Expert Tips for Making the Best Soil for Containers - Simplify Gardening (2024)

Creating your own soil mix for container gardening empowers you to manage the level of saturation, field capacity, and nutrients, which can influence the performance of your plants.

By adjusting the ratios of inert materials and organic matter in a container gardening soil mix, gardeners can manipulate water and nutrient flow and retention levels, a game-changer for root health, hydration, and nutrient availability.

What Kind of Soil Should I Use in My Container Garden?

This post takes you on a journey of discovery. We’ll start by studying what plants need and how nature meets these needs in garden soil.

The main idea of potting soil is to meet the plant’s needs. Understanding how nature does it allows us to mimic some of its mechanisms for optimal plant growth.

Smaller garden containers, especially indoors, limit our ability to replicate natural systems as some essential participants have restricted access. In these cases, we need to redesign the plans to endure our plants have the best environment possible, given the restraints.

11 Factors that Affect Plant Health and Performance

At the most basic level, 11 factors affect a plant’s performance. The primary element is the plant’s genetics, something a gardener has little control over. Five of the remaining time are above-ground factors, and five are below-ground.

What Plants Require from a Potting Soil

The best potting soil would replicate healthy natural soil in delivering what the plant needs to grow well. What happens above ground in nature affects what happens around the rhizosphere (root zone).

High levels of photosynthesis affect root health and even chemical root signaling, which affect the biota population’s responses, but that can be a topic for another post. We’re interested in meeting the basic sub-surface needs; nutrient availability, oxygen, and water.

Consistent soil temperature is related to the potting soil mix but less so. Without a diverse population of microorganisms (soil food web), the least we can do is add some organic matter – more on this below.

Garden Soil

Healthy garden soil is where structured solids, soil organisms, and chemical compounds interplay to create an environment where plants can thrive. Remove one of these three elements, and you have dirt or a science experiment in a Petri dish.

Healthy garden soil offers a dynamic interplay of plant roots, living microbes, water, air, and nutrients within structured solids (such as decomposing organic matter and weathered rock minerals).

Your Garden Soil Structured Solids

The minerals found in solid rock are crucial to plant health and soil function. About 45 percent of the material in soil comes from the worn particles of local parent rock material.

Soil structure is a product of clay, sand, and silt ratios. The National Resources Conservation Service (NRCS) of the United States Department of Agriculture (USDA) has an online Web Soil Survey where you can research the soil texture profile for your region (WSS)

The interplay of the ratios between clay, silt, and sand regulates water retention and drainage rates.

Sandy soil (0.05 to 2.0 mm) drains fast and contains a lot of space for air but cannot hold nutrients or water. It is low in organic matter, which compromises microorganism populations.

Clay, the finest-grained soil (0.002mm), is excellent at holding water and nutrients but has low air availability. It is high in organic (carbon) content, but soil organisms are mainly anaerobic (airless).

Silt grain sizes (between 0.002 and 0.05 mm) are better than sand in water and nutrient retention but can easily compact, expelling all the air.

Loam is a soil texture that balances the three particle sizes and offers the best growing environment for most plants.

Gardeners have little control over their soil structure as it’s a product of rock formations, environmental actions, and decomposition over the millennium. Adjustments can be affected by adding organic materials, but meaningful changes can take years.

A container garden is your best route if your natural soil quality is poor. It allows you to create and formulate different potting mixes to accurately meet other plants’ needs.

Garden Soil Fertility

Organic (containing carbon) materials are negatively charged, allowing them to attract and lightly hold cations in suspension, readily available when plants need them. This ability of soil to hold cations (positively charged ions) in abeyance is called cation exchange capacity (CEC).

Organic matter steadily releases nutrients for use by plants as it decomposes. Organic matter is also what microorganisms feed on, making them the carriers of several minerals.

More significant organisms, like soil mites, eat the microorganisms, releasing their nutrient load into the soil, usually in the rhizosphere (root zone).

