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Raised beds are a fantastic way to provide spacious growing areas for plants and vegetables.
Building robust, garden planters also allow you to choose a more generous and better-quality soil. This is particularly advantageous if your site has shallow, boggy or poor topsoil.
But what is the best soil for raised beds? The truth is there is no “one size fits all” soil type for filling raised beds. The best soil for filling raised beds will be moisture retentive but well drained and rich in Phosphorous, Potassium and nitrogen.
However the optimum soil mix will depend upon, the height of your planters, existing soil type, site hydrology and local climate.
Before you get worried it’s really very simple! In this article I will explain the best soil for raised garden beds in every scenario.
Most plants and vegetables like moist but well drained topsoil. Therefore, the best soil for raised beds will drain well.
However, soil which is too free draining can dry out very quickly in summer and leach nutrients quickly.
Therefore, the best raised bed soil will have both free draining and moisture retentive particles.
This is why typically the best soil type is a clay loam with some sharp sand particles.
If your locality naturally has very boggy soil prone to flooding raised bed soil needs to be more free draining. This will mean more sharp sand, grit, organic matter and less clay.
In such circumstances raised beds should also be built higher.
What is the best height for a raised bed?
There are many reasons why you may want your raised beds at a particular height.
Firstly, raised beds are generally integrated into a garden design and may have a more spatial or aesthetic function.
For example, raised beds of around 600 high can also provide good seating especially for small spaces.
However, in regard to soil, the height of your bed should reflect your existing soil type and site conditions.
If you have naturally free draining soil and a climate with little rainfall beds will dry out easily.
Therefore, free draining topsoil with more clay articles should be added to raised beds at a lower height. At the other extreme areas with boggy soils and higher seasonal rainfall can be built much higher.
Of course, what you grow in your beds will also have an impact. If you have an alpine or succulent raised bed, you will want the topsoil as free draining as possible.
If you want to grow record breaking carrots, tall beds filled with a sandy loam will be better.
Is clay soil good for raised beds?
Clay soil is good for raised beds if it is used in the lower half of a tall, raised bed.
Here it can help to retain moisture and nutrients for better topsoil above. If clay soil is well mixed with sand and organic matter it can become perfect for raised beds.
It is well known among many horticulturalists that a clay loam is the best soil for raised beds.
What is the best soil for raised beds?
The best soil for raised beds is a free draining but moisture retentive, fertile, clay loam.
It should be rich in the three essential nutrients for plants: potassium, phosphate and nitrogen with a crumbly texture.
However the fertility level, PH and free draining qualities will depend upon what you wish to grow and your specific region.
What to add to raised garden bed soil?
If the soil in your raised bed regularly dries out, you should mix in some clay, topsoil, loam and organic matter. This will help to absorb and retain more moisture.
If your soil is often boggy and stagnant you should mix in plenty of sand, grit and organic matter. This will create more air spaces for water to pass through.
In such a scenario you may also want to create more drainage holes and raise the beds height.
Can you fill a raised bed with just compost?
It is ok to fill a raised bed with just compost especially on top of boggy, clay, subsoil.
However a tall, raised bed just filled with compost will probably dry out too much during the summer.
If you are going to fill a raised bed with compost fill half of it first with a clay-based topsoil.
What is the best time of year to fill raised garden beds?
You can fill raised beds at any time of year but filling during the spring and summer may hinder your growing season.
However filling beds during the winter can be messy, especially if you have to track wheelbarrows across lawn.
During wet winters dirty wheelbarrow wheels can grind dirt deeply into patios and paths. This will normally require pressure washing to clean.
I find the best time to fill raised beds is late summer and early autumn while the weather is still warm.
This results in less chance of mess and boggy conditions while not interrupting your growing season.
It is also advantageous to fill soil before frost and allow soil to settle before the next growing season.
Do you have to put plastic inside raised garden beds?
Many decide the wish to line the inside of a timber raised bed with plastic. This is because a theory has developed that this will prevent wet soil degrading the timber.
Such a strategy is commonly implemented in beds with thinner walls. However, lining beds with plastic is not a good solution.
Plastic can limit a beds ability to drain leading to stagnant and boggy soil.
Furthermore, water will normally sit within the vacuum of where the plastic meets the timber.
This can lead to excessive moisture around fixtures causing corrosion. The best solution for building timber raised beds is using robust materials.
Thick timber sleepers are the best as they take many years to degrade. The edges and timber joins also allow excess moisture to seep out greatly improving drainage.
Should you add drainage holes to your raised beds?
If your raised bed is in contact with the surrounding soil and is naturally free draining, you do not have to have drainage holes.
Timber sleepers naturally drain through the gaps and joins of the timber work.
However it is always advised to add drainage holes especially if you are building with masonry materials like brick and concrete.
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Hyde Heath, Amersham, Buckinghamshire