Choosing the Best Soil for Raised Garden Beds

Importance of Choosing the Right Soil

Have you built your raised bed? Ready to fill with soil? At first glance, soil just looks like—well—dirt. But not all soils are created equal, and some soils aren’t suitable for raised gardens.

Although raised beds are famous for their quick drainage and ability to warm up fast in spring, a lot of that depends on the soil they’re filled with. If you put the wrong type of soil in raised beds, that soil can get soggy fast or become too dense for plant roots to push through.

Aside from soil structure, some plants require higher nutrient or pH levels than others, and they won’t grow well in soil that doesn’t meet their needs. Finding that perfect balance of loose yet well-structured soil, with the right nutrient and pH levels for the plants you want to grow is key if you want to keep a successful and prolific raised garden!

Soil Characteristics to Consider

Quality raised-bed soils can be composed of several different materials and varying concentrations of ingredients. We’ll advise on the best raised-bed soil in the next section, but first, it is important to understand what qualities your soil needs to help plants thrive! 

Drainage

Soil is a mix of particle sizes (sand, clay, and silt). A nice mix, called loam, allows enough air and water to reach plant roots. Raised bed soil must be well-draining and relatively loose, with a fair amount of sand or grit to enhance drainage. Raised beds can become waterlogged and overly compact if they have too much clay or you don’t use the right soil mix. This is why most raised-bed soil mixes at the garden store include sand in the ingredient list.

Nutrient Content

Besides mineral particles, air, and water, healthy raised-bed soil contains live microorganisms, like worms, fungi, and good bacteria. However, you need to amend the soil with compost to feed these organisms. Many experts recommend mixing your raised bed soil with 25% to 50% compost. Compost, which helps anchor plant roots, provides organic matter for the live soil microorganisms to break down into compounds that plants can absorb. 

pH Levels

Most plants prefer soils with a soil pH between 6.0 and 7.0, and soils that are too alkaline or acidic can lead to poor plant growth and nutrient deficiencies. Raised-bed soil mixes have an optimal pH for most garden plants. Find out the pH of your garden soil by getting a soil test. This is important because plants can not uptake nutrients in soil with the wrong pH, no matter how much you fertilize.

Rocks, Weed Seeds, and Other Contaminants

Plant roots can not reach down into soil that has rocks. In addition, rocks, weed seeds, bits of plastic, and other contaminants can sometimes be mixed into poor-quality bulk or bagged soils. Purchasing soil and amendments from reliable sources is the best way to avoid these issues!

Types of Soil for Raised Garden Beds

Raised-bed soil is typically sold in bags or bulk from garden centers and landscaping companies, but it can also be created at home if you have the right ingredients on hand. In the list below, you’ll find some of the most common soil varieties and materials that can be used to fill up raised beds. However, as you’ll soon see, some of these materials work better than others!

Native Soil 

The soil already in your garden, or native soil, may be suitable for growing inground plants. But it’s not recommended for raised beds!

Unfortunately, native garden soil is far too dense for raised gardens and can become waterlogged and overly compact in raised beds. Not to mention, garden soil often contains weed seeds that can transform raised beds into weedy jungles in no time flat. If you do decide to use native soil in raised beds, mix it with some compost and coarse sand before applying, and only add a few inches of the blend into the very base of your raised beds. 

Topsoil 

Topsoil can refer to the top few inches of native soil naturally found in your garden, but in the gardening world, the term is more commonly applied to a type of soil that can be purchased in bags or in bulk from landscaping companies. Quality topsoil usually contains a mix of sand, silt, and clay, which gives the soil structure and increases water retention. However, topsoil is heavy. If you want to use topsoil in raised beds, mix it with equal parts coarse sand and compost first!

Sand 

Similar to peat moss and coconut coir, sand doesn’t contain many nutrients, but it is an essential part of healthy soil and is the best product for boosting drainage in raised beds! Most well-balanced raised bed soils contain a mix of coarse or horticultural sand, topsoil, and compost.

