Best Soil For Raised Bed Gardening: In this article, I describe the The Perfect Soil mixture for a raised bed vegetable garden I use to fill a raised garden bed for cheap.
Raised bed gardening is increasingly popular in the United States and around the world. This method of gardening has several benefits for the plants and the people who grow them.
Ease of access, improved moisture control, reduced weed problems, and superior soil content are just a few of the factors driving its popularity.
If you are thinking about taking the plunge into raised bed gardening, you might be wondering what type of soil you should use for vegetable garden in raised bed?
While you might be able to get by for the first year with ordinary soil from your garden, you will inevitably get better results by taking a more custom approach.
Amending your soil will go a long way toward making sure you enjoy gardening success for many years to come.
So, What soil is best for raised beds? If you have the budget the best soil mixtures for raised beds is a 50/50 blend of high-quality bagged garden soil, blended with mature compost. This will give you the ideal mix of soil nutrients and organic matter. However, this might be cost-prohibitive for deep and large frame gardens that need a significant amount of soil to fill them.
In this article we'll take a closer look at soil, it's primary combination for Building Soil for Raised Bed Gardens, and how to adjust it to meet your raised garden bed ready for a new planting season.
Should You Fill Your Raised Bed With Bagged Garden Soil?
Garden centers and greenhouse supply companies sell bagged garden soil. In terms of a raised gardening bed, you can use this bagged soil for the top six to 12 inches.
The careful blend of soil nutrients and minerals will feed the roots of seedlings and give your plants a good base to draw from.
High quality bagged garden soil has a careful mix of screened top soil, mixed with compost, and other micro soil nutrients.
This often includes strains of endo and ecto mycorrhizae, which are the good types of soil fungi, as well as earthworm castings.
Of course, the cost of bagged garden soil can be a little high for the average home gardener to fill up an entire frame.
This tempts many raised bed gardeners to consider their other soil alternatives for filling up the deeper layers of their frames.
Best Soil Combination For Raised Garden Beds - Fill Your Garden For Less
Not all soil is the same. Chances are if you took a shovel and started digging around your yard you would likely find variations in soil composition in different locations and certainly at different depths.
The three major soil components you will find in just about any garden soil sample are sand, loam/humus, and clay.
As you might imagine, sand is largely made from ground silicate, stone, and other eroded minerals.
This means that sand offers very little when it comes to water retention. Sandy soil tends to let water pass right through it.
Which often carries any nutrients with it into the subsoil layers that a garden plant's roots can't access.
Now, this isn't to say that sand is all bad. You want a little bit of sand to keep plants from being waterlogged.
Indeed, there are some plants that prefer well-drained soil with above-average sand content.
With a raised bed garden, you want around 30% sand content. If you plan on growing plants that need very good drainage or “Sandy Soil” you can go as high as 50% sand. More than that and your soil likely won’t be able to retain enough moisture and nutrients to support healthy plants.
This is essentially an organic material that has been broken down by beneficial microbes.
In some cases, it could be active compost or simply organic material that no longer resembles its original structure.
This is what feeds the plants the nitrogen, phosphorus, and potassium they need to grow. The organic material also helps with water retention.
In a raised bed garden, the organic material in the soil can become compacted over time. Without some form of an amendment, a single rainy summer can compact the organic material in the soil to the point that a plant’s roots will struggle to expand.
When this happens, lack of access to available air and key nutrients can affect fruit production, foliage growth, and even lead to soil-borne diseases.
Clay is essentially fine grain particles and minerals. There are different types of clay. Some of which even hold key minerals and micro-nutrients.
Clay also does a great job of retaining water. Soils with a heavy clay content can even lead to problems with aeration and water saturation.
A little bit of clay thoroughly disbursed throughout the soil, can be beneficial for raised garden beds. Yet you don’t want to go beyond 10 to perhaps 15%.
Plants like lettuce, kale, and chard like soils with around 20% clay content. Yet they tend to be relatively shallow-rooted plants.
This allows you to make shallow adjustments without having to worry about incorporating clay into the lower soil levels of the raised bed.
Can I Use Garden Soil In Raised Beds?
Soil can be very different from one location to the next. If you have an enormous available budget, you could use an expensive mixture of garden soil from a garden center, and high-quality potting soil.
Yet, for most this cost is prohibitive. At the same time, soil from your lawn and garden will likely become compacted over time.
If you have an established garden and you want to add raised beds, you can use that soil as a base, but you will have to amend it with things like sand, vermiculite, perlite, and other additives.
The goal is to make sure that you have a decent nutrient base, while also preserving aeration and proper drainage.
