Chris Sherwood is a project manager by day and avid home and garden scholar by night who loves to share his trials and success with others.
Gardening is one of the easiest ways to not only save money but also quickly put you on the path to self-sufficiency. However, tough native soil conditions such as heavy clay, too much sand, rock, or poor nutritional content can cause many gardeners to give up before ever reaching harvest time. Luckily, a raised bed can quickly overcome any poor soil condition by allowing you to create the perfect soil mixture above ground.
So if you have the chance to create any soil mix you like, why would you throw that opportunity away by adding poor soil to your boxes? Raised beds give you the perfect fresh start that can lead to a thriving garden in no time with the right soil recipe. Though you can end up spending a fair amount of money creating the perfect soil, this guide will provide budget options to help get you off to a great start in your quest for fresh vegetables, sustainable living, and just living an overall healthier and more rewarding lifestyle.
Start With the Right Box
Before you start adding your soil mixture, it's important that you begin with the right raised bed for what you're planning to grow. Unless you plan on allowing your plants to dig into the soil below and have little issues with weeds or native grass invading your bed, it's important to have the right bed depth. While a 6"-high bed with a layer of cardboard in the bottom might be enough for shallow-rooted vegetables like greens and beans, you'll want a deeper box if you plan on growing items like tomatoes or longer-root vegetables like carrots. A deeper box also makes it more difficult for weeds or unwanted grasses to grow up through your soil mixture. Aim for at least 12" of height to your garden bed.
You should also make sure your bed is not made from potentially contaminated materials, such as railroad ties or treated wood. Treated wood has come a long ways towards becoming less toxic, but older or reclaimed wood might still contain harmful substances like alkaline copper quat (ACQ) or copper azole (CA-B), which are typically no longer used in the treatment process.
Raised Bed Soil Recipe
Now to your basic starter recipe. Add the following items in the estimated quantities below. This is just your foundation mix. Feel free to add or subtract additional amendments depending on your local climate and what you plan on growing. Different plants can require different soil pH levels. So always make sure you test your final soil and amend to fit the environment of what you want to grow. But by and large, this mix should get you jump started to a great harvest for many seasons to come.
About 50 percent of your raised bed should be made up of good-quality organic compost. Be wary of compost dealers that are unable to give you a good idea of what their compost is made from. If they use grass in their mix, make sure they have made sure that any grass clippings that have been added to their compost were not sprayed with fertilizers or herbicides that can impact the health of your plants. Ideally, you want a compost that is made up of a ratio of 4:1 browns (carbon) to greens (nitrogen). Browns are items like well-rotted leaves and weed-free straw, while greens are items like grass clippings, manures, and food scraps. You can make your own or purchase some from a local supplier or a garden center.
Compost not only provides an amazing amount of nutrients to your soil, it also helps provide structure to the soil, allowing it to better hold moisture while still allowing proper drainage and creating the perfect environment for garden helpers like earthworms to thrive.
Good Top Soil: 40%
Another 40 percent of your soil mix should be good quality top soil. DO NOT use fill dirt. Test your top soil by squeezing a handful in your fist. The soil should hold together when you open your hand, but it should also easily break apart again when you touch it. If the soil doesn't hold together, it most likely has too much sand in it. If it doesn't break apart easily, it may cause issues with drainage. If you have suitable native soil on your property, you can save some money by mixing that soil in with your compost on the bottom portion of the bed. Make sure to mix it in to avoid leaving a poorly draining layer at the bottom if you have naturally heavy clay soil.
Coconut Coir: 5%
Coconut coir doesn't add a lot nutritionally to your soil, but it does act as a sustainable soil aerator that can simultaneously hold both air and water in the soil. These traits can break up tough or compacted soil, while giving better structure to your raised bed and a better ability to hold onto water. This is especially helpful in drier climates with regular drought conditions or restrictions on watering.
Nutritional Soil Bomb Mixture: 5%
The final five percent should be a mixture of items that help rocket your seeds or starts to the finish line while maintaining a healthy root system and lots of nutrients. Remember, plants take up a lot of their nutrition through the soil. The better your soil, the better your fruits and vegetables will taste and the potentially higher nutritional value they'll contain.
Over time, worms will hopefully make a home of your garden bed, especially since you're creating the perfect environment for them. Worms digest organic matter in your raised bed and leave behind their nutrition-rich castings. You can buy these castings in bags from most garden centers or online. A little bit goes a long ways with worm castings. To save money, you can also add worm castings to the bottom of your planting holes to target the nutrients to your growing area.
Mushroom compost is not made from mushrooms, but is the byproduct of growing mushrooms made up of the materials the mushrooms are grown in, such as corncobs or hay. Mushroom compost provides both potassium and nitrogen as well as a small amount of phosphorous to your soil mix.
Rock Dust (Azomite)
Rock dust provides additional important minerals and trace elements to your raised bed soil that are often depleted in many soils mixtures today. Those who use Azomite in their soil also report better tasting fruits along with larger harvests both in quantity and in size.
Mycorrhizae may already be present in your compost or top soil mix. If not, you can add this beneficial fungi to your soil recipe. This fungi colonizes your plant roots and helps the roots better uptake nutrients from the soil in exchange for taking carbon from the plant. This increases the availability of nitrogen, phosphorous, and other important micro-nutrients.
Keep in mind that the type of soil you need may vary slightly depending on your climate. If you're in an area like Phoenix, you'll want to bulk up on items that help retain moisture, like well-rotted organic matter, or add a layer of mulch to the top of your soil. In wet areas like Seattle, you'll want to add plenty of compost and potentially some sand to help break up the soil and allow for better drainage with the heavier amounts of rain.