How to improve sandy soil

Two different problems

Sandy soils are common along the coasts of Australia and New Zealand. Perth's Swan Coastal Plain is the best-known example, and there is plenty of sand in coastal South Australia, Victoria and southern Queensland. They cause trouble in two separate ways, and it helps to fix them separately:

• They hold very little. Sand grains are large, so the gaps between them drain freely. Water and dissolved nutrients go straight past the roots. This is a permanent property of sand, and only adding fine material (clay and organic matter) changes it.
• They can refuse to wet at all. In many sands a waxy coating from decomposed plant matter builds up on the grains and makes them water-repellent. Water beads, runs off to the lowest point or tracks down a few channels, and most of the root zone stays dry even after a long watering. This part is treatable with a wetting agent.

Test for water repellence

Scrape back the mulch and take some dry soil from the top 2–3 cm. Put a few drops of water on it from a dropper or teaspoon and count how long they take to soak in:

Another sign is dry soil under a green lawn or a mulched bed the day after heavy rain. Dig a small hole with a trowel and look: if the soil is dry 5 cm down, the water never got in.

Wetting agents

A soil wetting agent is a surfactant, a detergent-like compound that lets water spread over the waxy coating and soak in. It does not fix the sand. It makes the water you already use reach the roots, which is why water utilities in dry regions recommend it.

• When: Western Australia's Water Corporation recommends applying a wetting agent at the start of winter and in early summer, and as directed on the product.
• How: granular products last longer and suit beds and lawns; liquid hose-on products act faster. Either way, water it in straight away. The Water Corporation's advice is to keep watering until it foams, which shows it has activated.
• How long: most products wear off within a few months as soil microbes break them down. That is why the repellence comes back, and why the long-term fix is clay and organic matter.

Adding clay

Even a small amount of clay mixed evenly through sand makes a lasting difference. The sand holds more water and nutrients and is less prone to repellence. WA's clay suppliers sell two main types. Bentonite is a swelling clay with very high water and nutrient holding. Kaolin is a finer, non-swelling clay that spreads through sand more evenly.

More is not better. Heavy single doses of bentonite form sticky lumps that do not spread through the sand. Mix it evenly, and keep the rate in the range above.

Compost and mulch

• Compost: 50–100 mm dug into the top 20–30 cm when you make a new bed, and 25–50 mm a year on top after that. Organic matter breaks down fast in warm, well-aerated sand, so sandy gardens need compost every year, not every few years. The compost calculator converts the depth into bags or cubic metres.
• Mulch: the Water Corporation recommends 5–10 cm of a coarse, chunky mulch rather than a fine one, which lets water through and can save up to a fifth of garden water. Fine mulches can themselves become water-repellent when they dry out. The mulch calculator works out the volume.
• Avoid pure potting mix as a "soil improver" in garden beds. It is bark-based, often becomes water-repellent itself when dry, and breaks down quickly.

Watering sandy soil

Sand holds little water per centimetre of depth, so the only way to store more is to wet it deeper and let the roots follow.

• Deep and infrequent beats a daily sprinkle. A short daily watering wets the top few centimetres, most of it evaporates, and roots stay shallow.
• Drippers under mulch put water exactly where the roots are and avoid runoff from a repellent surface.
• Water in the morning, and check how deep it went with a trowel an hour later. That tells you more than any watering chart.
• Check your local rules. Perth and much of WA have rostered sprinkler days and a winter sprinkler switch-off; drip and hand watering have more flexible rules.

If you collect rainwater for the garden, the rainwater tank calculator shows how many weeks of watering your tank holds.

Worked example: a new vegetable bed on Perth sand

1. A 4 m × 1.5 m bed, 6 m². The drop test takes over a minute, so the sand is strongly repellent.
2. Wetting agent now, at the label rate, watered in.
3. Bentonite at 4 kg/m²: 24 kg, spread evenly.
4. Compost at 75 mm: 6 × 0.075 = 0.45 m³, which is 450 L or 15 × 30 L bags. Spread it over the clay and mix both into the top 20 cm.
5. Drip line, then 75 mm of coarse mulch. Repeat the wetting agent at the start of winter and add 25–50 mm of compost each spring.

If building up is easier than digging in, a raised bed filled with a good soil-based mix sidesteps the sand for shallow-rooted crops. See the best soil mix for raised beds.

Questions