Succulent Care Series: The Dirt on Soil

In this blog post, I aim to tackle the age-old question of "What is the best soil for succulents?" As an avid succulent grower for nearly 30 years, I have learned a lot through trial and error. However, I want to emphasize that the advice I provide is based solely on my personal experience and may differ from what others suggest. Nonetheless, I hope that the insights I share will be useful to those looking to optimize their succulent-growing practices.

Soil is an essential part of a plant’s reality. Soil is a crucial component of a plant's existence. While animals move around and seek out food, plants are rooted to the ground, relying on soil to provide the essential nutrients necessary for growth. In addition to nourishment, soil serves as a habitat for plants, allowing them to photosynthesize and create the sugars they need to survive.

Habitat soils. Take a moment to observe the soil around you. Next time you're outside, pick up a handful and examine its texture and smell. Personally, I love the scent of fresh soil, which comes from compounds like geosmin produced by microbes within it. In nature, soil is a complex mixture of organic matter, minerals, water, and air that varies depending on factors such as climate, geology, and vegetation. There are numerous types of soils, and the study of soil science is dedicated to understanding their properties and characteristics. 

In their native habitats, succulents may grow in sandy, silty, rocky soils with a high content of mineral matter.

The picture above shows a Pleiospilos I encountered in it's natural habitat in South africa in 2006. Haworthia bayeri grew nearby. This picture is from a recent trip to cactus and succulent habitats in Palm Springs:

These pictures give you an idea of the mineral-rich composition and low organic matter content of soil in natural succulent habitats

Bagged soils. Packaged soil mixes are a convenient option for many gardeners, including those who grow succulents as well as other plants like peppers and zinnias. Unlike natural soil, these mixes are specifically designed to provide the right balance of nutrients, water retention, and drainage for potted plants. They are typically made by blending materials such as peat moss, vermiculite, perlite, compost, sand, and other organic or inorganic components in varying proportions to create a soil-like mixture.

While some succulents, such as Kalanchoe daigremontiana (mother of thousands) and some of the common big box store succulents can thrive in packaged soil mixes, most of the choice succulents like Euphorbia obesa and Haworthia require faster draining and less rich soil mixes. Even those mixes labeled as "cacti and succulent" can be too rich and retain water for too long, which can be detrimental to the health of the plants.

What soil mix should I use for my succulents? There's no one-size-fits-all answer since the ideal soil mix can vary depending on the species of succulent you're growing. That said, I recommend making your own soil mix, or you can buy reliable pre-made mixes like the ones I sell. Later in the blog post I will provide recipes for soil mixes I find work for different succulents.

It's also important to consider the type of pot you're using, as this can impact the choice of soil. Some people prefer red clay pots, but for soil mixes that are highly free-draining, such as those I recommend, the soil can dry out too quickly in these pots. For this reason, I tend to use plastic pots for most of my plants, and occasionally glazed clay pots for show or special plants.

Importance of drainage holes. It's important to use pots with draining holes for long-term success with succulents, regardless of what you might read on social media. Unless you're collection consists only of the impossible to kill Mother of Millions, it's best to use pots with proper drainage. Drainage holes have a significant effect on the water table within the soil, providing an outlet for excess water to escape and allowing the soil to drain freely. This prevents the buildup of excess water in the bottom of the pot, promoting healthy root growth and preventing the soil from becoming waterlogged.

In addition to preventing water buildup, drainage holes also allow air to circulate through the soil, promoting healthy root growth and preventing soil compaction. Compacted soil can inhibit root growth and nutrient uptake, so ensuring that your soil has adequate drainage is crucial for the health of your succulents.

Understanding the Water Table in Pots The water table in pots refers to the level of water within the soil that is held in the container. When you water a potted plant, the water initially saturates the soil and fills any available pore space. Any excess water that cannot be held in the pore space will accumulate at the bottom of the container, creating the water table.

If the water table is too high, it can cause the soil in the pot to remain wet for too long. This happens because the water table will be in contact with the bottom of the soil, preventing any further drainage of excess water. This can cause the soil to become waterlogged, leading to a lack of oxygen in the root zone and promoting the growth of harmful microorganisms that can cause root rot and other plant diseases.

