Optimising Soil pH

To achieve a healthy productive garden, you must know about soil pH. The health of the soil — the absorption of nutrients by the plants — depends on the pH, which directly affects the development and the harvest.

You may be gardening in a vegetable patch, an autumn vegetable garden, or testing the idea of companion planting tomatoes, but the right pH can spell the difference. Whether you use an organic fertiliser or a natural soil booster, understanding pH will help your plants thrive.

To learn more about what soil pH is and how it will influence the state of your plants, read on to find out how to get the balance just right.

What is Soil pH?

Soil pH, simply defined, is the measurement of soil acidity or alkalinity.

pH relates the acidic or the alkaline nature of something which registers zero–14 points where the halfway point 7 is neutral. A value of less than 7 is acidic and that of above 7 is alkaline (base).

To make it simple, imagine it to be a colour chart:

  • Red as acidic

  • Yellow-to-green as neutral

  • Blue to violet as alkaline

The majority of plants like moderately acidic environments of between 5.5 and 7.5. Suppose your soils are at 5 it is too acidic; 8 too alkaline. Regardless, you will require a soil treatment to get it back into the ideal range.

How to Test Soil pH

Excessive acidity or alkalinity of soil makes it impossible to have plants absorbing nutrients.

It is always better to test before applying further fertiliser — even recycled organic fertiliser like biofertiliser.

A self-test kit can be purchased in nurseries or hardware shops and instructions followed. Growth is balanced at a pH between 6 and 7, so this is an ideal pH range in most cases.

Reducing pH in the Soil

Some plants such as azalea, rhododendrons, camellia, and blueberry are acid-loving plants.

pH of soil can be reduced by adding:

  • Compost

  • Manures

  • Leaf litter

  • Mulch

  • Natural pH correction with a biofertiliser that is biologically active such as Food2Soil

In the case of extreme cases, the powdered sulfur or aluminium sulfate can be beneficial but make sparse use of it.

Raising the Soil pH

Most of the soils in Australia are already acidic hence the increase in pH is a common practice.

Try adding:

  • Lime

  • Bone meal

  • Wood ash

  • Ground up oyster or eggshells

The techniques also enhance alkalinity besides being a soil booster to the soil due to minerals such as calcium.

Factors Affecting Soil pH

The following factors affect the soil pH:

  • Minerals of the parent materials

  • Rainfall and temperature (Acidification can be caused by high rainfall)

  • Breakdown of organic matter

Leaching also occurs to create natural long-term acidification in moist climates.

Soil Amendments and pH

Soil pH and alkaline Soil

Plants that love alkaline soil thrive in alkaline soil but no-one likes extreme PH. When the pH value is above 7, it is deemed to be alkaline and is referred to as sweet soil and the nutrients supply is restricted. A few of the plants that like such a condition include hydrangea, gardenia, lavender, sweet pea, salvias, clematis, cauliflower, parsley, oregano, celery, cucumber, cabbage, beets, asparagus, dandelions and chickweed.

Clay Soil and pH

Clay soil is invariably alkaline more or less.

Lime and pH

Lime is a natural mineral but it possesses alkaline character hence it is applied to correct acidic soil and supply calcium.

Adding Calcium to Soil

There are various ways in which calcium can be introduced to any soils that lack it and they include eggshells, crushed oyster shells, lime, gypsum and bone meal. The calcium can also be added in lower amounts, through wood ash of hardwood fires and diatomaceous earth.

When amending a soil with a source of calcium, be sure to remember your soil pH since most tallies of calcium will render a soil alkaline in nature. As a matter of fact, most people put lime around chook yards to neutralise the acidity of the accumulation of faeces and the overall chook detritus!


Now you know what soil pH is and how to take control of it, so it is time to set your plants up in the ideal environment — whether it be a green veggie garden, a beautiful flower garden, or a bountiful autumn vegetable garden.

Using the right organic fertiliser, a targeted biofertiliser, and a quality soil booster will give your plants the nutrients they need while keeping your soil’s pH in the perfect range.