Making Your Own Potting Soil – How To Mix Potting Soil For Indoor Plants

What Are The Basic Components Of Potting Soil?

There are four basic parts of potting soil that you need to account for if you decide to make your own potting soil mix. Different materials can be substituted depending on product availability and your gardening or environmental priorities, but your potting soil mixture will need to contain the following components:

• Base Substrate
• Moisture Retention/Aeration
• Nutrition
• Drainage

Containers are great for a number of reasons, namely that they’re versatile, space-saving, and allow you to be more efficient in your use of resources. If you have a small garden space or live in an apartment with a balcony or rooftop, containers are for you.

Choosing A Base Potting Soil Substrate

This is the foundation of the potting soil. The substrate is the most abundant item and will give your soil its bulk and its primary structure. Substrates are generally inert, meaning they offer no nutritional value to your plants. Substrates are largely responsible for the physical form of your potting soil and provide a base to incorporate the other elements of your potting mix.

There are basically two options for substrates: 

Coco Coir-Based Potting Mix

This material comes from the husk of the coconut. Coco coir was treated as a waste product for decades until its use as an excellent potting soil substrate was realized. It has fantastic water retention properties and a texture that encourages healthy root development. In recent years, there has been a shift towards greater adoption of coconut coir-based potting mixes, since it is generally considered a more environmentally sustainable option than the other most popular soil substrate option, peat moss. It is usually packaged in compressed blocks that need to be hydrated in order to expand to its full volume.

Peat Moss-Based Potting Mix

Also called sphagnum peat moss, this material is the product of slowly decayed and compacted moss in peat bogs. These centuries-old swamps are harvested by draining and drying the soil so that the peat can then be vacuumed up for transportation to be processed and distributed. The product is a fine, soft, and airy material that has been the go-to substrate for container gardeners since, well, long before anyone was saying “container gardening.” Peat moss holds on to nutrients and water very well but needs additions to provide aeration and better structure for plant roots. Peat moss is also slightly acidic, so it needs to be cut with a substance like lime to neutralize the soil pH.

Adding Moisture Retention And Aeration To Your Potting Mix

Good soil doesn’t just absorb water but will hold on to that moisture over time and release it to healthy plant roots as they need it, while simultaneously not drowning the plant’s roots and causing root rot or a myriad of other issues. Aeration is very important for healthy plant roots, therefore a balance must be struck between soil hydration and aeration. There are a number of soil amendments that help to moderate moisture and air in the root zone and do so simultaneously.

The two most popular amendments for adding moisture retention and aeration to a homemade potting soil mix are:

Vermiculite 

Vermiculite is the product of silicate materials that are heated to expansion, creating tiny worm-like grains that can be easily mixed into the soil (‘vermiculare’ is Latin for ‘to breed worms’). Vermiculite is excellent at holding on to water in such a way that roots can easily access it without the risk of being drowned, making it an excellent addition to your homemade potting soil mixture. This ties into vermiculite’s other great quality: it provides water retention while also being an aerator. Vermiculite’s porosity is great for retaining moisture while also creating space for air to reach healthy plant roots. Vermiculite’s chemical structure means it will also hold on to soil nutrients for easy access by your plant’s roots. It is slightly more expensive than Perlite, which is why you are more likely to see perlite in store-bought bagged potting soil mixes.

Perlite 

Perlite is another product created by heating an inorganic substance to expand, in this case, volcanic glass. Perlite is extremely lightweight and consists of more air than anything else. Its pore structure releases water to plant roots very easily, although it does not hold that water nearly as effectively as vermiculite. Due to its airier nature, perlite is best utilized in a potting mix where light soil structure and drainage are important, such as with cacti and succulent potting mixes. 

Ensuring Your Potting Soil Has Sufficient Nutrition

While soil structure and moisture retention are extremely important, a plant can only get so far without nutritious soil. Since most soil amendments are inert, some kind of nutrient source will need to be added for the sake of healthy roots and plant growth. There are many ways to get nutrients in the soil, and the best kinds for your system are subject to what you are growing and your ultimate goals. We will cover the most popular and feasible for the home gardener. You can also read up a little more on fertilizers here, where we spoke with Tim Flood, of Nature Hills Nursery.