In the spring, you can watch the want ads for landscaping companies, co-ops, and dirt works companies who are selling “Screen Black Dirt.”
They typically sell it by the cubic yard, which means the price per volume is going to be better than what you see in the bags sold in hardware stores and garden centers.
This is a decent base, but you are still likely to see it compact over time.
Chances are the company or individual selling the black dirt will also be able to provide you with other soil amendments or point you in the right direction of someone who can.
How Can I Determine My Soil Content?
If you are trying to save some money by using existing soil on your property, you are going to need to get an idea of its basic composition.
Knowing how much sand, soil, and organic material there already is in the soil, will help you better dial in just which amendments you need to add.
There are test kits sold in hardware stores and garden centers that can help with this. Many County Extension offices also provide tests, which can include things like pH and nutrient profile.
If you want to “Ballpark It” there is a simple test you can do to determine the basic amount of sand and clay in your soil.
- Step One: Lightly water the area you intend to sample the soil from the night before.
- Step Two: Dig a sample from at least 4 inches below the ground level.
- Step Three: Collect a sample about the size of a ping-pong ball.
- Step Four: Feel the external texture of the ball if it feels overly gritting, it might be high in the sand. If it feels slick or overly wet, the soil might be high in clay.
- Step Five: Gently shape it into a rough cylinder.
What The Results Tell You
- -If it crumbles into modest chunks it likely has a proper balance of sand, clay, and organic material.
- -If it crumbles into gritty or mealy small chunks it is likely high in the sand.
- -If it holds its shape firmly, then the soil is likely high in clay.
Are There Different Soil Combinations I Can Try?
One of the advantages of a raised bed garden is that you can potential tailor the soil to the composition that works best for the plants you want to grow.
Common Amendments Or Ingredients For Raised Bed Soil
If you are using garden soil, or you sourced some screened black dirt then chances are you will need to amend it with other materials.
The goal is to balance the soil for proper aeration and moisture retention as well as making sure it has enough rich organic material to support vigorous plant growth.
Assuming that you have a basic idea of the clay and sand content, you should next move to test the pH and basic nutrient levels.
There are simple test kits for this in hardware stores and garden centers. Following the instructions on the package will give you a better idea of your soil’s pH, as well as the available nitrogen, phosphorus, and potassium or soluble potash levels.
The Following Are Some Common Soil Amendments To Help Adjust Your Test Results.
You can buy sand in bulk or in bags. If your soil is high in clay, you will need to blend a significant amount of sand to improve drainage and make it easier for roots to expand. This is admittedly a lot of back-breaking work.
This is a type of sand that has a slightly higher level of potassium or soluble potash. Adding it with the sand will also help condition the soil and can adjust a slightly acidic pH.
This is soft, and spongy little bits that look like pebbles. It’s made from super-heating mica. Blending it into your soil helps improve aeration. It also helps retain moisture.
Is harder than vermiculite and is made by super-heating volcanic glass. While it helps with aeration, it’s not as good as vermiculite when it comes to moisture retention.
is essentially hydrated sodium calcium aluminosilicate which contains minerals and other trace elements. It can be a useful alternative to perlite if your soil tests came back low in mineral content.
is prized for it’s arability as well as it’s moisture retention. Young plants like it for it’s ability to let tender new roots grow.
If you intend to grow plants that enjoy moist soil in a particular location, peat moss is preferable to clay. Just keep in mind that peat moss has very little in the way of available nutrients.
Helps to boost the nutrient profile of the soil. It certainly helps increase nitrogen levels. It can also help improve the soil’s ability to retain moisture, without having to add clay.
Is compost that hasn’t fully matured yet and tends to be high in nitrogen. You shouldn’t use this in the uppermost, shallow layers of the raised bed.
It’s best to lay a thick layer of green compost at the lower layers, to help feed deep-rooted mature plants in the peak of summer. This will give it time to mature.
Just like green compost, this is something that shouldn’t be added to the top layers of the soil. If you can get your hands on some chicken manure, you can mix it with ground oak leaves and layer the lowest level of your raised bed with the mixture.
As time goes on this potent manure mixture will mature into compost that plants can nutrients up for a year or more to come.
Alfalfa And Clover
When dried and crumbled into the mid-to-upper levels of the soil, it helps boost nitrogen levels. This can be very helpful for shallow rooted plants and seedlings.
The Best Soil Mix For Raised Bed Gardens (Recipe)
Healthy and nutritious vegetables thrive in properly balanced organic soil that is loose as wel as rich in nutrients and essential minerals.
Not only does this type of soil support vigorous plant growth, it also helps prevent some common plant diseases and conditions.
