Soil and mulch

Soil decides more garden outcomes than climate does. Two blocks in the same street can have free-draining sand and heavy reactive clay, and they need different plants, different bed construction and different watering. Before you choose anything, find out what you have — it takes about ten minutes and a bucket of water.

Identifying your soil

The ribbon test. Take a small handful of moist soil from about 100 mm down, work it into a ball, then squeeze it between thumb and forefinger to push out a ribbon. If it will not form a ball at all and feels gritty, it is sand. If it forms a ball but the ribbon breaks at under about 25 mm, it is a sandy loam. A ribbon of 25 to 50 mm is a loam. A smooth ribbon over 50 mm that feels slick rather than gritty is clay.

The drainage test. Dig a hole 300 mm deep and 300 mm across. Fill it with water and let it drain away completely, then fill it a second time and time how long the second fill takes to disappear. Under two hours is free draining, sometimes excessively so. Two to four hours is good. Over four hours means you have a drainage constraint that will decide your plant list. Over twelve hours means you need to raise beds rather than plant into the existing grade.

The dig test. Push a spade in to its full depth. Note where resistance changes: that is the boundary between topsoil and subsoil. If you hit a hard layer at 100 or 150 mm, you have a compacted pan — common on new estates where topsoil was spread over ground that heavy machinery had driven across. That pan needs breaking before anything else is worth doing.

Sand, loam and clay in practice

Sandy soils drain fast, warm up early in spring and are easy to dig. They also lose water and nutrients quickly, because there is very little for either to hold onto. The remedy is organic matter, added every single season: compost, aged manure, composted leaf litter. It breaks down and disappears, which is why it has to be repeated rather than done once. Mulch matters more on sand than anywhere else. Banksia spinulosa, Correa alba, Kennedia prostrata, Carpobrotus and Hakea laurina all suit sand naturally.

Loams hold moisture while still draining, and are what most garden advice quietly assumes you have. If you have loam, the main risks are compaction from foot traffic and the slow depletion of organic matter under continuous cultivation. Keep beds narrow enough not to step in, and add compost each season.

Clay soils hold water and nutrients well — that is their advantage, and it is a real one. The problem is structure: the particles are extremely fine, they pack tightly, and when saturated there is no air left between them. Roots suffocate. Worked when wet, clay smears and sets like concrete. Worked when bone dry, it comes up in bricks. There is a narrow window when it is just moist, and that is the only time to dig it. Callistemon citrinus, Melaleuca thymifolia, Carex appressa and Lomandra longifolia all tolerate it. The full treatment is in working with clay soil.

Water-repellent soil

Sandy soils in southern Australia commonly become water-repellent, or hydrophobic, after a dry summer. The symptom is unmistakable: water applied to the surface beads and runs off, or soaks in for a few seconds and then sits, and when you dig down after watering the soil 50 mm below is completely dry. A waxy organic coating on the sand particles is repelling the water.

The practical fixes are mechanical and cultural. Break the surface up with a fork before watering so water has a way in. Water slowly and repeatedly rather than in one burst — several short cycles let each one soften the surface for the next. Keep the soil covered with mulch so it never dries out to that point again, and build organic matter into the top 150 mm over successive seasons. Ground that is never allowed to dry out completely does not become repellent in the first place.

Compaction

Compaction is the most under-diagnosed soil problem in domestic gardens. It comes from foot traffic, from building work, from mowing wet lawn, and from machinery during construction. Compacted soil has no pore space, so it holds neither air nor water properly, and roots simply do not enter it.

You can find it with a screwdriver: push a long screwdriver into moist soil in several places and note where it stops easily. Sharp resistance at a consistent depth across an area indicates a pan.

Relieving it means physically opening the soil without turning it over. Push a garden fork in to full depth every 200 mm and lever it gently back and forth to crack the profile, working across the area. On lawns, do the same and brush a sandy top-dressing into the holes. Then keep off it: the only lasting solution to compaction is not walking on the soil, which is why vegetable beds should be no wider than you can reach across.

Organic matter, compost and manure

Organic matter improves every soil type, for different reasons. In sand it holds water and nutrients that would otherwise drain past. In clay it separates the fine particles and creates the pore space that clay lacks. It also feeds soil life, and soil life is what maintains structure over time.

Compost is the most useful form because it is stable, weed-free if made properly, and can be applied at any time. Aged manure works well but must genuinely be aged — at least six weeks broken down before planting, and never applied fresh to a bed about to be sown, where it burns seedlings and causes forked roots in carrots. Composted leaf litter, or leaf mould, is slower and lower in nutrients but excellent for structure, and it costs nothing if you rake your own leaves. Making your own is covered in making compost at home.

Where a fertiliser is used, a slow-release complete fertiliser applied at the start of the growing season is enough for most home gardens. This site does not recommend brands or publish dosage schedules. Many Australian native plants, particularly Banksia, Grevillea and Hakea, are adapted to soils very low in phosphorus and do poorly when fed with general-purpose fertilisers; the safer approach with those species is compost and mulch, and nothing else.

Mulch: what it does

Mulch does four things. It cuts evaporation from the soil surface substantially. It keeps soil temperature down, which protects the fine surface roots most plants depend on. It suppresses weed germination by keeping light off bare soil. And, if it is an organic mulch, it slowly breaks down and feeds the soil beneath it.

The right depth is 50 to 75 mm settled, and it should be kept 50 to 100 mm clear of stems and trunks. Mulch piled against bark holds moisture there and causes collar rot, which is a slow and entirely avoidable way to lose a plant. Deeper than about 100 mm and mulch starts to shed light rain rather than let it through.

Choosing a mulch

Coarse bark and woodchip is the general-purpose choice. It lasts, it lets water through, and it does not mat. Coarser grades last longer and shed less in wind. This is the default for shrub and tree beds in every zone.

Straw and lucerne break down fast and feed the soil quickly, which suits vegetable beds where the mulch is replaced each season. They are too short-lived for permanent plantings.

Composted mulches sit between the two and are good where you want both cover and soil improvement, though they break down within a year and need topping up.

Gravel and pebble do not break down at all and suit arid gardens, where an organic mulch would decompose or blow away. They do not feed the soil, and they hold heat, so they are wrong under plants with soft foliage.

Fine mulches — sawdust, fine bark fines, grass clippings applied thickly — are the one category to avoid. They mat together into a layer that repels water and starves the soil beneath of air. If you use grass clippings, spread them thinly and let each layer dry before adding another.

Timing matters as much as choice. Mulch in early spring in temperate, cool and arid gardens, before the dry months. Mulch at the start of the dry season in the tropics, and again before the wet. Renew twice a year in the tropics and subtropics, where organic mulch breaks down two to three times faster than it does in the south. The application detail is in mulching properly.