Soil pH and soil types

Many gardening problems start in the soil. Plants that sulk, yellow or die despite watering and feeding are often struggling with soil that drains badly, dries out, has the wrong pH or lacks organic matter. Understanding your soil's texture and pH helps you choose the right plants and fix problems at the source.

Soil texture

Soil is made of mineral particles of different sizes: sand (largest), silt and clay (smallest). The proportions give soil its texture.

Sandy soil. Gritty, drains quickly, warms fast in spring and is easy to dig, but holds little water or nutrients. Common on coastal plains, including much of Perth.

Clay soil. Sticky when wet, hard when dry. Holds water and nutrients well, but can drain poorly, become waterlogged and compacted, and crack in summer. Common in many suburbs on older landscapes, including parts of Sydney, Melbourne and Adelaide.

Loam. A balanced mix of sand, silt and clay with plenty of organic matter. Drains well but holds moisture: the ideal garden soil.

The squeeze test

Take a handful of moist soil and squeeze it. Sandy soil feels gritty and falls apart. Loam holds together but crumbles when poked. Clay feels sticky and smooth and can be rolled into a ribbon between your fingers. The longer the ribbon, the more clay.

Soil pH

Soil pH measures acidity or alkalinity on a scale from 0 to 14, where 7 is neutral, below 7 is acidic and above 7 is alkaline. pH matters because it affects how available nutrients are to plants. In very acidic or very alkaline soil, plants may show deficiencies even when nutrients are present.

Many vegetables and garden plants grow well in slightly acidic to neutral soil. Some plants, such as azaleas, camellias and blueberries, prefer more acidic soil. Others tolerate alkaline conditions.

Testing. Home pH test kits are cheap and easy to use. Test several spots across the garden and at root depth, because pH can vary.

Adjusting pH

To raise pH (make soil less acidic): apply garden lime (calcium carbonate) or dolomite (which also adds magnesium). Follow label rates, water in and retest after some weeks before adding more.

To lower pH (make soil more acidic): sulphur, acidifying fertilisers and organic matter can lower pH gradually. Very alkaline soils, such as those over limestone, are hard to change much, so it is often better to choose plants that suit them or grow acid-loving plants in pots.

Change pH gradually. Over-liming can cause problems that are hard to reverse.

Improving clay soil

• Add organic matter, such as compost and well-rotted manure, regularly.
• Use gypsum on sodic clays that disperse and set hard; it doesn't help all clays.
• Avoid digging or walking on clay when it's wet, which destroys structure.
• Mulch to protect the surface.
• Build raised beds or mounds for plants that need good drainage.

Improving sandy soil

• Add plenty of organic matter to help hold water and nutrients.
• Mulch to keep soil cool and moist.
• Use slow-release and organic fertilisers, because nutrients leach quickly.
• Address water repellence with organic matter and wetting agents.

Water-repellent soil

Some soils, especially sandy soils with waxy organic coatings, repel water when dry. Water beads on the surface or runs off. Adding organic matter, keeping soil mulched and moist, and using soil wetting agents according to the label all help.

Nutrients

Plants need major nutrients (nitrogen, phosphorus and potassium) and smaller amounts of others such as calcium, magnesium, sulphur, iron and zinc. Many Australian soils are naturally low in nutrients, especially phosphorus. Compost and organic fertilisers supply a broad range. Many Australian native plants, especially in the Proteaceae family (banksias, grevilleas, waratahs), are sensitive to high phosphorus and need fertilisers formulated for natives.

Soil testing services

For a detailed picture of nutrients, salinity or contaminants, laboratory soil tests are available. They are worth considering for new food gardens in older suburbs, where lead from old paint and past fuel can contaminate soil near houses and roads. Raised beds with imported soil are an option if contamination is a concern.

Common mistakes

• Adding lime without testing pH.
• Digging clay when wet.
• Expecting gypsum to fix all clay.
• Using high-phosphorus fertiliser on sensitive natives.
• Leaving sandy soil bare and unmulched.

Drainage test

To check drainage, dig a hole about 30 centimetres deep, fill it with water, let it drain, then fill it again and time how long it takes to empty. If water is still sitting there many hours later, drainage is poor. Choose plants that tolerate wet soil, improve the soil, or build raised beds.

Salty soil

In some areas, soil salinity from rising groundwater, salty irrigation water or coastal spray can damage plants. Signs include white crusts, burnt leaf edges and poor growth. Leaching with good-quality water, improving drainage and choosing salt-tolerant plants can help. Agriculture departments have information on salinity in their regions.

Soil and climate across Australia

Australia's soils are among the oldest and most weathered in the world, which is one reason many are naturally low in nutrients, especially phosphorus. Soils differ greatly between regions: deep sands on the Swan Coastal Plain around Perth, heavy clays in parts of western Sydney and Melbourne's north and west, alkaline soils over limestone in parts of South Australia and western Victoria, and rich volcanic soils in some areas. Knowing the soil typical of your area, and then testing your own garden, helps you make good choices.

Improving soil for vegetables

Vegetables need fertile, crumbly soil. Each season, dig in compost and well-rotted manure, keep beds mulched, avoid walking on beds by using paths or stepping stones, and grow green manure crops between seasons. Raised beds filled with good soil are a quick fix where native soil is very poor, contaminated or waterlogged.

Signs of pH problems in plants

Plants often show when soil pH is wrong. Yellowing between the veins of young leaves, particularly on azaleas, gardenias, camellias and citrus, can mean iron or manganese is locked up in alkaline soil. Poor growth, stunted roots and leaf discolouration in very acid soil can point to aluminium or manganese toxicity, or a shortage of calcium and magnesium. Hydrangeas show soil pH in their flower colour: many varieties flower blue in acid soil and pink in alkaline soil. These symptoms can have other causes, so test the soil before treating it.

How fast pH changes

Lime and sulphur work slowly, over weeks to months, and fine products act faster than coarse ones. Make modest adjustments, re-test after a season, and repeat if needed rather than applying large amounts at once.

Key points

Work out whether your soil is sandy, loamy or clay, test pH, add organic matter every season, adjust pH slowly, use gypsum only on suitable clays, mulch and choose plants that suit your soil.