Garden Soil Aeration

These microorganisms are also responsible for creating macro- and micropore spaces in the soil, which are held together by the glue they release.

These pores are essential for trapping air in the soil during extended rain or watering sessions. A soil’s saturation porosity quantifies the air it can trap when flooded.

What is the Best Potting Soil for Container Gardens?

As we saw above, the best potting soil would provide the following essential elements.

1. Soil Biota
2. Root Temperature
3. Nutrient Availability
4. Oxygen
5. Water

Let’s look at these five elements individually, focusing our attention on the bottom three as these are more directly related to the quality of a potting mix.

1. Does Potting Soil Have Microorganisms?

If you’re growing a raised bed garden, even one containerized, there are most likely diverse populations of micro-, meso, and macro Y, and they’re in your container garden.

Microorganisms and organic materials are closely (almost inseparably) linked, which is why organic matter decays. Smaller containers have a lower population and soil biota diversity but still hold hundreds of billions of microorganisms.

The only way to avoid microorganisms in container gardening is to sterilize an inert growing medium. This is often the case in hydroponics, where all the plant’s needs are artificially met.

The challenge with that is that inert materials (in the absence of carbon) have no cation exchange capacity, meaning that all the water and nutrients must be supplied in a continuous flow to allow the plants to access them on the run (*Tony imagines a fast-moving sushi conveyer belt).

2. How to Manage Soil Temperatures in a Container Garden

The advantage of growing vegetables in containers is that it allows you to extend your growing season. This is because the soil is lighter and dark containers heat faster, and the moisture in the soil then retains that heat stabilizing root temperatures.

The advantage to cool-season vegetable plants like beets, brassicas, carrots, Swiss chard, lettuce, radishes, and spinach, is that they can be planted in situ or transplanted earlier in the growing season when soil temperatures are in their seventies (above 21°C).

In some crops (like radish), you can grow plants more often and longer. To learn about growing vegetables in containers, check out my post How to Grow Vegetables in Containers: A Beginner’s Guide.

Soil temperature is integral to growing vegetables and plants. While a container garden can warm up faster than a traditional garden bed, it’s essential to manage potting soil and raised bed soil temperatures through proper hydration combined with organic mulch.

3. Nutrient Availability for Growing Plants

As mentioned earlier, nutrient availability in a potting mix (or any soil) depends on the cation exchange capacity, which depends on the levels of organic matter and microbial activity.

You can add extra nutrients using bone, blood, and fish meal to boost the vibrancy and health of growing vegetables. Excessive use of fertilizers can lead to a salt build-up, so ensure pots and containers have good drainage and occasionally flush the soil mixture.

4. How to Increase Oxygen in Your Potting Mix

Most potted plants fail to thrive because even the best soil can retard water drainage. As soon as the pore spaces collapse, the soil can no longer aerate (draw air in) after being watered, causing the roots to suffocate.

Under these conditions, plant pathogenic fungi like Rhizoctonia sp., Pythium sp., Phytophthora sp. and others thrive, causing root rot. Root rot symptoms resemble dehydration, often prompting gardeners to water more, exasperating the condition.

While organic material benefits in a container garden are applauded, natural decay and the absence of soil particles to form aggregates cause soilless mixes to compact over time.

Mix potting soil with inert materials that won’t decay to solve this problem. Not all inert materials can balance saturation porosity and field capacity levels equally. The table below may help.

Field Capacity is water content remaining after saturation and drainage.

Saturation Porosity is the air content remaining in the soil after saturation and drainage.

Improving Water Retention in Your Potting Mix

Field capacity (water remaining after drainage) and saturation capacity (air remaining after saturation) go hand-in-hand. The water remaining in a potting mix for the few days after watering ensures plant turgidity and is available for photosynthesis and transpiration.

Most of the water is lost through transpiration, and plants with shallow roots (like lettuce and some of the brassica plants) are the first to need a fresh supply.

