Soil in your Garden - plant food | Gardening Lessons

The soil in your garden is probably the most important factor in how well your plants will grow well – or not thrive at all.

“Earths and soils being the laboratory wherein the food or nutriment of the plant is prepared and the media through which it is conveyed to the roots; it becomes a matter of primary importance to the gardener”

John Towers
The Domestic Gardeners Manual, 1830

Plants take up nutrients (their food) and water through their roots,so the soil the plant roots are sitting in is crucial to their well-being.

A real gardener takes care of the soil. It is generally considered to be the most important element of successfully growing any plants, be those ornamental flowering shrubs or rows of cabbages.

Although we could delve deep into the science of soil, for most amateur gardeners the main questions regarding soil are–

What type of soil do I have?
How deep is my top soil layer?

The Soil in your Garden – what type of soil is it?

Clay, sandy , silt or loam? Do you know what type of ‘dirt’ you have in your garden?

When we take on a new client, whether this is for garden design or planting design, we carry out a site survey. This includes looking at the soil, with at least simple pH tests to check the acidity and alkalinity; and the presence of the three main elements or nutrients required for plant growth as well as the soil profile and texture. A more involved soil survey would include the topography of the garden, testing for trace elements and investigating the history of the area.

In Plews bespoke gardening lessons, we look in more depth as to why we need to test the soil. Then we test your own garden soil. So you learn what to do once you know the results, as well as different ways of testing.

The various types of soil have –

Different natural pH, ie how acid or alkaline it is, which affects the plants you can grow;
Differing abilities to absorb heat, which affects, for example, how quickly it warms up in the spring;
And the ability to retain water also varies with the type of soil.
The type of base rock way below your feet;
Whether you live in an old river bed;
The use the land was put to before the houses were built, for example agricultural pasture land;
Whether a gardener lived there before you;
Whether the builders dumped rubble and subsoil where they shouldn’t.

Most soils contain four basic components: mineral particles (45%), water (25%), air (25%), and organic matter (5%).

Soil mineral particles

This is the part we’re talking about when we refer to a soil being clay, sand or silt. This is also known as the soil texture and cannot be altered as it based on the parent rock.

Clay soils have the smallest particles and sand the largest. This is why sandy soils are very free draining as the large particles have large gaps between them. Clay soils have smaller gaps and are also a different shape, which increases their surface area and enables them to stick to each other and have other mineral particles stick to them. A clay soil does not drain well, which is why it gets waterlogged easily; it is also why it cracks in drought conditions. Sandy soils have much less variation.

The Soil in your Garden – what is the difference between top soil and sub soil?

For the purposes of most gardeners, all they need to know is that soil is roughly divided into –

Top soil – which is the section that the plants roots are in and should be full of plant food nutrients.
Sub soil – the lower section; which has different layers, but importantly for most gardeners does not have much in the way of nutrients for plant growth.

But how deep is your top soil layer? The easiest way to discover this is to dig a hole in your garden. This will show you the soil profile, or soil layers of both the top soil and sub soils.

Top Soil

The top soil should be dark, rich with organic matter and friable, that is easy to dig, with no large lumps. Your top soil may be as shallow as 4 inches or as deep as two foot.

Broadly speaking most people find the top soil in their garden is between one or two ‘spits’ deep. A ‘spit’ is the length of your spade blade, and generally taken to be 8 -10 inches.

Sub Soil

The sub soil will be an obviously different colour and texture to the top soil. There are various sub soil layers, reducing in nutrient the deeper you go. The higher sub soil layers will have some tree roots and micro-organisms.

There are various ways to increase the depth and quality of your top soil. We’ll look at this in detail in another garden soil blog, as it’s a big topic, and I’ve suggested some related blogs that will also be of interest. For now, let’s look at a couple of easy ideas for improving the top soil in your garden.

Increasing the depth can be achieved by building raised beds and bringing in new, fertile soil. This can be a good idea if your soil is all subsoil and rubble and is probably more economic than trying to dig out and remove sub soil from your garden and replace it, or spending years adding organic matter.

Improving the Soil in your Garden

However, all soils are improved by the addition of organic matter. If your top soil layer is shallow, digging in homemade or bought in compost will improve the quality of what little you have, and, longer term, increase the depth of the top soil. The compost can be added a mulch, laid on top of the soil for the worms to take down. It can be dug into the soil, either lightly forked in if your soil is reasonable or added so it is at root level straight away.

By adding compost, sandy soils are encouraged to absorb and retain water and nutrients, and become more fertile. Silty soils are fairly fertile to begin with but their ‘slipperiness’ is toned down with organic matter making a more workable soil. Clay soils are encouraged to be more free draining.

If you have a choice, go for a garden with clay soil as this is generally the easiest to turn into that elusive, fertile, friable ‘fine tilth’.

And finally…

Lastly, what is this loam that so many gardeners hanker after? The majority of garden soils are a mix of clay, sand and silt in differing proportions. Loam soil – a true loam soil – has equal proportions of all three and the best of all, so it is free draining, quick to warm up in the spring, water retentive, nutrient rich. A gardener’s dream.

For further gardening advice and inspiration, check out Plews Potting Shed blogs, including the selection below. Plus you’ll find our monthly Tipsheet has both topical gardening tips and design ideas. And you could come and find us on Instagram Pinterest and Facebook too.

If you would like help with learning more about your garden and gardening, why not ask about our bespoke Gardening Lessons, where your classroom is actually your own garden. We can help you learn gardening basics, and also show you how to plan an ornamental border or kitchen garden