The Best Raised Bed Soil 

As any gardener knows, good, healthy earth is the foundation of a healthy, productive garden.

One of the main benefits of creating a raised bed garden is that it enables you to fully control the quality of your growing medium. This is ideal if your garden earth is poor, filled with thick tree roots or too dense to successfully grow anything. Alternatively, a heavy, densely packed growing medium that is slow to drain can be difficult to improve. Filling pots and planters with your own potting medium mix is also a good solution if you are concerned about pollutants in the earth.

As well as being a good way to counteract poor earth, a raised bed garden also makes growing flowers and vegetables a lot easier, improving access for elderly people or those with mobility issues. With a little careful planning, a raised bed garden can also be used to make the most of a small or awkward space.

Most crops grow in good raised bed soil.

Many row crops such as beans, flax, potatoes, sugar beets, sunflowers and corn often struggle to grow in the ground. This is because they have to compete with the roots of other plants, bushes and trees for nutrients. Growing these crops in raised bed soil means that there is less competition for nutrients, helping them to thrive.

Once your planters are constructed, the most important element is the growing medium. Our guide to raised bed soil will explain how to work out how much potting medium you need, how to make your own blend and how to fill your elevated planters.

How Big Should my Planters be?

Whether you have chosen to build your own using our plans or used pre-cut kit such as a Cedar Raised Bed Planter Box, knowing the size of the planter enables you to work out how much potting medium you need.

A planter can be any size or shape. A standard, rectangular planter is typically 3 to 4 ft wide, 6 to 8 ft long and 10 to 12 inches tall. A planter this size is ideal for growing most plants. It also allows you to sit on the edge and easily reach all areas of the growing area without too much stretching. This makes tasks such as planting and weeding easy.

Being able to easily reach all areas of your elevated planter also means that you do not have to stand on the earth. Regularly walking over the earth in a planter compacts the growing medium. Raised bed soil that is not trodden on remains loose and friable, making it ideal for growing a range of flowers and vegetables.

In good raised bed soil, beans and other vegetables thrive. 

How Much Raised Bed Soil Do I Need?

The simple answer is more than you think.

How much growing medium you need will vary depending on how large your planters are. In general you need more earth than you think.

Before purchasing your growing medium measure the length, width and height of the raised bed. Unless the planter is very thick there is no need to worry about inside or outside measurements.

Remember to take all measurements in the same unit before multiplying them together. For example, if the height of your planter is 16.5 inches, convert this to feet before beginning your calculations.

A simple calculation for working out how much raised bed soil you need to fill a rectangular or square planter is to multiply the Length of the planter by the Width by the Height.

V = L x W x H

Using this equation we find that a planter that is 6 ft long, 3 ft wide and 1 ft tall measures 18 cubic ft.

To convert this to cubic yards, divide the answer by 27. That gives you a measurement of 0.66 cubic yards. So to fill an elevated planter that measures 6 x 3 x 1 ft you will need 0.66 cubic yards of growing medium.

Calculating the Volume of L-Shaped Planters

Calculating how much raised bed soil you need for an L-shaped planter is slightly more complicated.

Divide the planter into two four sided parts. Measure these as explained above.
Once you have the two separate sets of measurements use the formula explained above to calculate the volume of each shape. Add the two figures together.

V = (L x W x H) + (L x W x H)

Again, to convert the figure to cubic yards, divide by 27.

Work out how much growing medium you need to fill your planter.

Calculating the Volume of U-Shaped Planters

While U-shaped planters are a convenient shape, how much raised bed soil you need to fill them can be difficult to work out. The process can be simplified by dividing the planter into 3 square sections. Measure each of these before following the formula outlined above.

V = (L x W x H ) + (L x W x H) + (L x W x H)

Divide the final figure by 27 to convert it into cubic yards.

Calculating the Volume of Hexagonal and Octagonal Planters

A regular hexagonal prism can be calculated using the following formula:
V= 3/2 (√3) s2h

Here V equals volume, S equals the length of one side, H equals the height of the planter.

If, like me, you struggle with complicated equations, or you just want to check your workings out, run your measurements through an online calculator for regular hexagonal prisms.

Similarly the volume of an Octagonal planter can be calculated by using the following formula:

V= 2 (1 + √2) S2H

Again V equals volume, S equals, the length of one side, and H equals height.

