Qualities of Raised Garden Bed Soil

Proper Drainage

One of the most important considerations for vegetable soil in a raised garden bed is going to be proper drainage. Without proper drainage you will encounter problems with root rot, tomatoes splitting, pepper plants suffering, dampening off and a whole host of potential issues. Components such as sand, perlite, vermiculite, and sphagnum/peat moss all help keep the soil lose and enhance drainage.

Minerals and Nutrients

Another highly important aspect of raised garden bed soil is going to be nutrients. The best way to achieve this as you can imagine is through the use of compost. You can either make your own compost or purchase it. Even if you make your own compost, it may not be a bad idea to occasionally incorporate some compost from different sources (mushroom compost, composted manure, etc.) to be sure you’re providing a nice array of nutrients. The reason I say this is that your homemade compost may always be produced from the same type of plant material and the nutrient type may or may not include everything your plants need.

Moisture Retention

While it is very important to have your soil drain properly it’s equally important that the proper level of moisture is retained as well. The goal is to have soil that drains away excess water while holding onto enough water to keep your plant’s roots damp without causing problems. By providing a soil that maintains a consistent level of moist after draining away the excess your plants will have a more consistent growth and require less watering. Sphagnum/peat moss help hold onto moisture and break down very slowly in the soil so it won’t rob your plants of nitrogen. Perlite and vermiculite seem very similar and are both created from expanded rock however they are not equal when it comes to moisture retention. Although perlite is a little more cost effective and works well for enhancing drainage it doesn’t hold much moisture so if you’re looking for something to retain moisture then vermiculite is the correct choice of the two.

Raised Garden Bed Soil Mix

There are plenty of pre mixed garden soils at the local nurseries and big box stores that should be suitable for use in raised garden beds but if you’re like me then you might take more satisfaction with your own custom soil mixture.

Mel’s Mix: Square Foot Gardening Soil

If you’ve ever read the Square Foot Gardening book by Mel Bartholomew you’ll see his recommendation for raised bed garden soil mix is three equal parts of compost, vermiculite, and peat/sphagnum moss. In the gardening world this has come to be known as Mel’s Mix.

While Mel’s Mix sounds great, we filled our raised beds with our variation which included sand. The reason for the variation was twofold. First off vermiculite is expensive, even from the cheapest source. The second reason is that our native soil is a heavy clay at more than a shovel depth, another reason is we use raised garden beds. Hoping to improve drainage a bit, we also turned over the soil and amended it with compost and sand before setting up the raised beds. IAs years go by the sand and compost should continue to improve and loosen the soil beneath the raised beds.

Our Raised Bed Soil Mixture Ratios

The primary component in our soil mixture is compost. We used 50% compost for my soil mixture. We wanted to ensure that I provided plenty of organic nutrients especially when using the high intensity planting method of square foot gardening which will require more nutrients than traditional row spacing. The other 50% of our raised bed soil is equal to sand, vermiculite, and peat (sphagnum) moss.

We’ve used this mixture as container potting soil in the past and it has provided us with excellent results. Considering that raised garden beds are just large pots or containers we felt the ratio would be suitable. To this day we’ve had much success using this ratio as our raised garden bed soil, the plants love it. Don’t believe me? Check out my huge sweet potato harvest using this soil.

Do you mix your own soil? What are the components of you raised garden bed soil? I’d love to hear what you use in the comments below. Feel free to post questions as well.

UPDATE: We recently started using outdoor hydroponics in addition to our raised garden beds. Perhaps you’ll be interested in this post about the best outdoor hydroponic system.