Which Type of Soil is Best for Container Gardening?

Choosing the right container and soil will set your container garden up for success!

Container gardening is a fantastic way to bring the joys of gardening into small spaces like balconies, patios, and sunny windowsills. This approach makes cultivating herbs (and other plants) very attainable, even when space is limited. Once we have settled on what we want to grow, the selection of a healthy, nutrient-rich soil medium is essential so that our plants thrive and produce an abundant harvest.

Soil terminology is often used interchangeably which can be confusing or misleading.

There are so many soil variations and some are not even soil!

Here are a few terms that will help us choose or create an appropriate soil medium for our container garden:

• Seeding soil: A lightweight, soil-less blend, formulated for starting seeds. It has a fine texture that allows for easy root penetration and seedling development. Sterile, so it does not contain insects, diseases, or weed seeds. Recommended for starting seeds indoors. 

• Potting mix: A sterile, soil-less blend of organic materials, such as peat moss or coconut coir, along with perlite or vermiculite. It is lighter than potting or garden soil and provides optimal drainage and aeration for potted plants. Excellent for indoor or outdoor plants.

• Potting soil: A mixture that may or may not contain actual soil, and can be designed for container gardening or raised beds. Check labels for container vs. in-ground and indoor vs outdoor use. Usually consists of a blend of organic matter, perlite, vermiculite, and other amendments (see below for common amendment definitions). Provides good drainage and aeration, can be fully organic, and is more compact than a soilless potting mix. Again, check the labels before using inside or expect new flying friends.

• Raised bed soil: Soil specifically formulated for raised bed gardening, typically a combination of topsoil, compost, and other organic materials. It provides good drainage and fertility for plants grown in raised beds. Without serious amendment, it is too compact for container plants and isn’t a sterile medium so even when heavily amended, it should be reserved for outdoor use.

• Garden soil: The natural soil found in outdoor garden beds, is typically a mixture of sand, silt, clay, and organic matter. Garden soil can vary widely depending on location and may require amendments for optimal plant growth. It is too compact for container gardening and not sterile.

With all this in mind, we want to use sterile seeding soil to start our seeds indoors (or purchase seedlings directly from a garden center). Since we are focused on container gardening, once our seedlings are ready to be transplanted, we’ll need a high-quality potting mix specifically formulated for our chosen plants.  Container soil options are designed to provide excellent drainage while retaining moisture, which is crucial for healthy root growth. Again, avoid using garden soil for containers as it can become compacted and may contain pests or diseases.

Soil amendments provide nutrients, aeration, and water retention.

While there are plenty of commercial potting mixes available for purchase, we can also create our own. This DIY approach allows us to customize a soil medium that suits the specific needs of our chosen herbs (or other plants) and can be a cost-effective option depending on what we have access to.

Some common medium alternatives to be familiar with are:

• Peat moss: A natural organic material derived from decomposed sphagnum moss. It is commonly used as a soil amendment to improve moisture retention and aeration. Peat moss helps create a light, fluffy texture in soil and is often used in potting mixes and raised beds. Peat moss is acidic and will lower pH levels.

• Coconut coir: Also known as coco coir, it is a natural fiber extracted from coconut husks. Coconut coir is prized for its excellent water retention properties and ability to promote root growth. It is a sustainable alternative to peat moss and is commonly used as a component in potting mixes and hydroponic systems 

• Perlite: A lightweight volcanic rock material that has been processed into small, porous granules. Perlite is added to soil mixes to improve drainage and aeration. It helps prevent soil compaction and promotes healthy root development by allowing air and water to move freely through the soil.

• Vermiculite: A mineral material that undergoes expansion when heated, resulting in lightweight, absorbent flakes. Vermiculite helps retain moisture in the soil while also improving aeration. It is often used in seed starting mixes and as a soil amendment for plants that require consistent moisture. 

• Compost: A nutrient-rich organic material produced from the decomposition of organic waste, such as kitchen scraps, yard trimmings, and manure. Compost improves soil structure, fertility, and microbial activity. It provides essential nutrients for plant growth and helps retain moisture in the soil

• Worm castings: Also known as vermicompost, worm castings are the nutrient-rich waste produced by earthworms as they digest organic matter. Worm castings are a highly concentrated source of plant nutrients, beneficial microbes, and enzymes. They improve soil structure, enhance nutrient availability, and stimulate plant growth when added to soil or used as a top dressing.
  

When choosing or creating our container mix, we should be mindful of our plant’s particular growing requirements.  Unfortunately, there isn’t one magic soil formula that will fit every plant’s needs. Different growing mediums will affect soil pH levels, aeration, water retention, and a plant's ability to absorb nutrients. By choosing the right soil mediums and pairing appropriate companion plants together, we can maximize the growth potential of each container.

For example, let's say we are planting lavender and rosemary together. These lovely companions prefer a slightly alkaline, dryer, well-draining soil that can be obtained in various ways.

Lavender and rosemary make great companion plants and are container-friendly.

First, we’ll need a large container with good drainage holes. Choose a container that correlates with the growing space needed for your plant varieties. Lavender, for example, can range from 12 to 36 inches wide when fully grown and we don’t want to have to transplant once established.

Lavender thrives in large planters.

Next, we need a suitable potting mix. Lavender and rosemary are Mediterranean plants that flourish in full sun and sandy-loam soil. Sandy loam mix usually consists of about 50% soil solids (such as sand, silt, or clay), 50% pore spaces (peat, coir, perlite, bark), and water. A more dense clay would require additional pore space for aeration, while fast-draining sand will need pore space that leans towards water retention. Remember, there is no one right answer here, rather a good balance. Let’s look at different ways to obtain a slightly alkaline, well-draining, moderately nutrient mix for our lavender-rosemary container companions.

One possible lavender-rosemary soil combination could include:

• Sand: 1 part

• Perlite: 1 part

• Compost: 1 part

• Coconut coir: 2 parts
  

As we consider various soil-medium options, we may select neutral coconut coir over acidic peat moss for structure. Though coconut coir retains moisture, its air-to-water ratio will allow our lavender and rosemary roots enough time to absorb nutrients without becoming water-logged. Adding sand and perlite are sound options for aeration, while compost or worm castings will provide adequate nutrients.

Another lavender-rosemary soil combination could consist of:

• Sand: 1 part

• Vermiculite: 1 part

• Worm castings: 1 part

• Peat moss: 2 part

• Limestone: See below
  

Sand will ensure a fast-draining soil and worm castings will provide nutrients and serve as a pH buffer. Moisture-retaining vermiculite will allow lavender and rosemary roots time to absorb nutrients, especially in hot climates where potted plants dry out faster. Here, we counter the acidity levels of peat moss with limestone. While approximately ¼ cup of limestone is recommended for every 6 gallons of peat moss, desired pH levels may vary. Test soil pH and shoot for slightly alkaline, 6.5-7.

Proper growing conditions will lead to big harvests! What will you do with all that lavender and rosemary?

Understanding what any given plant needs to thrive is key and will arm us with the confidence to create appropriate soil mediums for our plant babies. Numerous soil medium combinations will sustain our plants and with a little understanding, we can adjust as needed to fit our growing conditions, climate, and product availability. Here’s to the start of a thriving container garden!