Plant Soil and Other Additives

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Plants, generally speaking, need soil of some kind to grow in. While some plants can grow in unusual substrates, most need organic matter such as soil. This shouldn’t come as much of a surprise to most people. What can be surprising is the amount of different soils and soil components available today! 

Plants are incredibly adaptive and can tolerate many different soil substrates. For many plants, you can use various different soil substrates and the plant will do just fine. Some species are more sensitive and require certain substrate conditions such as lower or higher pH. It is important to do a bit of research into the substrate needs of your plant to see if an ideal growing media can be identified. 

While many pre-made soils exist today such as ‘seed starter’ or ‘cactus mix’ soils, you can always learn about each soil component and get creative with your own substrates! In this article, we’ll be covering some of the basics of creating a quality substrate as well as some common additives. 

For a more in-depth look at the myriad soil components that can be used, check out our glossary of soil components here! 

Plant Soil and Other Substrate Characteristics and Considerations

A few basic characteristics of any soil substrate that need to be considered are: soil aeration, water drainage rate, chemical adsorption, and soil pH. Each of these characteristics are important to consider when choosing the substrate in which you’ll grow your plants. Luckily, each of these characteristics can be adjusted and tweaked by using different soil components.

Soil Aeration

Soil aeration is very important for healthy plants because it helps the plant uptake oxygen and helps protect against root rotting. If a plant doesn’t get enough air, it can damage the plant and inhibit its growth. If the lack of aeration is severe, the plant is susceptible to death. Ensuring your soil substrate provides ample room for aeration is a rather straightforward process.

To increase the amount of aeration in your substrate, you need to increase the pore spaces within the media. One of the simplest ways to do this is to add a hard rock type material such as pumice, perlite, or lava rock to the soil mix. Added bonuses of adding these components are increased water drainage (helpful for most all plants) and the adsorption of chemical nutrients. Rock materials that is porous such as lava rock has an additional benefit as beneficial microbes are able to colonize in the pores.

Water Drainage Rate

Water drainage rate is a critically important consideration for growing plants, especially if the plants are grown in pots or containers. If water does not drain from the pot quickly enough, the soil near the roots will become waterlogged. Waterlogged soil suffocates roots and provides the perfect conditions for root rot to set in. To increase water drainage, ensure the soil substrate has lots of porosity by including things like rocks, sand, or bark chips.

Pot size plays a role in the retention of water as well. There is a reason for root-soil volume ratios. Too much soil per root mass can quickly lead to the soil becoming waterlogged as the roots aren’t able to absorb all the water. If you have a plant with a small root mass in a large pot, ensure the soil drains very well. Always be mindful of how much you are watering your plants and try to not overwater!

Chemical Adsorption

Chemical adsorption is the scientific terminology used to describe the process by which chemicals such as plant nutrients (N, P, K, etc.) or contaminants (mercury, lead, etc.) are held onto by materials’ surface in a thin film rather than washing away. This is beneficial to plants as the nutrients are accessible to the roots rather than simply washing out of the soil substrate (also known as leaching). Adsorption is very important if you use water soluble fertilizers when watering your plants. 

Porous materials are excellent at trapping nutrients through adsorption. Adding porous material to your soil mix not only manages the water drainage rate but chemical adsorption efficiency as well.

Soil pH

Soil pH is also very important for optimal plant health. The pH of the soil substrate is simply a measure of how acidic or alkaline (basic) the material is. Some species have specific pH ranges or values they thrive in such as blueberries and great basin bristlecone pine trees. Most often, a species’ pH preference will be based on its native range. An example is the great basin bristlecone pines that live in areas with very high calcium carbonate content in the soil. As a result, this species is very suited to alkaline soils (high pH).

Other Soil Additives to Supplement With

Depending on your plant, there are various soil substrates you can add to increase or decrease certain soil properties. Water drainage rate, pH, and water and nutrient retention are just some of the properties that can be adjusted based on adding other soil substrates. But first, what are some common soil substrates that are added to plant soil?