Compost 

A gardener’s best friend, compost not only provides fantastic nutrition for your plants, but is a great way to recycle food scraps, yard waste, and other organic matter you may otherwise discard. Even if you don’t compost at home, you can still find compost for sale at garden and hardware stores (although we definitely recommend looking into more local sources such as municipal compost or nearby farmers). Compost is full of macro and micronutrients, as well as beneficial microbes and fungi to help your soil stay healthy and be the most productive for your plants.

Organic Fertilizers 

Organic fertilizers can consist of many different things, from agricultural or animal byproducts to mined materials and plant-derived substances. All organic fertilizers have different nutrient compositions, NPK ratios, and benefits to different plants, and there are far too many to discuss in this overview. You can find a great organic fertilizer recipe here. It is probably wise to add organic fertilizers to all potting soil mixtures, homemade or not, at some point in their life. As plants grow, they consume the nutrients in the soil, and these nutrients have to be replaced somehow in order to maintain productive soil.

Worm Castings

Worm castings are an excellent addition to any soil mixture. Also called vermicompost, worm castings are the product of worm digestion. Worms eat organic matter such as food waste, and they produce nutrient and microbe-rich material that resembles humus or topsoil. This material is well-balanced and rich in nutrients and releases nutrients slowly over time so you don’t have to worry about over-feeding your soil. Worm castings are not only a great organic nutrient source, but they also help close the loop in a circular economy of food production, making them a personal favorite soil amendment here at Urban Leaf.

Composted Manure 

Animal manure must first be composted before you can use it for gardening purposes. Composting breaks down any harmful microorganisms so all that’s left is good, rich plant food. Manure from cows and chickens is most popular, although horse manure is a common choice as well. Fresh manure is very high in nitrogen and can ‘burn’ plants if applied directly. Composting manure makes it a slow-release fertilizer that won’t nutrient burn your plants as fresh manure would. Composted manure provides good soil structure and helps with moisture retention in addition to its great nutritional value for your plants.

How To Add Drainage To Your Potting Soil Mixture

Good soil drainage is the result of a well-balanced soil structure in general but can be modified depending on your plant’s needs. All plants need some degree of drainage in order to develop healthy roots. No soil should be waterlogged and unable to drain, but some plants like cacti and succulents prefer very well-drained soil.

Sand 

Sand is the most common soil drainage amendment for container potting soil mixtures due to its relative abundance, effectiveness, and ability to blend well into soil mixtures. The small sand grains distribute well and provide dispersed drainage benefits to any potting soil mixture. Sand is also inert, meaning you don’t have to worry about it affecting nutrient availability or soil pH. Coarse sand is generally best.

Pea Gravel Or Small Stones 

Any kind of small pebbles, gravel, or stones work well to increase drainage in a soil mixture. For most potted plants, these are best used, if at all, in the bottom of the container to allow drainage. Though not necessary for most edible herbs and vegetables, gravel is important for providing the level of drainage preferred by most cacti and other desert plants. Just make sure you rinse any store-bought gravel or pea stones before using them, since they may be covered in silica dust which can present a health hazard.

Pumice 

Pumice is a lightweight volcanic rock with a porous structure that loosens soils and allows for exceptional air circulation. The larger pores create air pockets in the soil, as well as a foothold for beneficial microbes and mycorrhizal growth. Pumice can be used to break up clay soils, and serves cacti and succulent potting mixes well, although it is generally unnecessary for vegetable and herb container gardening.

Wood Chips And Bark 

Wood chips composted pine or fir bark, and other similar wood products are common gardening materials due to their low cost and accessibility. Although they are great for mulching and providing aesthetically pleasing ground cover around outdoor greenery, they should not be used for container gardening. While they can help provide drainage in moderation for trees and large shrubs, non-decomposed woody matter ties up nitrogen in the soil, making it inaccessible to plants and potentially necessitating the addition of nitrogen fertilizers.