Divide your answer by 27 for the cubic yardage.

An octagon volume calculator can be used to make sure your figures are correct.

Knowing how much raised bed soil you need can save both time and money. Correctly filling your planters also helps you to make the most of your space.

Sourcing Raised Bed Soil

Once you know how much raised bed soil you need, it is time to source the growing medium. Ideally you want to fill the planter with a good quality mix. A blend of topsoil and compost is ideal.

You can purchase bags of growing medium from garden and hardware stores. If you are filling a larger planter, or planters, it may be easier to arrange a delivery. You can also remove topsoil from your garden to reuse in the planting area.

Compost

An essential ingredient, when it comes to creating the ideal raised bed soil mix, rich organic matter, or compost, is important. Not only does it help the growing medium to hold moisture it also provides your growing plants with lots of nutrients.

If you are purchasing bags of compost, look for products labeled organic. An organic vegetable and herb mix or organic planting medium is ideal. To further enrich your purchased growing medium, you can work in homemade compost.

If you don’t have a compost pile in your garden, some municipalities sell or give away compost in the spring. If you have the space, a compost pile is a great addition to the garden.

A healthy compost pile gives you a ready supply of fresh, nutrient rich organic matter throughout the growing season. This enables you to top up your raised bed soil with more material and nutrients, ready for the next planting.

A compost pile is also a good way to reuse your kitchen waste.

A productive compost pile is a useful addition to the garden.

Your compost should be full of macronutrients, NPK (nitrogen, phosphorus and potassium), micronutrients, trace minerals and microorganisms.

Home improvement stores, hardware stores and plant nurseries all sell good quality potting mixes and compost. Large bags are often more economical to purchase than smaller bags. You can store what you don’t use for use at a later date.

Topsoil

Lining the bottom of the planter with topsoil reduces the amount of raised bed soil that you need.

You can reuse the topsoil in your garden to fill your planters.

When you are positioning your planters, cut out the sod from underneath the planter. Flip the pieces over and place them grass side down at the bottom of the planter. Over time the grass breaks down.

If you are ordering topsoil, try to find out where it is from before ordering. Topsoil is often taken from land that has been developed for subdivisions. It may have sat in storage for a long period, losing its nutrients.

Good topsoil is a vital ingredient.  

The 4 Types of Soil

The type of raised bed soil you use depends on what you are planting. Before filling your purpose built planter, research what potting medium best suits the plants you want to grow. For example, certain crops such as strawberries and blueberries like an acidic growing medium.

Plants take all their nutrients from the ground. Using the best raised bed soil is vital to cultivating a productive garden.

You can use improvement techniques to ensure that the planters remain healthy and productive.

In general there are 4 basic types of earth. The raised bed soil that you use should be one of or a mixture of these.
These are:

• Sand,
• Clay,
• Silt,
• Loam.

Sandy potting mediums provide good drainage. Unable to retain moisture for a long period, because they are made of the largest particles, a sandy potting medium is ideal for plants that are prone to developing rot in damp earth.

Succulents and cacti in particular thrive in sandy potting mediums. When combined with other types of growing medium, sand is a great addition to a planter or growing area.

Cacti and succulents thrive in sandy conditions.  

Clay is the complete opposite of sand. Composed of lots of small particles, it is a dense, heavy medium which is easily compacted. It retains both water and nutrients well. However, if the earth is too heavy, root growth can be impaired. Additionally, a heavy growing medium is more likely to retain water and remain soggy for long periods of time. This can cause some plants to develop issues such as root rot.

Silty potting mediums retain water well and are fertile. The medium sized particles are prone to compacting, so try not to put too much pressure on it.

Loam is the favorite potting medium for gardeners. A mix of the other 3 types, a loamy growing medium is ideal for planters because it is full of microorganisms that nourish growing plants.

For planters, the best growing medium is a mix of loamy soil and different types of compost. You can also use this mix to fill planters and window boxes.

While loamy growing mediums are versatile, you may need to research what growing medium is best for certain plants. For example, succulents will be happier in a sandy growing medium while lavender and some herbs prefer a well draining growing medium.

Filling a planter with loamy growing medium enables you to plant a range of plants.  

A Premium Blend

This is a raised bed soil mix that is geared towards growth. Rich in nutrients it is ideal for using in small or medium sized planters.