It's worth noting that the dimensions of the pot can affect the water table. Surprisingly, very shallow pots can tend to remain wet longer than deep pots. This is because water can get trapped in the water table of shallow pots, preventing proper drainage. In contrast, gravity forces water down in deeper pots, ensuring that excess water drains away from the roots and does not get trapped in the water table.

Using a soil mix that is rich in mineral matter, such as pumice, can be beneficial for maintaining a well-draining water table in the pot. Mineral matter creates air pockets in the soil, which allows excess water to drain away more easily and prevents the buildup of water in the pot.

Soil mix ingredients:

Peat moss. Peat moss is a type of partially decomposed plant material that is commonly used in packaged soil mixes. It has a fluffy texture and can retain moisture and nutrients, which is beneficial for some plants. However, there are several drawbacks to using peat moss in soil mixes.

Firstly, peat moss will break down over time and can eventually compact the soil. As it breaks down, it seems to metamorphose into a substance that is toxic to the roots of many succulents.

Additionally, the harvesting of peat moss is environmentally unfriendly and unsustainable. It involves the removal of large amounts of peat from bogs, which can damage delicate ecosystems and release large amounts of carbon into the atmosphere.

For these reasons, I recommend avoiding using peat moss as a soil ingredient for succulent mixes. A better option is to use coir, a sustainable and environmentally friendly alternative made from coconut husks. Coir has similar moisture-retention properties to peat moss but does not break down as quickly or release toxic substances into the soil.

Coir.

Coir, also known as coconut coir or coco coir, is a natural fiber that is derived from the husk of coconuts. It is a byproduct of the coconut industry, which typically discards the husks after the edible meat and water have been harvested. More and more soil mixes are using coir instead of peat moss. In my experience it won’t break down as quickly as peat moss and does not break down into something that is toxic to plant roots. One of the ingredients I use in my soil mixes is a local product, “Ultra Potting Mix” by American soil products in the CA Bay Area. The main ingredient in this mix is coir and it lacks peat moss.

Perlite. Perlite is a naturally occurring volcanic glass that is mined and processed into a lightweight, sterile, and porous material that is commonly used in horticulture and gardening. It’s a common ingredient in packaged soil mixes where it helps improve drainage and aerate soil mixes. Although perlite can improve drainage, the way it sticks together and interacts with the walls of the pot can also have the opposite effect, by trapping water in the moisture table of the pots. I recommend avoiding perlite as a soil ingredient. Pumice is a much better option.

Pumice

Like perlite, pumice is a type of volcanic rock with a lightweight texture and is used as a soil amendment to improve soil structure, aeration, and drainage. It has many advantages compared to perlite. It is heavier than perlite and wont float to the top of pots, It is harder than perlite and wont break down the way that perlite can, it retains moisture slightly longer than perlite which can dry out too quickly, it will not trap water in pots like perlite sometimes can and it is better at retaining nutrients. It is also a key component to successfully growing succulents, at least for me. When I first started growing succulents I had terrible luck with root rot. Frustrated, I switched to growing them in pure pumice with a layer of gravel on top to keep to keep it from drying two quickly. Many succulents will thrive planted in 100% pumice. At some point I started adding coir and other ingredients to the pumice to keep my soil from drying too quickly and optimize my mixes.

Turface and clay pellets. These are both fired clay products but are produced in different ways and are different sizes. Turface is a brand of fired clay soil conditioner that is widely used in horticulture and sports field management. The size of the particles ranges from 0.25-5 mm. Fired clay pellets are commonly used in hydroponics. The size of the particles ranges from 4-16 mm. It is commonly used in hydroponics. It is made from fired clay and is available in a wide range of sizes and shapes. I have experimented with adding both to my soil mixes but I don’t currently add them to my current mix because I prefer the qualities of pumice.

Decomposed granite 

Decomposed granite is a type of natural rock that has been weathered and broken down into small particles over time. It is an ingredient that most resembles the natural soils where succulents grow.  It is typically reddish to tan colored and is heavy with a gritty, silty almost clayish texture with small, granular particles that range in size from 1/4 inch to powder-fine. When used as a soil amendment, decomposed granite can provide a number of benefits, including improved drainage, increased soil porosity, and enhanced soil structure. I suspect it also provides a slow supply of micronutrients to plants. I add decomposed granite to my soil mixes and it is the main component of the soil mix I use for lithops and other mesembs.