High Volume All Purpose Organic Soil
Let’s say you want to fille a 4-foot by 8-foot planting bed with one foot of soil. By the basic calculation of volume 4 x 8 X 1 = 32 cubic feet of soil. In a situation like this you might want to consider the following “Soil Recipe
- 11 cubic foot bags of peat moss
- 3 cubic feet of organic compost
- 4 cubic feet of worm castings
- 3 cubic feet of organic chicken manure
- 4 cubic feet of vermiculite
- 6 pounds of Azomite
- 2 pounds of organic kelp meal
- 6 pounds of calcium rich oyster shell flour
Make A Lasagna Garden In A Raised Bed
Lasagna gardening is another type of soil strategy that can help you fill the volume of your raised beds without breaking the budget. It’s largely designed to make the most out of the materials you have on hand.
With a lasagna garden, you first want to make sure that all the underlying weeds or grass have been removed from the intended bed location.
You can do this by renting a sod cutter, or simply by laying out overlapping sheets of carboard, and newspapers that have been printed with soy-based ink. You then start building the soil layers from the bottom up.
The Lowest Layer should be made of carbon rich materials like crumbled leaves, small twigs, and straw. The goal is to provide around four to five inches to increase the volume.
The Second Lowest Layer should be some “Green” or “Hot” compost that is rich in nitrogen. Spread a good two to three liberal inches over the carbon-rich material. If you have some on hand, you could use chicken or cow manure that has been allowed to age for two to four months.
The Top Layer is your primary soil. If you are trying to save money, screened black dirt will work. If you’ve still got some wiggle room in your budget, you should consider using a high quality bagged garden soil, or a custom organic soil like the one described earlier.
The total depth of the top layer can vary depending on what you want to grow. At a bare minimum there should be six inches of soil.
This will give the compost and carbon layers time to break down before the roots of the plants grow down to that layer.
If you want to grow root vegetables like potatoes, or a plant that needs deep planting like tomatoes, you might want to make the top layer 12 or more inches deep.
With the top layer you can also to play with other amendments that are tailored to moisture and nutrient requirements of the plants you intend to grow.
For example: If you wanted to grow shallow rooted lettuce and mesclun greens, you could mix some peat moss into the bagged soil.
If on the other hand you were planning on growing tomatoes, potatoes, or onions which prefer loose well drained soils, you might want to consider working in sand or perlite into the top layer of the soil .
Prevent Weeds From Growing In Your Raised Bed Soil
Weeds can rob the nutrients from even the most well-balanced and nutrient dense soil.
If your raised bed is 24-inches or deeper, then chances are that the underlying weeds and possible grass from your chosen location won’t be able to push all the way up to the surface.
Less than 24-inches and you may end up being able to establish a presence in your soil.
If you are planning to build your raised beds over a previous section of lawn, you might want to rent a sod cutter, to remove the top two or three inches of grass and turf.
This is essential if you the depth of your raised bed will be less than 15-inches.
In a pinch, you might be able to block out weeds and grass by putting down layers of cardboard and newspaper. Just make sure that the newspaper is printed using soy-based ink.
Mulch Is Very Important!
A mulching film with sealed or stitched edges is also a great way to block out weeds. It also allows excess water to drain out of the raised bed soil.
You should avoid using plastic sheeting, as this will gradually allow excess water to build up in the lower layers of the soil.
In a particularly wet summer, it could suffocate deep-rooted plants and could lead to other plant diseases.
Do I Need To Replace The Soil In My Raised Bed Garden?
As time goes by the plants growing in your raised bed will gradually start to deplete the available nutrients. At the same time, even most well-balanced and aerated soil will eventually start to compact.
As I mentioned earlier, if you are using a mound system without a frame, you can simply till up the rows, and augment them again each spring.
You can still do this if you are using a more popular framed raised bed, it just requires a little more labor. It usually requires you to lean over and shovel out the excess soil.
With a new raised bed, you can probably get away with simply hand tilling the top layers for year one and year two.
By the time you get to year three, you should strongly consider digging deeper to loosen all the soil and amend it again.
You should also use a soil test kit to measure the nutrient profile and pH of the middle and lower layers of the soil.
This will help you dial in which soil amendments you need to add. If you’re lucky, incorporating a yard or two of mature compost each year will boost you raised bed soil to give you vigorous plants all summer long.
From his childhood obsession with gardening to the decade he spent operating a hobby farm, Eric has developed over four decades of experience in self-sufficiency. Not only does this include the organic elements of growing and tending plants, but it also includes a wealth of experience in
maintaining lawns, landscaping, and equipment.
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