Soil For Seedlings, Container Gardening, and Raised Beds

Good soil is essential for healthy plants. Soil for seedlings, container gardening, and raised beds is different than the dirt in your garden. Container gardening soil needs to be light, be able to retain moisture, and have enough nutrients to keep the plant growing. 

Here we share two mixtures for different situations and uses. Mel’s mix is great for seedlings, container gardening inside and outdoors, and also a mixture of freely available materials that is great for raised garden beds. If you are not sure if you have a container garden or a raised bed garden, head over to read Raised garden beds VS Container gardening. 

We are addressing the subject not as professionals, but as end-users. DIY container soil, how to plant in containers, so that we get the most benefits and how to re-use the soil.

Basics about container soil

Container soil or potting soil usually has 3 components, a light growing medium, a moisture retainer, and a fertilizer. Some of the component’s functions overlap.

Common growing mediums that lighten the soil and add volume are Peat Moss, Coco Coir, leaves, straw, sand, etc.

Vermiculite, perlite, or wood (as sawdust, small twigs, wood chips) are used to absorb and release moisture.

Natural compost, composted manure, worm casting, etc are used to add nutrients to the soil to feed the plants.

You can buy ready-made soil from a garden center, however, you most likely will not know what has been used in that soil. 

A growing medium like Growing Mix or Pro-Mix is just a growing medium, it has only two of the three components a plant needs to grow in. You will have to add your own fertilizer to help plants grow. Organic potting soil may have organic fertilizer in it, non-organic may have some chemical fertilizer in it. However, you have to read the labels (and know what they mean) to know what is in them.  

We prefer to mix our own soil that does not require any chemical fertilizer. It helps to keep things simple with great results.

Make your own soil for containers

The DIY soil mixture that we use and have made great experience with comes from the All new Square Foot Garden book.

The book teaches how to make your own Mel’s Mix. You will need

• 1/3 compost (preferably different varieties)
• 1/3 vermiculite
• 1/3 peat moss

All of these are measured by volume in its loose form, not by weight. For example, use a bucket to fill it with vermiculate, then use the same bucket to fill it with loose peat moss and add it to the vermiculate, and finely measure in the same bucket different types of compost and add that to the mixture as well. Now mix everything together and there you have it, a light, nutrient-rich soil. We have been using this mix with great results. 

We use Mel’s mix for all kinds of container gardening and also for the raised bed that is not connected to the topsoil. The advantage of Mel’s mix is that it is reusable for many years. We talk more about it below. 

Soil for seedlings and indoor gardening

If the soil is for growing indoors or starting seedlings indoors, you want to make sure it’s sterilized, so it is bug-free. 

Mel’s mix is great for all kinds of indoor growing. When mixing it yourself, the only part that could have bugs or seeds in it is the compost. You will need to sterilize only the compost. 

A simple way is to heat the compost to 82C (180F). Put the compost into an oven-safe container and heat at 82-93C (180 to 200F) for 30-60 minutes, or when soil temp reaches 82C. Or dedicate an old slow cooker to soil sterilization (that’s what we do), about 2 hours on high will reach the temperature. A thermometer can be helpful. 

When starting seeds in Mel’s mix, you will not have to worry about fertilizing. Natural compost is available to young plants to use nutrients as needed. It simplifies the process a lot. Read more about starting seeds indoors here. 

Free soil for raised garden beds

Raised garden beds can be filled with all-natural materials from your garden. If those natural materials are widely available to you, very much like you would build a Lasagna Garden. For example, layer some straw, to make the soil light then add some compost, for the bottom layer the compost does not have to be of top quality. You can repeat the layers as many times as you need to fill the container. Top up with good fine compost to plant in.

Whenever possible we recommend building your garden beds on the topsoil, so that worms, fungi can enrich it.

Using natural material for containers

This natural mixture can also be used in a container bed, meaning raised beds that are not open to the soil. In an enclosed container you might want to start with twigs at the bottom for drainage, followed by some smaller twigs or wood chips to absorb moisture.

At the end of the growing season add the mixture to the compost pile, and start new next spring. You don’t lose the materials, and if they are freely available to you, go for the natural way. There is no need to buy expensive peat moss or vermiculite. 

In a warmer climate, you might get away using the soil mix for a few years, however, in a cold climate all soil life that is trapt in the container will die over the winter. 

Container garden planting

Not all plants need the same amount of nutrients. There are those that are heavy feeders and even those that provide nitrogen to other plants.

Heavy feeders are plants that have a particularly high nitrogen requirement. These are plants like artichoke, broccoli, cucumber, potatoes, cabbage, corn, pumpkin, tomatoes, zucchini. If you want to grow them, you have to make sure there is enough compost in the soil. We usually add compost, or worm casting into the individual hole at planting time. 

The most common nitrogen fixers in the vegetable garden are legumes. They can be planted together with the heavy feeders. For example corn with beans, or cucumber and peas, indeterminate tomatoes with bush beans. In these combinations, the nitrogen fixers aid the need for the heavy feeder.

Companion planting in containers looks good and plants can help each other to grow and avoid problems. Basil and tomatoes, dill and cabbage, garlic and potatoes, cucumbers and radish are great examples.

West Coast Seeds has a good list for companion planting. Find the plant you want to grow and see what does well with that plant. 

How to re-use container soil

As we said above, if using Mell’s mix as a base, you do not need new soil year after year. Depending on the plants and the size of the container or raised bed, you want to use different methods, but you can grow in the same soil for several years.

Let’s go through three scenarios, here we share our experience. 

Raised garden bed

In our raised garden bed we mostly grow tomatoes, so at planting time we add compost or worm casting to the hole before planting. We might also mulch the bed with wood chips, so we do not have to water as much. In the fall we take out the roots when plants are done and add a layer of compost if we had not mulched it during the summer. The idea is to add some new organic material every year. Since our tomatoes are usually disease-free, we grow them in the same bed year after year.

Grow bags and bigger containers

The soil for grow bags and bigger containers get mixed new every year. However, compost is the only new material that is being added. We found it easiest to deal with container soil to just empty all the containers or grow bags, add a good amount of compost, and mix it thoroughly. At this time it does not have to be 1/3 compost to the volume of the soil, depending on what you grow you might want to add more or less. But mixing it anew loosens the soil and it’s just like the first year.

Small containers

The soil from smaller containers might be done after one year of growth. Just use common sense. If the soil is completely full of roots, it is better to add it to the compost pile. This way it stays in your garden, only that the roots decompose before you use it again.

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