Healthy soil: healthy garden

Soil colour

Colour is a simple method of classifying soil. Black/dark brown soil usually indicates the presence of decaying organic matter so is generally fertile. Pale brown/yellow soil often indicates that organic matter and nutrients are low and this generally means poor fertility and structure. Pale soil needs plenty of organic matter and mulching.

Red soil usually indicates extensive weathering and good drainage, but often needs nutrients and organic matter. The red colour is due to the oxidising of iron compounds ('rusting') in the soil.

Soil composition

Soil is composed of mineral particles, organic matter, water and air.

Mineral particles

Mineral particles in soil are defined by their size. Sand is 0.02–2 mm in diameter, silt is 0.002–0.02 mm diameter, and clay is less than 0 .002 mm diameter. The mixture of these particles gives soil its texture. The ideal texture for plants is loam, a mixture of clay, silt and sand.

Organic matter

Organic matter is the lifeblood of productive gardening soil. It stores nutrients for plants, improves the structure of the soil, holds moisture and provides food for earthworms and microorganisms which further improve soil fertility and structure. Organic matter is any material that was once living, and includes leaf litter, compost, mulch, lawn clippings and animal manures. A soil without organic matter is a starving soil.

Water

A moist but freely draining soil holds 25–35% water by volume. Water drains through large spaces (macropores) in the soil such as cracks, earthworm tunnels, and spaces between lumps of soil (aggregates). Water is held in the soil in tiny spaces (micropores) within soil aggregates where it is used by plant roots. Loam is a good gardening soil because the sand in it allows water to drain freely while the silt and clay stores water in micropores. Organic matter also helps retain moisture in soil.

Air

A moist but freely draining soil has 15–25% air by volume. This air is held in the macropores and micropores and provides oxygen needed for roots and for soil microorganisms. When these pores fill with water and the soil becomes saturated, most plants die because there is not enough oxygen available for their roots.

Soil structure

Particles in soil are bound together by clay and organic matter into aggregates. These are the crumbs or lumps the soil breaks into naturally when you dig. The technical term for these is peds.

Well-structured soil has adequate space (pores) between the aggregates to allow water and air to enter the soil and drain easily, but holds enough water in the aggregates to maintain plant growth.

You can damage your soil structure by too much cultivation. Soil particles break off the aggregates and block the soil pores, forming a crust which water and air cannot enter, and seedlings cannot break through. Organic matter can help improve structure.

Soil pH

Soil pH is the measure of acidity or alkalinity in the soil. Below pH 7 is acid, above is alkaline and pH 7 is neutral, like pure water.

pH affects the availability of nutrients in the soil. Below pH 4.5, aluminium and manganese can rise to concentrations toxic to many plants.

North Coast soils are naturally acid, around pH 4.5 (CaCl lab test) or pH 5.0 (kit test). They need to have lime added to bring them up to at least 5.5 (kit test). Fine grade Ag lime is best. Though builders' lime ('hydrated' lime) can be used, it can burn plants and your skin. Lime should be mixed into the soil at least a week before planting.

You can test your soil pH with simple field test kits available from most farm supply stores. Some plants, such as azaleas, thrive in acid soils. However, most vegetables and plants will benefit from a pH of 5.5 to 6.5 (kit test). At this level, most soil nutrients will be available to plant roots. Hydrangeas can be an indicator of pH levels. In many soils, blue hydrangeas indicate acidity and pink hydrangeas indicate a more neutral or alkaline soil.

Soil nutrients

Soil nutrients are provided by organic matter (rotting leaves, dead roots, and so on) and fertilisers. If you use water-soluble fertilisers, apply them in small amounts frequently so that plants can use the nutrients as they are applied. Leaching of excessive applications of some soluble fertilisers, especially nitrogen fertilisers, can acidify your soil, burn plant roots and leach into water supplies.

Native plants are often very sensitive to phosphorus, so only use fertilisers low in phosphorus or special formulations for native plants.

Organic fertilisers such as manures release their nutrients slowly so that plants take up all the nutrients. If you use plenty of organic matter and compost, your soil will not need many other nutrients.

You can have your soil tested to find out the nutrients it contains.

Soil organisms

Soil organisms can represent 3% of the dry matter in your soil. They include bacteria, fungi, mites, ants, millipedes, beetles, earthworm, slugs and snails. Soil organisms derive their energy and nutrients from breaking down plant and animal material. When digesting this material they release oxygen and mineral nutrients that plants can use. Whey they die they decompose and release more nutrients, so are valuable contributors to soil fertility. They also help improve soil structure with their tunnelling and burrowing, and by converting fine particles into larger crumbs. Soil organisms need large supplies of organic matter to live on, warmth (but not extreme heat), moisture., oxygen and a near-neutral soil pH.

Troubleshooting

My soil is either rock hard or a bog. What can I do?
If it is in a garden bed, see if you can raise the level of the garden bed to provide better drainage. Dig sand into clay whenever you work it and you will see a gradual improvement. Add plenty of organic matter and mulch, as this helps open up clay. With lawns, leave the lawn clippings where they are mown, as this provides organic matter, and pour sand down the cracks which open up in the dry periods. Gypsum will improve sodic clay soils, but these soils are uncommon on the North Coast.

My soil is very acid. How can I improve it?
Add lime every year. It is most effective if cultivated into the soil. A pH test will tell you just how acid your soil is and how much lime to apply. Note that gypsum does not change soil pH.

Why does water run off my soil without soaking in?
This indicates your soil has formed a crust. Small soil particles have blocked the pores between the aggregates and the water cannot soak in. This may have been caused by: the soil drying out and breaking up, too much cultivation, which also breaks up soil aggregates, or low levels of organic matter. You may have to cultivate the soil gently to get rid of the crust. Then thoroughly wet it so that the water soaks in easily, and cover with mulch. The mulch will stop the soil drying out and crusting. You will find the water will soak through the mulch and into the soil.