Recommended: Raised Bed Mix

Made with sand, topsoil, and compost, store-bought raised-bed soil mixes are the absolute best choice for raised gardens—unless you make your own soil blend. Raised-bed soil is heavier and provides better structure for plant roots. It’s also lighter weight than topsoil and less likely to cause drainage issues!

Tip: If you want to purchase raised-bed mix in bulk to fill your beds, you can calculate the cubic feet you need to order by multiplying the length by width by height of the beds you’d like to fill.

DIY Soil Mix

Raised-bed mixes are more cost-effective than filling beds with potting soil. But you can make raised beds even more budget-friendly by creating your own DIY raised bed soil with equal parts topsoil, sand, and compost. If you wish, swap out the compost for vermicompost or aged manure, or blend in some peat moss or elemental sulfur to lower the pH for acid-loving plants, like potatoes.

Saving Money

In addition to making your own soil blend, you can cut costs by filling up the lower 1/3 to ½ of raised beds with organic materials, like straw, chopped autumn leaves, logs, twigs, or pinecones. These natural items will eventually degrade into a rich soil, and they’ll save you money by reducing the amount of soil you need to purchase! 

Potting Soil (Not Recommended)

Note: Any bag labeled “Potting Soil” is intended to be used “as-is” and doesn’t necessarily need to be blended with other soil additives. However, potting soil is most appropriate for containers, pots, and smaller planters, and is not a cost-effective option for more extensive raised gardens. Beyond the expense, potting soil is fluffy and doesn’t hold onto plant roots or water well in raised beds.

Soil Amendments

Compost

We discussed the importance of amending your soil with compost. Compost can be made at home from yard waste and kitchen scraps, or it can be purchased in bags or in bulk. Packed with the nutrients that plants crave, compost acts like a slow-release fertilizer and is always appropriate to add to raised beds. That said, compost needs to be mixed with soil as it doesn’t hold onto water well on its own.

Vermicompost

Made from the leavings of composting worms, vermicompost is rich in plant nutrients, and can improve the structure and health of raised bed gardens. But like compost, vermicompost is intended to be used as a soil additive, not a soil substitute. For best results, use vermicompost in combination with raised bed soil, topsoil and sand, or blend it into a store-bought raised bed mix for extra nutrients.

Aged Manure

Fresh manure can burn plants and pose health and safety risks. But properly aged manure can be an asset to raised beds and provide lots of nutrients to growing plants. Aged manure is usually used in place of compost or vermicompost as a soil additive, but it needs to be mixed with coarser soil and sand for proper drainage!

Peat Moss or Coconut Coir

Peat moss and coconut coir are often added to soil blends to increase water retention and aeration, and acidic peat moss can also be used to lower soil pH. However, these products don’t contain many nutrients, and can make gardens soggy and overly acidic if they’re applied in high concentrations. If you want to use either of these products in raised beds, mix them with topsoil, compost, and sand before use, and consider using more sustainable coconut coir in place of nonrenewable peat moss.

Maintaining Healthy Soil in Raised Beds

Choosing a quality raised-bed mix for your garden is step one. We also recommend mixing your raised bed soil with compost when you first get established with new beds. 

But after that, you’ll want to maintain the health of your soil by topping your garden beds off with a 1- to 2-inch layer of compost, vermicompost, or aged manure, followed by a layer of organic mulch, at least once a year in spring or fall. This top-dressing of organic matter and mulch will keep raised beds from becoming nutrient deficient, and protect the soil from the elements, leaching, and weed seeds.

Summary

One of the best things about raised bed gardening is that it gives you complete control over your soil. While most raised bed soils contain about equal parts topsoil, compost, and sand, you can completely customize raised bed soils to suit the specific needs of the plants you’d like to grow. Try adding soil acidifiers like peat moss to lower the soil pH for acid-loving plants, blend in extra sand to increase drainage for Mediterranean herbs, apply additional vermicompost, compost, or aged manure to fuel to growth of heavy feeding crops like pumpkins, or keep it simple with a store-bought raised bed mix. It’s up to you!