Common Soil Substrate Additives

Bark is a common additive to soils for it’s ability to increase soil water drainage due to its ability to increase the pore spaces in the soil column. In addition, as bark decomposes over time, nutrients stored in the bark tissue are released into the soil column where they can be assimilated by the plants roots, nourishing the plant over time. Bark chips can be mixed directly with the soil.

Charcoal is often added to plant soils to reduce toxins in the soil column due to charcoal’s ability to adsorb various compounds to its surface. This binds the toxins to the charcoal and helps reduce the likelihood of them being assimilated by the plant’s roots. Charcoal can be added as chips or pieces that can be mixed directly into the soil.

Mycorrhizae are fungal organisms that form mutualistic relationships with plant roots via their hyphae. These fungal associations are present in nearly all natural ecosystems. The fungi help the plant access nutrients in the soil that would otherwise be inaccessible to the plant.

Perlite is a small porous rock additive that also increase soil drainage to ensure your plant’s soil does not retain water long enough to cause rot. Perlite comes in a variety of sizes which is ideal for adding it to different mixes. Small perlite can be added to slightly increase drainage or large perlite can be chosen to add significant drainage.

Pumice like perlite, is a rock additive that increases soil drainage. Additionally, the pores of the pumice rock are ideal colonization sites for beneficial soil bacteria. These beneficial microbes get more shelter from water running through the soil in the pores and are able to remain in the soil longer.

Worm Castings is an additive that is loaded with nutrients that help make your plants happy and healthy. The material itself is actually the waste from worms that have eaten decaying material in soils. Worms digest the organic matter and excrete the remnant as worm castings. In addition to the nutrient rich nature of worm castings, this additive is great at retaining moisture and can be used to increase moisture longevity in the soil column.

The Importance of An Ideal Plant Soil Mix

The soil substrate you choose for your plants is one of the most important choices you can make for their health. As long as you keep these main characteristics in mind, you can feel confident you are choosing the best soil substrate to keep your plants happy and healthy. If you are crafting your own specialty soil substrate, keep these factors in mind and match them to whichever plant you are planting. With a bit of experimenting and practice, you’ll find the ideal soil mixes for your favorite leafy family members.

Of course if you don’t want to experiment and just want some great pre-made ideas, check out our list of homemade soil substrates for various different plant types.  

How to Choose an Ideal Soil Mix for Your Plants

When it comes to choosing the ideal soil mix for your plants, there are a few considerations to keep in mind. Like all other things regarding how to keep plants happy and thriving, most of the considerations below have to do with the natural environment the plants are native to.

Thinking of the plant’s natural environment and modeling your care and the conditions you provide it is one of the most effective ways to successfully raise plants. Think about it, if you bring home a tropical plant like a philodendron, you aren’t going to keep it outside in the Pacific Northwest during winter! Instead, you’ll focus on providing heat, humidity, and light; physical aspects of the tropical regions these plants are native to.

The same logic should be applied to creating the ideal soil mix for your plants. If you have a plant native to desert regions, you’ll want to focus on sand, rock, and humic (carbon rich) materials in that order. This provides optimal drainage with sufficient moisture retention and nutrient availability. Conversely, tropical plants prefer high nutrient (high humic content) soils that provide adequate aeration and drainage. 

Some plants have speciality needs for their soil mixes. Plants that live in unusual or extreme environments can benefit from you trying to mimic these unique conditions. Great Basin Bristlecone Pines for example live at high elevations where the soils are very rich in calcium carbonate; this increases the pH making the soil much more alkaline.

Plant Soil and Other Additives, Final Word

Soil is such a complex material that is should be one of the most thought-out and researched aspects of your intended plant growing efforts. An ideal soil is literally the difference between a plant that struggles and, often times, perishes and a plant that thrives for years to come. Soil additives can help turn your soil into the ideal growing substrate for your plants if formulated well.

Whatever plant you are intending to grow, researching that plant’s natural environment will help you to craft a soil mix that will keep your plant healthy and thriving. Most plants are complex and hardy and can tolerate various conditions but their natural environment is already well-suited to support them. As the old adage goes, if it isn’t broken, why fix it?