Filling Your Raised Box With Soil

One of the primary advantages of raised bed gardening is that you get to take control of your soil.  It seems obvious, but the quality of your soil is the #1 thing that will impact the productivity of your garden.  Water, sunlight, a little luck?  All those things are necessary to grow anything in the ground, but it’s that ground that provides the nutrients for your plants to grow big, tasty fruits and veggies. With that in mind, we’re going to look at what we want in our raised bed soil and how to get it.

What Makes Good Soil?

Your plants will get the nutrition they need to grow and produce from what they absorb through their roots.  Therefore, in the most basic sense, we want soil that includes the things plants need.  Fortunately, you don’t need a PHD in chemistry or biology to get started.  

At a basic level, soil should include a mix of the following in roughly these amounts: topsoil (60%), compost (30%) and amendments (10%).  Let’s break each of those down.  

Topsoil (60% of your soil mix)

Topsoil will make up the majority of your soil mix, but it’s more than just a filler.  Topsoil is collected from the first few inches of soil (hence the “top” of “topsoil”).  This layer of soil contains organic material that your plants will appreciate.  When picking out top-soil for your garden, look at 3 different things:

1. Color: Good topsoil will have a dark brown color.  If you’re looking at soil that is light brown in appearance, it is likely too low in organic material to be of much use.
2. Feel: When you grab a handful of top-soil you want it to be neither too loose nor too clumpy, but in between.  Good top-soil should clump sightly when you grab it but then break apart easily if you squeeze it.  If your soil is too loose and won’t clump, it’s too high in rock and sand.  If it’s too clumpy and doesn’t break apart easily it likely has too much clay.  
3. Smell: Last, but not least, your top-soil should smell earthy, not rancid.  It’s unlikely, but top-soil can contain harmful bacteria and other things that will negatively affect your garden.  While most top-soils can be “fixed”, if it smells rancid upfront, look elsewhere.

NB – Topsoil is NOT the same as fill dirt.  Fill dirt is composed of sand, crushed rock and dirt with little organic material.  It is light brown or gray in color and won’t do you any use in your garden.

Compost (30% of your soil mix)

A good compost will be chock full of organic material, contain lots of nutrients (primarily nitrogen) and have a pH range appropriate to what you’re growing.  Where possible, the best way to get compost for your garden is to make your own.  Not only does making your own ensure you’ll have high quality compost, but you’ll also reduce material in landfills and the amount of methane they produce.  

If producing your own compost isn’t in the cards, or if you just can’t produce enough for your whole garden, you can buy compost.  If buying compost, it’s important to do your homework before you buy.  Where possible, get a recommendation from a fellow gardener for a trusted, local source.  If that’s not available, here’s what to look for (and look out for) in purchased compost.

Good compost should have the following:

Organic products thoroughly broken down
Made from a mix of “green” (fresh grass clippings, discarded veggies) and “brown” (wood chips, saw dust) waste
Earthy aroma

Look out for compost that has these characteristics:

Fill dirt or rocks
Trash or ammonia smell (this indicates the compost is not ready)
Made primarily of manure (some is OK)
Sludgy texture or have large chunks of material not yet decomposed

Amendments (10% of your soil mix) 

Amending your soil can get rather complex so here’s the good news: if you’re using high quality top soil and compost as we’ve recommended here, you probably won’t need to add any soil amendments–at least not for your first season.  If that’s the case, add more topsoil and compost in a 2 to 1 ratio and get gardening!  

Amendments are typically used to make less than ideal soil better.  Some common issues that can require amendment either when initially filling a garden bed or that can develop over time are:

Issue	Amendment	Things to Look For
Soil too acidic (or not acidic enough)	Lime to raise pH, Sulfur to lower pH	Do a pH test every 2-3 years
Too much clay	Peat moss	Soil stays too wet and won’t drain
Poor crop growth	Fertilizer (natural or artificial)	Plants growing slowly or not producing

How Do I Get My Soil?

Here you have 2 options: you can buy a raised bed mix or mix your own.  We’ll go into the benefits and drawbacks of each option.

Raised Bed Mix

Raised bed mix is a commercially available pre-mixed combination of top soil and compost.  Each company will have their own blend of ingredients and additives.  There are 2 ways to procure commercial raised bed mix: buy it in bags from a nursery or get it in bulk from a landscape supplier.  

For relatively small amounts of soil (less than 2 cubic yards) it’s usually best to go with bags of raised bed mix.  Though it may be more expensive, bags are easier to transport than loose soil in smaller quantities.  National brands are available at big box stores, but we recommend checking with your local nursery as they’ll likely have a recommended brand best suited to your local climate.  

Before you head to the nursery to load up your SUV with soil, calculate the amount of soil you’ll need in cubic feet (use an online calculator like this one [LINK]).  Note that the maximum amount of soil that can fit in a typical pickup is a little less than a yard of soil and that will weigh about a ton.  Most places will offer delivery a fee so look into that, renting a flatbed truck or consider taking multiple trips.

For larger quantities of raised bed mix, head to a local landscape supplier.  These companies specialize in delivering large quantities of soil and other landscape supplies.  These places typically won’t make their own soil but get it from a local provider.  If they don’t have information on the soil breakdown (topsoil to compost ratio, amendments added) ask them to share their providers’ info so you can check with the source directly.  

Make Your Own

With a little time, planning and space, you can make your own soil mix for your garden beds.  Check with your local garden center and landscape supplies for topsoil, compost and any amendments you might need.  Mixing your own will allow you to shop around and pick the right blend of ingredients for what you want to grow.  Most suppliers will be busy leading up to and during peak planting seasons, so plan ahead and expect to order materials several weeks before you plan on planting.