Potting Soil for Planter Box

Question from Judy:
Great website. I just found you. My husband just built a planter box 38x48x12deep. It is on wheels so i can move it to get sun both summer and winter. My growing space is small with everything planted. I have removed much of the none editable landscape. If I can’t eat it I don’t want to waste water on it. Blueberrys are pretty shrub. and strawberrys a good groundcover. What I need to know is what type of soil mix would you put into this planter box that I will use for lettuce carrots spinich etc.?? I am thinking of a pearlite, peatmoss, topsoil combo??

Answer from Pat:
What your husband has built for you is basically a large, portable planting container, not a raised bed. If he had built a raised bed on the ground I would have told you to fill it with good quality amended garden soil and put wire on the bottom against gophers. Raised beds are the same as terraces and need to be filled with soil, not potting soil which is for containers. But this is not a raised bed, it is an enclosed container. All containers should be filled with good quality potting soil, not garden soil, since properly constituted potting soil is lighter in weight than soil and also provides appropriate drainage for use in containers. Garden soil, with mighty few exceptions, does not drain well in containers. (Some decomposed granite soils, such as those found in Claremont California, work well in containers but these soils are few and far between.)

It is possible for home gardeners to mix their own potting soils as you have suggested you might do, but I don’t recommend this practice. Also, you suggest including garden soil as one of the ingredients. I would strongly warn against this, since one of the advantages of growing plants in a container is that you avoid the possibility of pathogens and pests, such as nematodes, which might be found in garden soil. Also, garden soil can sift down and clog drainage holes. Additionally, it is heavy when wet. One of the advantages of using potting soil in containers is that it is lighter weight than garden soil.

Gardeners do love recipes and so I provide a good one for making homemade soilless potting soil on page 25 of in my month-by-month garden published this year. This recipe is based on a recipe originally devised by Cornell University, for making one’s own potting soil. I have made it myself on television years ago and things grew well in it, and you can do it to if you want to. But my own experience has shown me that even with a good recipe it’s a struggle for a home gardener to mix a good-quality homemade potting soil. Doing so creates a lot of dust which is not good for one’s lungs, and it’s actually cheaper in the long run to purchase a high-quality commercial bagged potting soil. It’s hard to make just the right amount needed and then you need to store the rest. Storage is a problem. Reliable companies that make potting soils subject their products to extensive tests before selling them to the public and you can purchase just what you need. Also, you can find potting soils that are especially designed for growing vegetables and say so on the package. If you cannot find one of these I suggest that you add a gallon or two of aged bagged chicken manure to the mix in the box.

Before filling your box with potting soil, be sure to cover each of the drainage holes with a large enough piece of broken crockery to permit water to drain out and the soil mix to be safely held back within the container. Even using potting soil in this container it’s going to be heavy when wet and due to its shape it will need several drainage holes distributed on the bottom. The advantage with potting soils is they drain well but also the shape and size of this container means it will dry out far quicker than in the ground. Be sure to water accordingly, perhaps even daily in warm weather. Be sure to add organic fertilizer recommended for vegetables according to package directions prior to planting. And each season before planting again, remove some of the top of the old mix and add some fresh potting soil on top.

I’m happy to know you like my site and thank you so much for saying so. I have a partner, Loren Nelson, who does all the web design and technical side. I answer all letters and provide all written content. The difference between my site and garden “forums” is that folks who write to me get prompt, practical, thoughtful, and reliable answers to their questions, that are often exhaustive and always based on the best scientific research and organic principles, plus good common sense. The only time there is a wait is when I’m on vacation.