Soil texture: digging deeper into the soil in your garden

Soil Texture - digging the soil


It’s hard to overstate how important it is for gardeners to understand the soil in their garden. The soil dictates the kind of plants that you can grow and has a huge bearing on how well your plants will grow once you have selected them.

As indicated in our introduction to garden soil, perhaps the two most important characteristics of soil that you need to understand are soil texture and soil structure. Here I dig a bit more deeply into the subject of soil texture.

What is soil texture?

Soil texture is the key feature that can vary between soil in different places. When we talk about soil texture we are concerned with the mineral particles in the soil and in particular with the relative proportions of clay, silt and sand.

Soil texture can be described as a being on sequence 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.

The reason that the different mineral soil particles affect soil texture is mainly because of their relative differences in size.

Sand particles are generally considered to be between 0.05 and 2.0 mm in diameter, silt particles are 0.002 to 0.05 mm and clay particles are below 0.002mm.

Thus, clay particles are hundreds of times smaller than sand particles.

Soil texture also describes how soil feels to the touch and how it feel when it is worked with tools.

We tend to talk about soil in terms of its weight, whether it is heavy or light. A light soil is one that is easily worked, i.e. easy to dig or fork over.

A light soil will be a sandy or silty soil. A heavy soil is one which is much harder to work. Heavy soils are dominated by clay.

You will also hear soils discussed in terms of whether they are coarse or fine textured.

Sandy soils are coarse because they are dominated by the relatively large particles of sand. Soils which are more finely textured are composed of more silt and clay particles.

Garden soil horizon

Soil texture classes

It is useful for soil scientists and even for gardeners to be able to describe soil texture accurately. Therefore various generally accepted soil texture classes have been introduced.

These are the most commonly recognised classes, listed according to whether they fall under the headings of sands, loams or clays and then listed in order of increasing silt and clay content:

Sands

Sand
Loamy sand
Clayey sand

Loams

Sandy loam
Fine sandy loam
Loam
Silty loam
Sandy clay loam
Clay loam

Clays

Sandy clay
Light clay
Medium clay
Heavy clay

You might wonder what the difference is between, for example and loamy sand and a sandy loam. The answer is that it is the second word of these descriptions that provides the basic characteristic of the soil. So, in this example a loamy sand is more sand than loam, whereas a sandy loam is more loam than sand.

Soil texture triangle

In a separate post on soil texture analysis, I show you how you can do your own test to work out which kind of soil texture you have.

It is also possible to work out your soil texture class if you know the percentage of clay, sand and silt in your soil, using the soil triangle below.

You can try to work out the percentages as follows. Take a a sample of soil and gradually wet it. While rubbing it between your fingers, you can try to estimate the percentage of sand, silt and clay according to how gritty or sticky the sample feels.

I must say, I find this quite hard to do, which is why I use the method described here. But, you only need to know two of the percentages, as the other can be determined by subtracting from 100. For example if you know or estimate your soil is 30% sand and 40% clay, the silt content will be 30%.

In this example, the soil texture triangle tells us that the soil is a clay loam.

soil texture triangle

Characteristics of sandy soils

Sandy soil is open textured because of the relatively large size of the sand particles that dominate sandy soils.

This means that rain water easily drains through sandy soils, so they are not prone to water-logging. The open texture also means that it is easy for roots to penetrate sandy soils.

The easy drainage also means that sandy soils dry out quickly and can also lack nutrients. The water draining through the soil dissolves nutrients and these can therefore be ‘leached’ away from the plant roots as the water flows through the soil and away.

Characteristics of clay soils

As we have seen, clay particles are tiny. They are also shaped like saucers. This means that they each have a relatively large surface area and they can pack tightly together.

In practice these characteristics mean that clay soils, when wet, hold water (and nutrients) well. But the tight packing of the particles means that water can find it hard to penetrate clay soils in the first place. Hence, we see puddling.

It also means that roots can find it hard to penetrate clay soils.

As gardeners, we should avoid working clay soils when they are very wet as we risk compacting them further, so that they become completely inhospitable to plants.

Characteristics of loam soils

Loam soils are the best soils that gardeners can hope for. They fall somewere between sandy and clay soils.

Therefore, they have enough clay to hold essential water and nutrients, but enough sand to allow for good drainage.

Can you change soil texture?

As gardeners we often talk about improving our soil, for example by adding organic matter or fertilisers.

These additions can indeed improve the soil structure as well as certain characteristics of the soil such as its fertility or its water holding capacity, but the soil texture itself will not be altered. This is because this kind of improvement does not add to or subtract from the mineral (sand, silt, clay) content of the soil.

It is possible in some cases to change soil texture at a particular site by adding significant quantities of differently textured soil to what is already in place.

But this does not really work when it comes to any attempts to increase the sand content of clay soil.

This is because, as indicated, texture is dictated by the relative proportions of the mineral particles. Because clay particles are so many times smaller than sand particles, it is virtually impossible, at least in a garden situation, to add sufficient sand to affect the relative proportions of clay to sand particles. Therefore, the texture remains unaffected.

Why does soil texture matter?

Why do you need to know the texture of the soil in garden with any degree of precision?

The honest answer is that you probably don’t need to know down to the finest detail. Whether your soil is a silty loam or sandy clay loam is not going to matter much to you, because those soils are quite similar as they are next to each other in the hierarchy.

However, it will repay you to know whether your soil is sandy, loamy or clayey, because the differences between these broad classes are marked and affect how we need to work with them as gardeners.

As we have seen, the key differences is drainage and, in this context, drainage is as much about air as it is about water. Plant roots (except for those of plants especially adapted to live in water or boggy conditions) need air as well as water.

Therefore, if soil is continually waterlogged the roots ‘drown’ and the plant dies.

If, however, water can drain away through the soil, the the space between the soil particles can re-fill with air, providing the oxygenation that is needed.

Importantly, although we cannot technically change soil texture. We can improve its structure by digging in organic matter, because organic matter has the seemingly impossible quality of being able to improve water holding capacity (and thus fertility) in sandy soils and improving drainage and thus aeration in heavy clay soils.

References

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