Every gardener knows the soil in their garden affects what they grow. But soil type plays a big part in fencing too.
So before you start your fence project have a think about your soil type. It will influence how easy it is to put in your fence posts. It will also determine how strongly the posts hold.
Below I will look at each of the three main soil types and how each affects the installation and stability of your fence. Where I can, I will offer a few tips to ensure your soil type does not prevent you from getting an easy installation experience and a good, strong outcome.
Just to remind you, if you put in your fence with FenceFins
you will just need to dig a small hole, nail the fins to the post and then put the post and FenceFins into the hole and compact the soil back firmly.
What soil types are there?
Without being too technical we can say that all soils are described by how much sand they contain and how much clay. Sand grains are coarse and either rounded or sharp. Clay particles are very fine.
In addition there will be decomposed vegetable matter which turns the soil into loam – the gardener’s favourite but not the fencer’s choice! Then there are chalky soils and soils with high contents of gravel, peat or silt.
Many soils will be a mix and if you need a little help to identify your soil type, take a look at this guide from Gardeners World.
The effect of each soil type on fence installation
1. Sandy soils
Installation. With sandy soils it will be pretty easy to dig holes for your fence posts. If the soil is very fine or dry, the sides of the hole may be prone to collapse. If you dig a slightly bigger hole than you need this will be fine (and easy to do).
Compacting the soil back in is best done if the soil is not too wet or too dry. Sandy soils drain well, so are unlikely to be too wet. If the soil is very dry add a little water as you compact it in.
Strength. We often think that building on sand is a bad idea. But in fact once it has been compacted a sandy soil will provide a very stable support. (Have a look at our ‘tug of war’ video to see what I mean, it was filmed in a garden with very sandy soil in Bournemouth, Dorset).
2. Loamy soils
Installation. Loamy soils are easy to work, so it should be easy to dig holes for your fence posts and compact the soil back in afterwards.
Strength. Loamy soils are unlikely to shrink if they dry out so are stable from that point of view. However the loam content gives the soil a degree of “softness” or flexibility and this is not ideal for the stability of any fence. My advice would be to provide as good a fixing for your posts as you can. For example, if you had planned to use 3”x 3” (75mm x 75mm) posts, why not consider 4”x 4” (100mm x 100mm) instead? You can also add some gravel, stones or rubble into the hole which will help strengthen the hold.
3. Clay soils
Installation. Clay soils are dense, sticky and hard to work. A soil with a high clay content will be easiest to work and to compact when the moisture content is neither too high (almost liquid) nor too low (brick-like). When using FenceFins in clay soils it is particularly important to compact all the soil back in really well, and if this is done they will work very reliably.
Strength. A soil with high clay content will hold water well and will not drain easily. And dried out clay soils are very hard and strong. However problems can come because as clay soils dry out, they shrink a great deal and this can move objects held in them. This is why houses built on clay can suffer badly from subsidence.
In clay soils the big advantage of FenceFins (over concrete or spikes) is that if severe soil shrinkage loosens the fence post it can easily be firmed by re-compacting the soil around the post. Wet the soil a little so it is easier to work and re-compact.
4. Other soils – gravel, chalk, silt
Soils with a lot of gravel or stones will be harder than loamy soils to dig. But this soil type has the advantage of providing very good support once your post and FenceFins are in the ground. Needless to say, this is a difficult soil to use spike post anchors in, as they will be hard to drive in straight and undamaged.
Silt is ground up rock with a particle size between clay and fine sand. A high silt content can result in a soil that is stable when damp or dry but can become fluid when very wet. Silty soil is therefore easy to work when moist and will provide moderate support to a fence post but when very wet will give poor support.
Chalk is mostly only found in certain areas and is often accompanied by large flint stones. Any post in soil with a very high chalk content will be hard work to erect but will be extremely solid. Soil which contains some chalk will present few problems.
A final word on compacting soil
We are often asked ‘how can ALL the soil that has been dug out be put back into the hole once the post is in there too?’ The answer is that it is generally possible to compact the soil to a better state than it had when undisturbed. To put back all, or very nearly all, the soil is a good indication that you have compacted sufficiently and you will be rewarded with a strong, stable fence post and fence.