Soil Types

Pore spaces can be classified as macropores, or micropores. Macropores are bigger gaps, as the name suggests - usually larger than 75 µ. These more open pores are critical for drainage and aeration. They allow free travel of water, oxygen, plant nutrients and microbial life through soil. Highly macroporous soil drains freely, but on the flipside, water and nutrients can be quickly depleted. This type of soil tends to erode quickly.

Micropores are tiny enough to impede the passage of water and oxygen, so they provide water retention to soil. But, you can have too much of a good thing: if soil is made up mostly of micropores (e.g. heavily compacted clay soil) it can hold onto moisture so it's constantly waterlogged. This can prevent oxygen from circulating to plant roots, which can drastically restrict growth. This type of anaerobic environment really doesn't suit beneficial soil microbes, so plants struggle even more without their symbiotic little friends.

Good garden soil contains a balance of macroporous and microporous particles, for the best of both worlds.

Beneficial soil microorganisms under a microscope.

Why Organic Matter in Soil is Important

There's another critically important component of soil: mixed in between all the mineral particles, water and air are particles of organic matter. This description includes all of the decaying dead material derived from plants and animals, along with the multitudes of living microorganisms found in soil.

For gardeners, the most useful examples of organic matter from plants are compost, or mulches (bark chip, straw, etc.). Organic matter derived from animals includes manure, or blood and bone fertiliser.

Adding any type of organic matter to garden soil has multiple benefits:

• It enables the soil to hold more moisture.
• It enables the soil to hold more nutrients.
• It improves the structure of all soil types.
• It improves the drainage of heavy soils.
• It provides plants with slow-release nutrients.
• It nourishes and attracts beneficial microbes and earthworms.

Organic matter naturally breaks down in the soil, so it should be regularly replenished in a productive garden.

Assess the Soil Type in Your Garden

You can identify what type of soil you have in your garden, by feeling it in the palm of your hand when the soil is slightly moist. Give it a squeeze, crumble it up, then consult this list to settle on the closest description:

• Sandy soil won't stick together at all. It feels coarse and gritty.
• Sandy Loam sticks together a little, but is very friable (easily crumbled) and feels slightly gritty.
• Loam sticks together, is friable but doesn't feel gritty.
• Clay Loam sticks together, is slightly friable but also holds its shape when squeezed.
• Clay soil sticks together, isn't friable at all, holds its shape when squeezed and feels sticky to the touch.

1. Sandy Soils: How to Build Them Better

Sandy soil - won't stick together at all when squeezed. It feels coarse and gritty.

Sandy Loam soil - when moist, it sticks together a little when squeezed, but is very friable (easily crumbled) and feels slightly gritty.

2. Loam Soils: A Gardener's Happy Place

These are both examples of Loam soil - it sticks together when squeezed, is friable (crumbly) but doesn't feel gritty.

3. Clay Soils: Improve Fertility, Make Them Easier to Live With

Clay Loam soil - when moist it sticks together, slightly friable (crumbly), but holds its shape when squeezed. Forms large clumps when it dries out.

Clay soil - sticks together when moist, holds its shape when squeezed and feels sticky to the touch. Clay isn't friable at all. When dry it forms large lumps that are difficult to break up.

Open Up the Structure and Porosity of Clay

Adding Gypsum is an effective way to improve and 'condition' clay soil. The calcium content in gypsum interacts with clay particles in a process called 'flocculation', which causes the tiny clay particles to bind together to form crumbs. These crumbs make soil more porous and open, allowing better air, water and nutrient movement through the soil. You can also use garden lime to break up clay soil, but gypsum has a big advantage: unlike lime, gypsum is pH-neutral, so it doesn't increase soil acidity or alkalinity.

Yates Gypsum Clay Breaker Soil Improver Granules is a great remedy for hard-to-dig soil. It contains a blend of gypsum, zeolite, lime and molasses, designed to help open up the structure of clay and nourish microorganisms. The granules are much less messy and dusty than traditional powdered gypsum. Apply gypsum at a rate of 260g per square metre. Ensure you rake it into the topsoil and water it in well.

The Easy and Effective Way to Add Organic Matter to Clay

Clay soils especially benefit from added organic matter, because it improves their structure: opening up the spaces between close-packed clay particles allows better air and water movement.

The traditional method to amend clay is to dig in a generous amount of compost along with gypsum (as described above). Fork them both all through the soil and mix them in well. Cover the soil surface with a layer of organic mulch, like bark chip or pea straw, then water thoroughly. Continue to water the soil over the next couple of weeks, to assist the organic matter to break down. After two or three weeks, use a trowel to explore the soil, to check that the compost has started to decompose. If the texture feels more crumbly, you're on the right track. If the soil still feels sticky and clay-like, you can repeat the process.

Always grab the chance to improve soil before putting new plants in, to avoid disturbing their roots later on. It's best to plan ahead for this job, because soil improvements can take time to bed in. Expect the process to take anywhere from 4-6 weeks, or longer depending on the overall condition of your soil.

A faster-acting solution is to rake or mix Yates Dynamic Lifter Organic Plant Food into the soil whenever you plant something new. You can also sprinkle it around the root zone of existing plants. It's good to do this several times a year in heavy clay soil. The earthworms that arrive to feed on the extra organic matter also help to aerate and loosen clay soils, via the tunnels they create and the worm casts they leave.

Long-term Improvement to the Structure of Heavy Clay

For extremely heavy clay soil, you can make a permanent improvement in its texture by adding a generous quantity of fine pumice and coarse sand. Spread the mix to a depth of 5–8cm, then dig it thoroughly into the topsoil to a depth of 15–20cm. This job is hard work, so it's easiest to combine it with the compost and gypsum solution described above, to reduce effort.

Pro Tip: It's easy to destroy the crumb structure of clay and clay loam by digging while the soil is wet, so wait for the soil to dry out a bit before digging. A rule of thumb when cultivating soil, is only dig the topsoil. Don't dig so deep that you bring subsoil up to the surface, because it's likely to be heavy clay.

Avoid planting directly into very clayey soil: many plant roots will rot. If you want to plant in that particular spot, add some garden mix to raise the soil level up in a gently sloping mound. Trees need good soil at least 45cm deep; most other plants will need 35cm. Lawns need a minimum 15cm depth of soil added on top of heavy clay.