Soils of Substance: A Guide by Jac Semmler

Great gardening starts with the soil. Understanding your soil prepares you, and the ground, for maximum abundance and beauty in the garden over the long term.

What makes up soil?

Soil is a composition of organic matter, minerals, water, air and living organisms. Organic matter is plant, animal and microbial material which has broken down, or is on the way to breaking down.

The amount of organic matter in soil is an indicator of quality. Organic matter can be added to soil by incorporating well-decomposed animal manures, which have had time to break down, leaf mould and worm castings. Air is often of unknown importance to plants; roots absorb oxygen through pockets of air in the soil.

Soil Texture

Soil minerals are classified into three particle sizes: clay, silt and sand. Clay is the smallest particle, and sand the largest. The percentages of these in the soil determine the soil’s texture. This can be sandy (or light), clay (or heavy) or in between – loam. Sandy, clay and loam soils all have different properties and benefits for plants. Sandy soil is free draining, with greater air pockets but less ability to hold water and often lower in natural fertility. Clay soils have greater water holding capacity, more natural fertility (complex but due to the cation exchange of smaller particles), but less drainage – they can get saturated, with fewer spaces for air pockets in the soil.

How to identify your soil texture

As a general test, take a soil sample and form a shape roughly the size of a golf ball in your palm. Then press it into a ribbon, if you can. Sandy soils will not hold a ball shape or form a ribbon, unlike clay soils that can easily form a ribbon. Loam soils are in between. 

If the soil is clay and heavy you might want to use gypsum, which reacts with larger clusters of particles and ‘breaks’ the clay. Gypsum does not react to all clays. Depending on the site, you may manually incorporate sand if heavy soil is an issue, but careful consideration is needed; you don’t want to create buckets in the soil where water collects. Forming mounds and planting plants into them can often be a good solution, enabling root systems are well drained and not waterlogged.  

The incorporation of organic matter can improve sandy soils before planting. Over time, keep an eye on how the soil absorbs moisture as sandy soils have a tendency to become ‘hydrophobic’ where water ‘beads’ off and doesn’t deeply penetrate the earth. Natural wetting agents can assist in breaking hydrophobia if it develops.  

Soil Ecology

Like all miraculous things in nature, the soil will always be greater in sum rather than its parts. Healthy soil not only holds water, minerals, organic matter but is a teeming microcosm of life from worms and insects to intricate webs of fungi, good bacteria and other microorganisms. To see healthy soil under a microscope is a true wonder. In a simple snapshot, this ecology provides networks of communication for plants, beneficial symbiotic relationships, extending root systems and, in essence, breaking down and making the goodness of organic matter available to plants. As a gardener, if you continue to focus on the basics of adding organic matter, gardening naturally and mulching your plants, you will be providing a helping hand to this sophisticated world in the soil. In turn, it will help your plants thrive.

The pH of soil. What is it?

The pH of the soil is simplify a rating of how acidic, neutral or alkaline a soil is. On a scale of 1 – 14, a neutral rating is roughly 6-7. As the rating decreases from 5 to 2, soils increase in acidity. Also, as the rating increases from 8 to 10, soils are increasingly more alkaline.    

The soil’s pH level is significant as it affects which minerals are available to plants from the earth. There are over 21 essential macro and micro-nutrients that plants require to be healthy, i.e. iron, magnesium, nitrogen, potassium etc. Plants can absorb these nutrients from the soil without intervention. This is why a rating of 6-7 is ideal, as all nutrients are available and easily absorbed at that pH level. Neutral soils are typically suitable for most plants, but many plants will tolerate and even prefer acidic or alkaline soils.

How to test for pH

Simple pH testing kits are available from good nurseries and garden stores and are the best way to diagnose pH. Following the instructions on the packet will usually involve taking a few soil samples from your garden.

When taking a soil sample, you want to sample from the root zone of plants – the depth of a typical root system in the ground between 15 and 30cm deep. A dye and powder are then applied to the soil to create a colour that can be matched to a pH chart.

Repeat the soil test in a couple of locations in your garden. It is not unusual in an urban or suburban garden to have different pH levels in different areas depending on the site’s history. The pH level will give a guide on what potential amendments might help prepare your soil before planting, and plants that may naturally thrive. For example, blueberries, proteas and rhododendrons love acidic soils, while currants, lilac, legumes and figs can tolerate more alkaline soils.

Note: if you are in an urban area or the site has a history of manufacturing or industry, and you are wanting to grow food, it’s worth ensuring there are not high levels of heavy metals, i.e. lead, in the soil.

Adding lime is not always the answer.

Adding lime sometimes seems like the ‘go to’ technique for any garden advice on preparing the soil. This can be detrimental if the soil is already alkaline. If alkaline, the lime will increase the alkalinity.  If the soils are acidic, garden and dolomite lime can be beneficial in increasing the pH of the soil closer to neutral.

If you find that soils are alkaline, the best natural option is to increase the soil’s organic matter. ‘Add organic matter’ is true wisdom for all soils, no matter their pH levels. A good proportion of organic matter in the soil always ‘buffers’ a plant from pH extremes, alkaline or acidic. It is one thing that will never be detrimental and will always benefit plants and soil ecology. So if in doubt of texture or pH or just unsure where to start with soils – add organic matter. Organic matter feeds the soil, and for a humble ingredient, its greatness is revered.

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