It’s every gardener’s dream to have good quality topsoil!
And good drainage is one of the essential characteristics of healthy garden soil.
Poor drainage can lead to waterlogging or, inversely, soil that drains too quickly. Pretty bad situations for most plants!
Most gardeners understand that compost is a valuable soil amendment for improving soil quality.
But is compost any good for drainage?
Let’s find out…
Does compost help with drainage?
Compost can help any kind of soil with drainage problems. Whether your garden soil is too sandy or you have heavy clay soil, adding compost improves the underlying structure. The various materials used in composting modify the soil’s properties and improve drainage.
Clay soil is typically dense and heavy. It tends to remain wet throughout winter and dries up during summer.
This can lead to wet and soggy conditions that can saturate plants or subsoil so hard that plant roots have difficulty penetrating.
Sandy soil, on the other hand, has difficulty retaining moisture. This means plants cannot take up adequate amounts of water and nutrients.
Both situations can restrict plant growth or even kill them.
So what can you do if you find yourself with poorly draining soil?
Compost soil drainage
Compost soil drainage methods involve fixing bad soil to create optimum moisture conditions for plants.
Whatever type of soil you have, probably the best overall method to improve drainage is to incorporate well-rotted organic matter in the form of compost.
It’s all about fixing your soil’s relationship with water.
Compost contains a multitude of organic ingredients which have decomposed over time. Some of these ingredients break down quickly, while others take longer. This results in a highly varied, coarse-textured material, with many crevices for water and air to permeate.
As an added advantage, the hard carbon-based materials found in compost will continue to decompose over time until they end up as humus. This has excellent sponge-like qualities that improve both drainage and water retention.
Furthermore, because compost is rarely wholly decomposed, it provides a feast for all kinds of macroorganisms. These creatures move around in the soil, creating even more permeability and better drainage.
How does compost improve soil drainage?
In clay soils, the particles are tiny and compact. They are so close together that there are no significant pockets or gaps for water to seep through. That’s why this type of soil retains too much rainwater, and you run the risk of drowning your plants!
When you dig mature compost into clay soil, it helps break up the structure and provides spaces for water to drain more freely.
This will make a noticeable improvement to your backyard soil which will have a more flaky texture and better drainage.
Sandy soil has larger, hard particles compared to clay soil types. This leaves spaces between the particles for water to drain freely. Not only does it leave your plants thirsty, but it also lets water-soluble nutrients leach away quickly.
Adding mature compost to sandy soil helps bind soil particles together.
The compost acts as a kind of framework to support the existing soil particles. It also helps reduce compaction to create gaps between the sand particles.
The overall effect is better-textured soil with improved drainage.
So as you can see, in reality, compost provides multiple benefits. It simultaneously enhances the draining properties of your topsoil, corrects water retaining properties and oxygenation.
You can test the draining properties of your soil by digging a small hole about 12 inches (30cm) deep and filling it with water. The first fill should drain away pretty quickly, so fill it again and watch what happens. If the water drains away quickly, then your soil might be too sandy. On the other hand, if only a couple of inches of water have drained off after an hour, you probably have a high clay content. Picking up a handful of soil and testing its texture is always a good starting point. Does it feel sticky or sandy and granular?
How much compost do I need to improve soil drainage?
To start correcting your soil’s draining properties, as a general rule, add 2 inches (5cm) of compost over your earth and dig it in. If this is the first time you try amending soil, you could go as far as to add 4 inches (10cm).
This can be repeated each year as a way to upgrade your soil continually.
But remember, compost isn’t soil! So don’t overdo it…
Soil contains nutrients essential for plant growth that you won’t find in compost. And if you add too much compost, microbial activity in the soil multiplies. These microbes feed off the same nutrients your plants need and can cause a drop in nutrient availability.
What to mix with compost for drainage
Some plants actually prefer shallow well-drained soil. For example, drought-resistant plants like lavender, hydrangeas, and teasels might require you to increase soil drainage for them to be happy.
In this case, you can amend your soil using free-draining compost.
What is free draining compost?
The term “free draining” means that rainwater soaks through fast. Free draining compost is a mixture of mature compost and an additive that opens up the structure even more to allow water to drain quickly.
This can be grit or something like pearlite or vermiculite.
Simply mix your compost with about one part pearlite or vermiculite to four parts compost.
Pearlite is excellent for making free-draining compost since it aerates the compost and increases the spaces for water to drain correctly. Vermiculite is a mineral that has the added characteristic of absorbing moisture, making rainwater less available to plants.
The ideal soil has a crumbly texture that drains well and retains just enough moisture for your plants to thrive.
Adding compost is the perfect solution to make better soil for most types of plants. Compost improves your soil texture and adds many other benefits, whether you have sandy or clay soil.