Sand and gravel. Sand and gravel are both composed of rock particles but the size and composition of the particles is different. Sand is composed particles that range in size from 0.063 to 2 mm. Gravel, on the other hand, is composed of larger particles that range in size from 2 to 64 mm. I strongly recommend against using fine sand in soil mixes because it will make the mix stay too wet. I am agnostic about the use of gravel.

This list of ingredients is not exhaustive but I tried to cover the most common ones that people might encounter. There are other ingredients I did not cover like vermiculite and fir bark but these are not things I recommend adding to your succulent mix.

Fertilizers and biological additives:

Slow release fertilizer. Slow-release fertilizers are fertilizers that release their nutrients gradually over an extended period, rather than all at once. They are designed to provide a steady supply of nutrients to plants over time, which can be beneficial for both the plant and the environment. Slow-release fertilizers are typically made up of granules or pellets that are coated with a polymer or resin. This coating helps to regulate the release of nutrients by slowing down the rate at which water and other environmental factors break down the fertilizer.

Biological additives. Biological products contain beneficial microbes that can be a useful addition to a plant care routine, as they can help to support healthy plant growth. They are somewhat controversial for some people and I have been accused online of promoting witchcraft for recommending them. Broadly speaking they can be divided into 3 different categories:

1. Mycorrhizae are a type of beneficial fungi that form a symbiotic relationship with plant roots. The fungi colonize the roots and create a network of hyphae (thread-like structures) that extend out into the soil, increasing the surface area of the roots and helping the plant to absorb water and nutrients more efficiently. In exchange, the plant provides the fungi with carbohydrates that are produced during photosynthesis.

2. Trichoderma fungi: These fungi can help to suppress plant pathogens and improve plant growth.

3. Beneficial bacteria: These are bacteria that can help to fix nitrogen, break down organic matter, and suppress plant diseases.

Biological soil additives are widely used in various agricultural industries to improve soil health and promote plant growth and these products are frequently used in the cannabis industry where growers are constantly seeking innovative ways to improve the growth and yield of their crop. 

I add two products to my soil mixes: “Great White” which is mixture of mycorrhizae and beneficial bacteria and “Root Shield” a Trichoderma based product.

Other mineral additives:

Mohr's salt (ammonium iron(II) sulfate). I started adding this to my soil mix from information I gathered from my consulting experience. It provides a very easily accessible form of iron and has successfully been used to provide iron in hydroponics. Since Haworthia and other south African succulents grow in red iron rich soils, I feel like they benefit from this. It is probably the trickiest to find of the ingredients I add to my soil. It is available on Amazon from different sellers located in India.

Calcium chloride. I add this to provide extra calcium to the soil. It is available on Amazon from different sellers located in India.

Azomite. A broad spectrum natural mineral product mined from a unique deposit in Utah. I started adding this product because I was involved in some consulting work for them. I add it a small amount to all of my mixes.

My Soil mix recipes:

I make my soil up in large tubs. My base mix is different ratios of pumice, “Ultra Potting Mix”(a local coir based soil mix), decomposed granite, with small amounts of Osmocote Indoor & Outdoor slow release fertilizer(handful), Azomite (handful), Great white (tablespoon), Root shield(handful) and very small amounts (½ teaspoon) of Mohr’s salt and calcium chloride (½ teaspoon).

My Haworthia mix is ~85-90% pumice, ~5% Ultra Potting Mix and ~5% decomposed granite with small amounts of all of the other additives.

My Euphorbia mix is ~75-80% pumice ~15% Ultra Potting Mix and ~5% decomposed granite with small amounts of all of the other additives.

My mesemb mix is ~70% decomposed granite, ~25% pumice and ~5% Ultra Potting Mix. The only additives I add are the Great white and Root shield

I currently sell 1 gallon bags of all 3 soils, see the “Pots and soil” tab for more details. Soil is shipped in small priority mail boxes but I am currently evaluating if other shipping options will be more cost effective to customers.