animal manures, leaf mulch or garden compost — will break up clay soil and increase pore space.
Gypsum is a clay breaker and fine to use in the edible garden. When the clay is moist, work it into the soil at the rate of 500g (or 3–4 handfuls) per square metre.

You don’t have to dig it in but you can easily “scratch” it in with a three-pronged hoe or wire rake. After a few weeks, if it’s moist again, repeat the process and you’ll notice your clay soil is becoming friable, rich and deep. If, after trying all this, you think you have a drainage problem, test for water-logging by digging a hole about 40cm deep and filling it with water. If the water remains there after 24 hours, some underground drainage will be required. If drainage is a challenge, why not create a raised garden bed for your plants instead?

Sandy soil

Sandy soil is characterised by its large, gritty grains of sand. This means the water leaches out easily and the soil dries out fast. So, while sandy soils will not have the drainage problem of clay soils, they are very poor at holding water and/or nutrients. But many of our loveliest native plants grow best in sandy, nutrient-deficient soils, so you might want to think about planting native species in your garden if your soil is the sandy type.

In the vegie patch, improve sandy soil with the regular addition of lots of organic matter. Incorporating organic matter into sandy soil improves its consistency and structure. This will allow roots to spread easily and both water and air to move through the soil. In other words, it becomes richer and more fertile.

Well-composted animal manures or organic composts can be dug in often. Don’t add fresh or uncomposted animal manure directly to your soil as it can damage the precious soil microbes and other living organisms dwelling in it, including worms. All animal manures must be composted with other organic materials before adding to soil.

Loam soil

Loam is often considered somewhere between clay and sand. When you scoop a handful, it will form a ball but crumbles if you squeeze it too tightly. Loam is usually brown and smells like mulch — a rich, earthy aroma. It’s widely considered the ideal garden soil because it’s alluvial and holds moisture and nutrients well.

Acid or alkaline?

Most soils range between slightly acidic and somewhat alkaline. Some are pretty neutral. For optimal plant health it’s essential that the pH of your soil, whether in beds or pots, is suitable for the plants you wish to grow.

In simple terms, pH is a measure of the acidity or alkalinity of soil. The pH scale is from 0 to 14 with neutral at 7; acid soils have a pH of less than 7; alkaline soils have pH of more than 7. A good rule of thumb is that most plants will do OK in soil with a pH of 6–7.
It’s really important to have an indication of your soil’s pH as it will influence your choice of plants and ensure those already established will flourish in the right soil mineral content.

Testing pH level

Testing soil pH is simple and interesting. You can buy a testing kit from nurseries and garden centres. These kits contain everything you need to conduct a soil test. The instruction booklet will take you through the easy steps of collecting a soil sample (take from a depth of about 10cm), combining it with a few drops of liquid formula, dusting it with a powder and comparing the result to the enclosed colour chart. You can then use this information to devise a plan for suitable amendments to adjust the pH of your soil.

Correcting soil pH

To fix or “sweeten” acid soils, you need to raise the pH. This can be done by adding agricultural lime (calcium carbonate) or dolomite (calcium carbonate and magnesium carbonate). Agricultural lime is the cheapest to buy and can be applied at the same time as fertiliser, but it takes some time to break down in the soil so is best applied in advance of planting. The quantity required depends on soil type, the amount of organic matter in the soil and the existing pH level. Your pH testing kit has a dosage guide for your reference.

Dolomite and lime are derived from crushed rock and are alike in that they will increase the pH of the soil and supply calcium, but dolomite also contains magnesium. Magnesium is necessary for plants to produce chlorophyll.

To correct alkaline soils, you’ll want to lower the pH. This is not always easy and the most forceful way is to apply elemental sulphur or flowers of sulphur (which are available at some pharmacies). Again, you should follow the recommendations on the pH soil-testing chart.

Another way to lower pH is to add organic matter, such as compost or composted manure, to your soil annually and mulch with acidic organic mulches, such as those containing pine needles. Adding organic matter slowly lowers your pH over time and improves the soil structure as well as increasing microbial life.

• 

• 

Improving soil

If you are starting a garden or vegie patch from scratch it’s easy to improve the soil when there are no plants around and you have a somewhat blank canvas.

If you are on a new house site, you probably have soil that has been excavated well into the subsoil — in other words, with the topsoil completely removed. This will need replacing with bought organic soil. Don’t be tempted to buy cheap soil or soil blends as they may contain diseased or chemically treated material.

If you are refurbishing a garden, you may need to top up or rejuvenate the existing soils. When digging soil it’s important not to mix the topsoil with the subsoil, especially on clay. You will see the difference between the two — they are usually different colours and textures. You need to add organic matter to help break up clay soil and it should be added to sandy soils to improve nutrient levels and water-holding capacity. You also will need to dig in lots of organic composts and composted animal manures.

Remember that soil is a living environment and it’s vital to familiarise yourself with the type of soil you have in your garden, know how to improve it and put in the effort to maintain it. You will be rewarded with healthy and health-giving fruits, herbs and vegies.