Plants with deeper roots (like pumpkins and tomatoes) have an extended moisture supply as gravity and evaporation dry to the top layer first.

Succulent plants retain moisture and need less watering (much less). For these plants, it’s essential that the water drains fast and dries out.

Balancing water retention and aeration depends on the ratio of carbon/inert materials. The higher the organic materials, the higher the water retention. As seen above, inert materials have varied field capacities in the short term, but all drain faster.

The Best Container Gardening Soil Mix

Different plants have different needs. As we saw, succulents need rapid drainage and low-field capacity soil. Most container vegetables need some water retention without comprising saturation porosity.

We’ve taken some time to cover the principles; let’s consider how to make it happen.

Best Soil for a Raised Bed Garden

Unlike other potting mixes, a raised bed (and larger containers) should include soil. Firstly, this will save you money, and secondly, it allows the soil biota to create aggregates that improve water management capacities.

Top Soil

Topsoil is a cheaper option than potting soil. Topsoil comes from sites where the top 4 to 6 inches of the surface have been removed, commonly using a grader, and then bagged for resale. Interstate regulations exist for soil migration.

Topsoil includes less organic matter than potting soil and thus has poorer initial drainage capacity. This will all improve with time as the soil biota becomes established.

Compost

Made of decomposed organic materials, compost enriches and enhances the soil and introduces a healthy soil biota. As we know, microorganisms abound where there are organic materials, as does the cation exchange capacity.

All potting soils, by their very nature, include soil biota. Sterilizing these out of the soil is short-sighted and adds unnecessary cost. If there’s soil in the mix, they’ll create micro- and macropores (aggregates) for improved water management, nutrient availability, and pH buffering.

Inert Materials

Raised beds are ideal for growing vegetables but can present drainage challenges. You may want to add some inert materials to improve drainage until the microorganisms do their thing.

Check out my The Bottom Line on Raised Garden Beds: What You Need to Know post for more helpful information.

Additional Organic Matter

Raised beds are at their weakest in the middle, where heavy soil can cause them to bulge. Adding lighter organic material can reduce the overall content weight. Pick from the list below.

Coconut Coir

Coir helps retain water in dry soil, improves soil drainage, is slow to break down (4 years), and creates pockets of air around the roots of plants, allowing excess moisture to drain away. It is 100% organic and eco-friendly, free from soil-borne pathogens and weeds, and has a neutral pH (7.0).

Forest Products

These are items like barks and leaves in a state of semi-decomposition. Semi-decomposed leaves and bark can act as a water-storing sponge. Forest products provide nutrition and good drainage to bridge the gap between organic matter and grit. Ideally, the particle size should be limited to a quarter-inch and exclude dust.

Carbon Activated CocoPeat

I have read that some growers include a carbon-activated cocopeat – a coconut coir brick with some activated carbon. Food manufacturing processes commonly use activated carbon as a filter material to remove impurities. Its use in horticulture is gaining popularity as a pathogen inhibitor.

Sphagnum Peat Moss

I added peat moss to the list to have the opportunity to advise you to avoid it if you can. Peat moss has a low pH (acidic 5.5 – 6.2), and most potting soils that contain it use lime to neutralize the acidity. Using coconut coir as a replacement lessens the chance of anaerobic conditions.

Leaf Mold

Contrary to popular belief, mold is beneficial to your garden lawn. As a result of the tree roots extracting minerals deep underground and transporting them to the leaves, leaf mold contains many valuable trace minerals.

Mold is one of the best soil conditioners you’ll ever use, despite the, and that’s cat’s connotations associated with the word “mold.” If not better, a mold spore can hold five times its weight in water, which is on par with peat moss.

Because of this, leaf mold is an excellent mulch to use around the garden. When added to sandy or clay soils, soil structure and texture significantly improve.

Other Potting Soil Recipes

Commercial Potting Mixes

Commercial potting soil is typically made by blending several organic and inorganic materials to create a nutrient-rich mixture that promotes healthy plant growth.

