Are your plants looking sad, yellow or sickly, no matter how much you feed or care for them? The answer often lies in the soil. Soil provides the foundation for plants to grow, so if the soil isn't healthy, your plants will suffer too. Soil pH – a measure of alkalinity or acidity in the soil – plays a key role in determining how healthy plants are, so it's important to know your soil pH and adjust the pH when required.
Soil is more than just dirt! It is a living organism, made up of minerals, organic matter, air and water, plus micro-organisms and larger soil organisms like earthworms. Soil minerals can heavily influence the levels of organisms, organic matter, air and water.
Soil can be divided into three main different types. The type of soil relates to the size of the soil particles. Smallest to largest soil particle size is clay, loam/silt and sand. This is also referred to as soil-texture because it affects how the soil feels.
Clay soils are made up of small particles, with minimal spaces between those particles. They store water well, often too well for good drainage and aeration. Heavy clay soils can be difficult to dig when they’re too wet or too dry. Clay soils are able to hold good amounts of nutrients.
Loam soils fall between the extremes of sand and clay and are mixtures of coarse and fine particles. They are divided into categories such as sandy loam (more sand than clay) and clay loam (more clay than sand).
Sandy soils contain large particles with large spaces, called pore spaces, between them. They drain readily, have good aeration and are easy to cultivate. They are often called ‘light’ soils. However, very sandy soils are not effective at retaining water and nutrients.
Healthy soil is rich in organic matter, has lots of beneficial organisms, is well-drained but still holds enough moisture to support plant growth and is well-aerated. No matter what soil type you have, it can always be improved (see Beginner's Guide to Soil & Soil Improvement). pH plays a significant part in soil health and if it's not in the ideal range, the result will be poor plant growth.
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.
Soil type (clay soil or sandy soil) does not determine acidity or alkalinity and soil pH can be quite varied. It primarily depends on what parent material the soil was formed from (e.g., limestone, granite, basalt) 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.
Many of the soils around Australia are acidic, except for much of South Australia and parts of Western Australia, where soils are typically alkaline.
pH can affect a plant's ability to access essential nutrients from the soil. A common example of pH affecting nutrient availability is with iron in alkaline soils. When soil pH is above 7, the uptake of iron into young leaves is adversely affected, leading to leaves turning yellow with dark green veins (known as lime induced chlorosis). Additional nutrient deficiencies in alkaline soil include manganese, boron, copper and zinc.
If the pH is extreme, particularly in acidic conditions, it can also make major plant nutrients (nitrogen, phosphorus and potassium) unavailable. Without adequate nourishment, plants will decline and potentially die. Too acidic or alkaline soil can also stunt or retard plant root growth and consequently, restrict water and nutrient uptake.
The addition of fertilisers and trace elements may help, but these are only temporary solutions. It's best to check the soil pH and make adjustments to bring it closer to neutral.
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 garden centre or hardware store. You can also use a soil pH meter. It's best to take a few soil samples from different places around the garden to have a good understanding of the 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 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 synthetic fertilisers can make soils more acidic, so it's a good idea to check your soil pH regularly and adjust as needed.
If your soil has a pH of 7.5 and above, then you will need to lower or acidify it. An effective way to lower soil pH is to apply Yates Sulfur Soil Improver Granules. These granules contain sulfur, helping to lower soil pH, plus bentonite, which improves soil structure and water retention. These granules are easy to handle and apply and are less messy and dusty than traditional sulfur powders. Reapply every 4 weeks until the required soil pH is achieved.
Another way to lower the soil pH (make it more acidic) is to use 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 four weeks until the desired pH is achieved. Take care not to over apply – this will not make the process faster and it can lead to issues with sulfur toxicity and cause the pH to drop too low.
Using aluminium sulfate-based products, such as Yates Hydrangea Blueing Liquid Aluminium Sulfate, is another way to help temporarily decrease soil pH. It is important to follow the directions for use to reduce the chance of overapplication and potentially toxic levels of aluminium. To promote blue hydrangea flowers, start applying every 3 to 4 weeks after winter pruning.
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.
Soil acidification with sulfur is the best option as this gives a more steady and longer-term pH reduction.
The list of plants below 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 nutrient availability and stunt plant growth.
If your soil pH is sitting below 6.5, then you have an acid soil and will need to increase the pH (make it more alkaline). This is also known as 'sweetening' the soil. 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.
An easy way to add both lime and dolomite to the soil is to use Yates Lime & Dolomite Soil Improver Granules. These granules contain a special 3 in 1 blend of lime, dolomite and molasses. The granules are easy to handle and apply and are less messy and dusty than traditional lime and dolomite powders. Reapply every 4 weeks until the required soil pH is achieved.
Another way to add lime to the soil and increase the soil pH is with Yates Hydrangea Pinking Liquid Lime & Dolomite, which is a liquid mix of calcium carbonate and magnesium carbonate. Apply every 3-4 weeks until the desired pH is reached.
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.
Beginner's Guide to Soil & Soil Improvement
Beginner's Guide to Soil & Soil Improvement
