Media for raised beds and containers

Asked April 16, 2020, 10:51 AM EDT

Hi, I've seen all sorts of opinions online about what "soil" to use in raised beds and containers. Last year we planted both in the ground and in Earthboxes and a three-tier raised bed I built into our hill. We ordered 3cu yards of a mix of 1/3 topsoil and 2/3 leafgrow compost from a local garden supply place. It seemed to work fine, and it was relatively affordable. This year we're even more ambitious. We're going to add a dozen self-watering containers (55-gal plastic barrels cut in half with 4" corrugated pipe in the bottom covered by landscape fabric, with a length of PVC pipe for refilling the water in the corrugated pipes). I've seen online experts(?) suggesting soil-less media (peat, perlite coconut coir, vermiculite, etc etc). But this seems only to come in bags, and it would cost a fortune compared to bulk. Questions: 1. What do you recommend as media/mix? 2. If you recommend soil-less, are you aware of anyone anywhere near us (we're north of Annapolis) who sells a bulk mix (or bulk unmixed)?

Anne Arundel County Maryland

3 Responses

Hi- the mix you describe- 1/3 topsoil and 2/3 compost- should work well for a raised bed sitting on soil or smothered turf. We recommend that people not use mineral soil (solid part made up of sand, silt, and clay particles) in containers, even if it's topsoil. The reason is that mineral soil is much denser and slow-draining than compost and soilless grow mixes. There is less gravitational pull in a container and water held tightly between soil particles can lead to saturation, root death due to reduced oxygen levels, and increased disease problems. If you are using even a small amount of topsoil, it will drain water more readily in a large, deep container compared to a smaller, shallow container.

We are intrigued that your mix last year worked well in Earthboxes, which are not very deep. Since the drums cut in half will be larger in volume and at least as deep as the Earthboxes, you should try that same mix. You may want to do a comparison with drums filled with a 50:50 mix of a soilless mix and compost. You may even want to fill one barrel planter with 100% compost. The most economical way to get compost, as you know, is by the cubic yard. For soilless mixes, try to find a local nursery or garden center that sells 3.8 cu. ft. compressed bales of Pro-Mix, Lambert, Fafard, or other brands. Relying on local topsoil and compost makes the system more sustainable than relying on soilless mixes.
https://extension.umd.edu/hgic/topics/soil-fill-raised-beds
https://extension.umd.edu/hgic/topics/soil-containers

I would love to see pics of your garden this summer (jont@umd.edu). Container gardening is very popular and we'd like to see and share the many cool ways people are going about it.
Jon

Thanks, mix may have worked since earth boxes water from below. We’ll experiment w media and send pictures, thanks again.