Are your plants looking a little sad? Are they yellow or sickly looking, no matter how much you feed, care or even talk to them? The answer often lies beneath, in the soil below. Soil is the foundation upon which plants grow, so if there is something not quite right with it, your plants will suffer too. So, it makes sense to understand how to take care of it. This includes understanding soil pH – a measure of alkalinity or acidity in the soil – as this significantly impacts soil quality and its uses.

What is soil and what makes it healthy

Soil is more than just dirt. It is a living organism, made up of microorganisms and larger soil organisms, minerals, organic matter, air and water. The minerals that make up the soil can heavily impact the levels of organisms, organic matter, air and water in the soil.

Soil minerals include clay, silt and sand and it is the varying percentages of these minerals that make up soil texture. For example, a heavy clay soil has very little sand and silt and due to the structure and minute size of the clay particles, there is very little room for air, water and soil organisms. Without these key ingredients, the soil quality is considered poor for growing. When the mix of these elements are well-balanced, it makes for a highly fertile soil that is perfect for planting. Even if your soil is considered poor – high in clay or high in sand – it can always be improved (see The Secrets in the Soil). Soil pH also plays a part in soil health and if it’s not in the optimum or ideal range, plants will not grow well.

What is soil pH?

pH stands for ‘potential of hydrogen’ and refers to the amount of hydrogen found in the soil or substance. The more hydrogen, the more acidic and conversely, the less hydrogen, the less acidic or more alkaline. pH is measured on a scale from 1 to 14, with 7 being neutral and anything above 7 considered alkaline and anything below 7 considered acidic.

Most soil pH sits between 3.5 to 10, with 6.5-7.5 considered neutral. Most plants thrive in the neutral pH and tend to suffer when it’s too high or low. However, there are some plants that prefer or can tolerate growing in ‘sweet’ (alkaline) or ‘sour’ (acidic) soil. Ensuring plants are growing in their preferred pH range is key for long term plant health and productivity.

What determines soil pH

Soil pH can be quite varied. It primarily depends on what parent material the soil was formed from (e.g., limestone, granite, basalt and many others) and what changes it has been subjected to over time, such as climate, farming, vegetation, fertiliser practices, mining or topography. These activities generally make the soil more acidic.

Most of the soils in Australia are acidic, except for most of South Australia and parts of Western Australia, where soils are typically alkaline.

What does soil pH mean for my plants

pH can affect a plant’s ability to absorb vital nutrients from the soil. If pH is too acidic or alkaline, this can stunt or retard root growth and consequently, restrict water and nutrient uptake. Additionally, if the pH is extreme, it makes major plant nutrients (nitrogen, phosphorus and potassium) unavailable. Without adequate nourishment, plants will decline and potentially die.

The addition of fertilisers and trace elements may help, but these are only temporary solutions. It’s best to check the soil pH and fix any issues at the source.

How to test soil pH

The easiest way to test soil pH is by using a soil pH tester or soil pH kit, which you can purchase at your local nursery or garden centre. You can also use a soil pH meter. It’s best to take a few samples from the area to have a good understanding of your soil pH. Ideally, it should sit between 6.5-7.5, but if it’s on either side of this range, then you will need to improve with specific soil conditioners, like liquid sulfur or lime.

It’s important to note that adjusting soil pH is not a quick fix, but may be an ongoing process to reach the desired pH. The continual cultivation of soils and addition of especially synthetic fertilisers can make soils more acidic, so it’s a good idea to check your soil pH regularly and adjust as needed.

How to make soil acidic or lower soil pH

If your soil is sitting above 7.5, then you will need to lower or acidify it. The most effective way to lower soil pH is to add Yates Soil Acidifier Liquid Sulfur, which contains elemental sulfur (S). Soil bacteria convert the sulfur to sulfuric acid and thereby, help lower the soil pH. This is highly dependent on soil bacteria, so it’s best to apply when conditions are warm. For best results, apply every 4 weeks until the desired pH is achieved. Take care not to overapply – this will not make the process faster and it can lead to issues with sulfur toxicity and cause the pH to drop too low.

Adding aluminium sulfate is a fast way to help decrease soil pH. However, it is easy to overapply and consequently, can lead to issues with aluminium toxicity. A build-up of aluminium in the soil will significantly impact plant health and vigour, so it’s not recommended as a long-term solution. Similarly, adding vinegar to lower soil pH is not recommended. It causes localised sharp pH drops, but the adjustment is not lasting as the organic acids are eventually broken down by various soil microbes.

The sulfur soil acidification method is the best option as this gives a more steady and longer-term pH reduction.

How to make soil more alkaline or increase soil pH

If your soil pH is sitting below 6.5, then you will need to increase it or make it more alkaline. Adding lime (calcium carbonate) or dolomite (calcium carbonate and magnesium carbonate) is the most effective way to amend soil acidity. When calcium or magnesium carbonate is applied to acidic soil, the ions separate into calcium/magnesium, and carbonate. Calcium and magnesium bind to the soil – this is ideal as these elements are essential for plant health – and are eventually taken up by the plant when required. Carbonate binds with the excess hydrogen in the soil, to form water and carbon dioxide, and thereby helps neutralise the acid in the soil.

The easiest way to lime the soil is with Yates Hydrangea Pinking Liquid Lime & Dolomite, which is a mix of calcium carbonate and magnesium carbonate. Apply every 3-4 weeks until the desired pH is reached.

Acid loving plants

These plants like acidic soil and grow better when the pH is lower. Ideally, soil pH should be between 5-6.5. Any lower and this can inhibit nutrients and stunt plant growth.

  • Azaleas
  • Rhododendrons
  • Gardenias
  • Camellias
  • Blueberries
  • Blue hydrangeas

Azaleas

Blueberries

Alkaline tolerant plants

These plants are more tolerant of sweeter or alkaline soils, so will still thrive if the pH is slightly on the higher side. A soil pH range of 7.5-8.5 is best. Any higher and this will impact plant health.

  • Buddleja davidii
  • Lavender
  • White correa (Correa alba)
  • Viburnum tinus
  • Pride of Madeira (Echium candicans)
  • Cockie’s tongue (Templetonia retusa)
  • Rosemary
  • Bauhinia

Echium

 

Lavender

 

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