Common soil types in the garden

Soil is made up of different mineral particles, which vary in shape, size and chemical composition. Particle size and shape is influenced by how easily each mineral breaks down from weathering and wear.

For example, particles of sand are relatively large, because they're durable and break down very slowly. Other minerals break down faster, to form much smaller clay particles. Clay particles are extremely small – many thousands of times smaller than coarse sand. This variability plays a big part in the different physical and chemical properties of soil. The size of the particles (and the proportions in which they occur) determine the texture of soil.

Each soil particle is solid, but when trillions of particles are stacked up on top of each other they have voids, called pore spaces, in between them. These pore spaces can contain water or air - this is how water and air move through the soil. The size of these gaps influences soil's interaction with air and water - they determine how efficiently soil drains, or retains moisture.

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 saturated. 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.

Sandy Soils

Sandy soils have large, macropore spaces in between particles. They drain quickly, have good aeration and are easy to cultivate with garden tools. For this reason, they're often called ‘light’ soils. However, very sandy soils are much less effective at retaining water and nutrients than other soil types.

Clay Soils

Clay soils have tiny particles and very little pore space. They store water well, but often too well to provide good drainage and aeration. Clay is very good at retaining plant nutrients; the particles attract and hold nutrients on their surfaces. Clay soils can be very hard work to cultivate and are often referred to as ‘heavy’.

Loam Soils

All soils between the extremes of sand and clay are referred to as loams. They are mixtures of coarse and fine particles. They are further categorised as sandy loam (more sand than clay) and clay loam (more clay than sand).

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.

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