Garden soil: a gardener’s guide

For a gardener, the garden soil is probably the single most important thing in your garden.

Everything good in the garden comes from the soil, so it’s important for gardeners, especially new gardeners, to understand their soil – we need to know the characteristics of the soil we have and we need to know how we can improve it, if necessary.

In this video you’ll find an introductory guide to garden soil covering the following topics:

• What is soil?
• Why does soil matter?
• Soil texture
• Soil structure
• Improving soil

A short guide to garden soil

Here is a summary of what is in the video.

What is soil?

Soil consists of 5 elements:

1. Mineral (rock) particles
2. Organic matter (humus, decaying plants and animals)
3. Water (soil solution)
4. Air, and
5. Living things – insects, microbes, bacteria.

Most of these elements are self-explanatory. But it is worth focusing on the following three, so that we can fully understand the components of soil.

Humus

Humus is formed from the decomposed remains of plant and animal matter. It is dark brown and high in nutrients that are critical for plant growth, especially nitrogen. It is difficult to visibly identify humus as a separate component in soil, as it is usually mixed in with the other components.

Humus is plays a critical role in soil structure (see more detail below). The more humus that is present, the better the soil structure will be.

Organic matter

Whereas, humus consists of decomposed plant and animal matter. Soil organic matter compromises plant and animal material that is in the process of decomposing, as well as soil microbes and the materials those microbes produce.

Like humus, organic matter is also critical to improve soil structure and fertility.

Soil Solution – water

The water in soil takes up the nutrients from other soil components. These are then dissolved in the water to form what is called the ‘soil solution’. It is the soil solutions that is taken up through the roots of plants to contribute to the plants’ growth and health.

Soil Horizons

Ultimately soil is derived from the bedrock below it, from the actions of weathering, erosion and soil movement over millennia and and the organic deposits and biological activity that occur within the soil.

The soil and rock beneath our feet are organised in layers – or horizons – as you can see in the diagram below.

The difference between top-soil and sub-soil

As we think about the soil horizons in our garden soil, it is worth remarking especially on the difference between top-soil and sub-soil, because these are our growing zones.

Top-soil (the A horizon) is the layer of soil nearest the surface. It has a significant amount or organic matter and humus within it. It is also subject to a high degree of biological activity, such as the insect and animal life, the activity of fungi and microbes, and the breaking down of organic matter.

Sub-soil (the B horizon) forms a layer beneath the top-soil. It has far less organic matter in it and is more lifeless – with much less biological activity.

Sub-soil is also much more compacted than the top-soil and may contain more small rocks and stones. In places where there has been significant construction work, it is not unusual to find the top-soil and sub-soil mixed together.

Why does your garden soil matter?

The type and quality of the soil you have determines which plants you can grow and also how well your plants will grow.

We can see this in nature because, along with factors such as climate, the quality of the soil can mean the difference between a lush green landscape and a barren scrubby one.

Soil texture

All soil is not the same. Soils from different locations, even locations quite close to each other, can differ widely.

This is because:

• In different types of soil, each of the five soil components mentioned above are present in different proportions
• In different places, the mineral particles which make up the soil are derived from different types of rocks
• Different plants and animal matter are present or have been present
• Different processes of soil formation may have taken place, e.g. weathering or water erosion.

Soil texture is the central characteristic which varies between different soils. It is basically a measure of the size and proportions of different particles in the soil, that is the relative proportions of sand, silt and clay particles.

Soil texture can be described on a continuum from sand through loam to clay. Thus the texture of an individual soil will be determined by the respective amounts of clay and sand content.

What difference does soil texture make?

Different soil textures give rise to different characteristics in the soil.

Sandy soils are open textured and free draining, so they do not hold water or nutrients well,

Clay soils are closed textured, and therefore can be come sticky and waterlogged when wet, and set hard when dry.

This is important to gardeners because all plants prefer the characteristics of particular types of soil.

For example, plants like succulents, cacti and many Mediterranean plants such as lavender and rosemary, need free draining soil. If you planted them in heavy clay they would not thrive and would probably die.

Other plants, such as potatoes, bananas or bamboo require heavier, richer soils that are more water retentive.

Most plants, e.g. Thalictrums or Dahlias like moist but free draining loams. If you have soil of that type, you can grow almost anything.

Soil structure

Soil structure is also a fundamentally important characteristic of soil.

Soil structure is how we describe the way that all the various components of soil are combined.

Generally, soil components combine to form aggregates or crumbs, with spaces in between.

The existence of these crumbs and the sizes of them determines the type of soil structure in place.

Good garden soil structure

Good soil structure:

• Has small crumbs, well bound together that do not break up when wet or when dug
• Has lots of spaces and channels between crumbs so that water and plant roots can penetrate
• Allows air into the root spaces
• Allows roots to grow easily
• Allows water to be made available to roots but also to drain away.

Poor garden soil structure

Soil with poor structure:

• Lacks small crumbs or aggregates
• Is hard and compacted or consists of large lumps
• Cannot be easily dug, but may be eroded by rain water or wind
• Means plant roots, air and water cannot easily penetrate.

Improving soil

Whether soil is sandy or clay, you can always improve soil structure with the addition of plentiful organic matter – compost, leaf mulch etc.

Sandy soils can be improved with the addition of clay topsoils.

Clay soils can be improved with substantial amounts of sand or grit, or sometimes with gypsum.

Improving garden soil is an ongoing process for gardeners and is a big subject in itself that will be covered in more detail in future posts.

Remember, nurture your soil it and it will repay you well.

References

• Ingram, D. S., Vince-Prue, D., & Gregory, P. J. (Eds.). (2015). Science and the garden: the scientific basis of horticultural practice. John Wiley & Sons.
• Handreck, K. A. (1993). Gardening down-under: better soils and potting mixes for better gardens. East Melbourne, Vic: CSIRO Publications.