A premium blend consists of a mix of peat moss, vermiculite and compost. While a mix of equal parts is ideal it is not a hard and fast rule. You can use 50% compost and 25% peat moss and vermiculite. Perlite or azomite can be used instead of vermiculite.

As well as promoting growth, a premium blend mix is also ideal for starting seedlings.

The Difference Between Garden and Raised Bed Soil

Garden earth is essentially topsoil with shredded wood products added. Raised bed soil also has peat moss and other nutrients mixed in. It also holds moisture and nutrients longer than garden earth.

If you choose to use freshly dug earth, in combination with compost, make sure that it is a native product, such as earth dug from your garden, that is pathogen or pest free. If you are unable to do this, it is better to avoid using earth dug from your garden.

How to Fill Your Raised Bed with Soil

Once you have decided what growing medium combination to use, it is time to start filling your planter.

To save money, you can scatter branches, logs, wood chips and sticks at the bottom of the planter before adding any raised bed soil.  These organic materials break down over time adding bulk to the planter.

Organic materials add bulk to your planter, filling the space and adding nutrients.  

Before adding any raised bed soil, laydown cardboard or newspaper to prevent weeds from growing. Keeswin Permeable Landscape Fabric, which provides a more permanent barrier preventing weed growth, can also be used.

Arrange organic materials such as hay, sticks or logs at the bottom of the planter before adding a layer of compost. As these materials break down they add carbon and nitrogen to the growing medium. Use a garden hose to water each layer to moisten it before adding next. Finally, fill the planter with a topsoil compost mix.

The Half and Half Technique

As the name suggests this is as simple as filling the lower half of your planter with compost and the top half with earth. Vermicompost can also be used in the lower half, as can topsoil from the garden.

If you are using topsoil, drench it with a dose of AgroThrive All Purpose Organic Liquid Fertilizer. A homemade liquid fertilizer can also be used. This enriches the earth after placing it in the planter. If you are using topsoil from the garden, make sure that it is weed free.

The Lasagna Method

This technique of filling planters with earth is ideal for tall planters. Filling a tall planter can be expensive. The lasagna method is a cost-effective solution that doesn’t sacrifice soil quality. It is also useful if you do not have lots of compost.

The lasagna method is simply layering compostable materials in the planter, slowly filling it up. These compost naturally over time, giving your growing plants access to a continuous supply of fresh, nutritious compost.

The lasagna method is all about mixing brown and green materials. Brown materials are carbon rich ingredients. These include:

• Dried leaves,
• Wood chips,
• Straw,
• Shredded bark
• Untreated cardboard or paper.

Green materials are things that are rich in nitrogen. These can include:

• Coffee grounds,
• Grass clippings,
• Egg shells,
• Fruit and veg scraps.

Do not use meat or dairy products.

To fill a planter using the lasagna method, start by laying down a brown layer. This should be a couple of inches thick. Then apply a green layer which is similar in thickness. Repeat this until 4 or 5 layers have been placed in the planter. The top layer should finish 6 inches below the top of the planter.

Place a layer of cardboard on top of the final layer and cover this with fertile, fresh earth. The cardboard prevents the earth from falling to the base of the planter too quickly.

In general the best mix to use as a top layer is a blend of compost and topsoil. A 2:1 ratio is ideal. Here the compost provides lots of rich organic nutrients, benefitting plant growth and productivity. You can also mix in shredded leaves, mushroom compost or vermicompost to increase fertility.

Once your planter is filled, allow the soil to settle before planting.

Raised Bed Soil After Care

Once filled, your raised bed soil needs a little regular attention to remain rich and healthy.

Good raised bed soil is ideal for growing a range of crops and flowers.  

In the fall, once the main growing season has ended, mulch the surface with chopped or shredded leaves. These can be sprinkled evenly over the growing surface. You can also use leaf mold.

In later winter and early spring, before you begin planting, you can also amend the raised bed soil with compost.

Regularly amending raised bed soil with organic matter helps to keep it healthy and rich. If you have more than one growing area, implementing a basic crop rotation system can also help to keep the growing medium rich and fertile. Growing cover crops is also a good way to return nutrients to the earth.

A great solution for gardeners with poor earth or mobility problems, elevated planters are a fabulous way to grow plants. Enabling you to control the quality of your growing medium, so that you can cultivate healthy, productive crops and flowers, raised bed soil is an essential part of the process.