Square Foot Gardening Potting Soil Mix

May 9, 2019 By

We had an interesting question come in to the Square Foot Gardening Foundation recently and thought it would be helpful for everyone.

"Does one ever need to change out the soil mix (Mel's Mix) in the containers completely? How does one amend the soil each year?"

This is a great question because if you're used to growing in something other than Mel's Mix™ you have to do a lot more work and amending. Switching to Mel's Mix is definitely a change, but a good one!

Let's investigate how Mel's Mix works and why you don't have to replace it constantly or amend it.

Mel's Mix components. Clockwise from top: compost, peat moss (or coco coir), and coarse vermiculite.

First, we're going to assume you're using Mel's Mix as prescribed in the All New Square Foot Gardening book which is, by volume,

1/3 peat moss (or coco coir)
1/3 coarse vermiculite
1/3 of a good blended compost made from at least 5 different sources, if possible or use your own homemade compost, which we recommend

Second, we're going to assume that if you are using Mel's Mix that the only thing you've added to your Square Foot Garden when you replant is a trowel of compost each and every time you've harvested a square — it's that easy!

Our advice is to not mess with success. . . so, if this is not what you've been doing, say you've added other potting mixes or your native soil to your garden bed, you probably will need to amend your soil, have it tested, add artificial nutrients, and so forth. It would be simpler to start over using Mel's Mix.

Gardening to Preserve Mel's Mix – Throughout the Season

Again, assuming you're using true Mel's Mix, here's what you need to do so that you won't have to completely change out the Mel's Mix for at least 10 years, and probably longer. As mentioned before, every time you harvest a square, you'll only need to add a trowel of blended or homemade compost and stir it in. This will naturally refresh the nutrients in the Mel's Mix.

When you harvest a square mid-season, or at the end of the season when you're cleaning your beds, you will want to knock off as much of the Mel's Mix back into the raised bed as possible. Some people shake the veggies into a bag to make sure they keep as much of that precious growing medium as possible. The stubborn bits of Mel's Mix clinging to the roots can be swished in a bucket of water and then used to water your garden. It's a great practice because, not only do you retain as much Mel's Mix as possible, your veggies will be cleaner when you take them into the house.

Of course, no matter how diligent you are, after a few seasons of being absorbed into the plant roots, you'll notice that the level of your Mel's Mix™ will naturally go down. When you lose an inch or two, you'll add more fresh Mel's Mix mixed in the proportions described above. You'll incorporate this into the older Mel's Mix. This will help bring up the proportion of peat moss and coarse vermiculite and keep your mix loose and friable. You should not have to completely replace it unless you notice that it is compacting too much. This usually means that you're adding too much compost after each harvest-just a trowel-ful is needed.

Amending the Soil – Is it necessary?

To answer the second question, one does not need to amend the Mel's Mix™ each year – that's done on a continual basis throughout the gardening season when you add compost. If you choose to not grow beyond the summer season, and decide to put your beds to rest over the winter, one common practice is to put dark plastic sheeting over the raised bed and weight it down. By doing this, your beds will be ready super early the next season.

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