The specific ingredients and ratios of these materials can vary depending on the type of plant being grown and the particular needs of that plant.

After reading this post, check the content if you ever purchase a potting mix. The content list usually includes a blend of some of the following ingredients:

• Finely ground Fir bark
• Sphagnum peat moss
• Sterile horticultural sand
• Humus
• Peat
• Other ingredients essential to organic gardening
• Redwood sawdust
• Washed sand
• Vermiculite
• Perlite
• Forest products
• Composted centrifuged sewage sludge
• Charcoal
• Organic materials
• Moisture retention materials
• Compost with N. Ca, K, P, Mg, and Fe added

Standard Potting Mix

Here’s a basic recipe for a potting soil mix that you can customize to meet the needs of your specific plants:

Ingredients:

• Two parts compost
• 1 part peat moss or coir fiber
• 1 part vermiculite or perlite
• 1/2 part sand (optional)

Optional amendments such as bone meal, blood meal, or greensand (follow package instructions for application rates)

Instructions:

• Start by gathering all the ingredients and mixing them in a large container or wheelbarrow.
• Add any optional amendments and mix thoroughly.
• Wet the mixture with water until it is moist but not soaking wet.
• Use the potting soil immediately or store it in a covered container until ready.

Note: If you are planting succulents or cacti, you may want to reduce the amount of peat moss or coir fiber in the mix and increase the amount of sand for better drainage. If you plant acid-loving plants like blueberries, you may want to add sulfur to the mix to lower the pH.

Working from a recipe is a good idea when making your potting mix. Once you experiment with your blends, try small test batches to evaluate the mix’s quality. See the recipes below to get started making your potting mixes.

Succulents Potting Mix

I use a simple, well-draining blend of a 50/50 split of organic/inorganic materials for semidesert climate cacti.

• 25 percent Potting Mix with no added fertilizer
• 25 percent Coconut Coir
• 20 percent Expanded Shale
• 20 percent Turface
• 10 percent Pumice

Don’t be tempted to switch the potting mix with peat moss. Peat moss is hydrophobic and difficult to re-wet. Also, be sure that the potting mix does not have any added fertilizer, as this will negatively affect the leaf growth of your succulents.

Ideally, a terracotta pot allows air and water movement through its walls. All pots must have draining holes, and these should not be covered. Test your mix for water retention and flow. Do not use stones or pebbles at the bottom of the pot.

When watering your pot, water should not sit on top but readily flow through the planter. Due to capillary action, some water will remain in the soil, but your mix of coconut coir and the other gritty material will ensure that the roots are not soggy yet retain some moisture.

FAQs on Expert Tips for Making the Best Soil for Containers

How do you make good soil for containers?

To make good soil for containers, start with a base of high-quality potting soil and mix in organic matter such as compost or peat moss. Adjust the ratios of inert materials like perlite or vermiculite to manipulate water and nutrient retention levels. Choose soil types that match your plant’s needs, and ensure proper drainage. Test the soil periodically and amend it to maintain optimal plant growth.

What is the best soil mix for containers?

The best soil mix for containers provides adequate drainage, water retention, and nutrients for plant growth. A good mix usually contains a combination of peat moss, perlite or vermiculite, and compost or fertilizer. Adjusting the ratio of these ingredients can help tailor the mix to suit the specific needs of different plants.

Do I need to add fertilizer to potting mix?

It depends on your potting mix and the plants you are growing. Some potting mixes already contain fertilizer, while others do not. Most plants will benefit from adding fertilizer to the potting mix. Still, it’s important to follow instructions and not over-fertilize, as this can damage the plant.

In Closing

Whether making a soil mix for growing container vegetables or succulent soil, doing it yourself is a natural form of self-empowerment. It helps you fully appreciate the symphony of parts that make up the greater whole.

Making your potting soil is fun!! Happy gardening, fabulous people.