As the saying goes, healthy soil, healthy plants, healthy people. Compost, topsoil and potting mix are three essential components in gardening and agriculture. They all play a vital role in supporting the growth and development of plants, but they are different in their composition and purpose.
The importance of soil health is something I focus on greatly in my book, Seed to Table and in the A Year in the Urban Garden Master Class. Garden success and bountiful harvests start with healthy soil.
In this post, I will explore the definitions of compost, topsoil and potting mix as well as their uses in the home garden.
What is compost?
Compost is a type of organic matter that is produced by decomposing food scraps, yard waste, and other organic materials. The decomposing process occurs naturally over time, but can also be accelerated with the help of composting bins or piles.
Compost is rich in nutrients and serves as a natural fertilizer for plants. It improves the structure and fertility of soil, making it easier for roots to grow and absorb water and nutrients.
Composting is a simple and sustainable way to reduce waste, conserve resources, and promote plant growth. By composting food scraps, yard waste, and other organic materials, you can create a nutrient-rich soil amendment that is free from harmful chemicals and synthetic fertilizers. Composting is also an excellent way to reduce your carbon footprint, as the decomposing process captures carbon and reduces greenhouse gas emissions.
There are several different types of composting systems, each with its unique characteristics and benefits.
- Tumblers are composting containers that rotate to mix and aerate organic material for faster decomposition. They are ideal for people who want to speed up the composting process and don't have a lot of outdoor space. Tumblers are great for turning food scraps and yard waste into usable compost within a matter of weeks.
- Piles are a traditional method of composting where organic materials are piled up in a designated area and allowed to decompose over time. Piles can be made in a backyard, and they are an excellent option for those with a large amount of organic material to compost. The process can take several months to complete, but it requires minimal effort and expense.
- Bins are enclosed composting systems that can be made from a variety of materials, such as wood, plastic, or metal. They help control pests and odors while retaining heat and moisture. Bins are great for composting in urban areas or small backyards, and they can produce high-quality compost in a few months.
- Bokashi is a composting method that involves fermenting organic material using a specific mixture of beneficial microbes. This process can break down a wider variety of materials and is often used in indoor or small-space composting. Bokashi is an excellent option for people who live in apartments or don't have access to an outdoor space.
- Vermicomposting uses worms to decompose organic matter into nutrient-rich soil. Worms consume organic matter, and their castings (waste) become a valuable soil amendment. Vermicomposting is great for people who want to produce high-quality compost indoors or in small spaces, and it can take as little as a few weeks to produce nutrient-rich soil.
Advantages of Compost
There are several advantages to using compost in the home garden that go beyond nutrition. These include:
- Providing the soil organisms and worms with organic matter that they can then turn into nutrients for our plants
- Can act like a mulch and helps suppress and control weeds
- Helps with water retention and balances moisture levels
- Helps reduce the likelihood of diseases as healthy thriving soil is less likely to succumb to disease
- Reducing your carbon footprint by reducing
Disadvantages of Compost
Despite its long list of advantages, there is some important drawbacks you have to consider. These include:
- Making your own compost is a time-consuming activity and can take months to get a finished product that is usable in the garden.
- Compost piles can attract rodents and can also smell. This is an issue for urban and suburban gardeners especially.
- It’s labor intensive. Turning tumblers, flipping compost piles, etc.
- Composting requires space, which is not always available to home gardeners in cities and suburbs.
What is Topsoil?
Topsoil, on the other hand, is the upper layer of soil that contains a mixture of organic matter, minerals, and other essential elements that plants need to grow. Topsoil is the main source of nutrients for plants, and it is crucial for their survival. Without topsoil, plants would struggle to absorb the necessary nutrients and water to grow.
Topsoil is usually sold to home gardeners in bags at hardware stores or garden centers. It can also be purchased in bulk (totes, cubic yards). There is some controversy with topsoil. The challenge with these products is that they are not the most sustainable because they are mined or scraped from various locations, bagged, and resold to consumers.
That being said, when you need to bulk-fill new raised beds, the options are limited especially if you’re on a budget.
Topsoil can also be included in specialized blends like garden soil. Different brands will have different formulations and naming conventions so you should definitely check out the ingredient list or talk to a garden center expert before buying.
Differences Between Compost and Topsoil
Compost is different from topsoil in that it is not naturally occurring. Instead, compost is created through the decomposition of organic matter, while topsoil is formed over time through the accumulation of organic matter and minerals. Compost is also much richer in nutrients than topsoil, making it an ideal supplement for soil that is lacking in nutrients.
Another difference between compost and topsoil is their use. Compost is most commonly used as a soil amendment, which means it is added to existing soil to improve its fertility and structure. It can also be used as a mulch to suppress weeds, regulate soil temperature, and retain moisture.
On the other hand, topsoil is used as the main growing medium for plants, as it provides some of the necessary nutrients and structure for their growth.
How to Use Compost and Topsoil in the Garden?
Topsoil can be used to add bulk matter to raised beds, flower beds, or garden beds. On its own, it doesn’t provide all the necessary nutrition your backyard plants need. This is why I typically recommend mixing topsoil with other materials such as compost, manure and coco coir (optional, for raised beds) when filling new raised beds.
Raised beds are an excellent way to help you grow healthy abundant edible gardens especially if you have clay soil below. It allows the plant roots to grow unobstructed.
Once the raised beds have been filled, there is rarely any need to add more topsoil to them. As the level of soil sinks in them year after year, you can add compost or manure to top them up. Compost can be added to existing soil. I prefer to layer it on top (using a no-dig method) so that I don’t disturb the soil. Compost is an essential ingredient for a healthy garden.
For in-ground garden beds, I like to spread a 2” (5cm) layer of compost in the spring. This will help with the overall garden bed fertility and set my plants off on the right path at the beginning of the season.
What is Potting Mix?
When it comes to pots and containers, I do not recommend you use any topsoil as it will be too heavy. Containers need specialized soil-less mixes. These mixes are called potting mix, potting soil, and container mix, to name a few.
These mixes are essentially growing media formulated for containers. Different brands have different formulations, but they typically contain peat moss or coco coir with added perlite, vermiculite, compost, and other products like slow-release fertilizers. Organic options are available and recommended
It is also possible to mix your own container growing media. I personally prefer to purchase commercial products that are trustworthy and ready to use.
A Note on Lawns
Though I am personally not a fan of growing lawns (I much prefer to grow native flowers, edible plants and other perennials), compost and top soil can be used to add fertility to your lawns in the spring. Spread 0.5” (1cm) of compost over the lawn surface. Use a leaf rake to spread it across the lawn and get it as even as possible. The best time to do this would be after aerating or dethatching your lawn.
You will often find bags labeled topsoil in garden centers. These products can be used to create new garden beds when mixed with other organic materials like compost and/or composted manure. The challenge with these products is that they are not the most sustainable because they are mined or scraped from various locations, bagged, and resold to consumers
One of the best sources of organic matter, compost is created by decomposing organic matter like food waste into simpler organic and inorganic compounds. Compost can either be purchased or made. I certainly recommend that gardeners think about how they can repurpose their kitchen and garden waste to make compost. There are various systems available like tumblers, bokashi, and vermicomposting (also known as worm farming).
A growing medium formulated for containers. It usually contains no topsoil at all. Different brands have different formulations, but they typically contain peat moss or coco coir with added perlite, vermiculite, compost, and other products like slow-release fertilizers. Organic options are available and recommended.
Plants, especially edible plants, need more than just compost to thrive. This is why compost is added to top soil to create a balanced environment that provides plants the structure and nutrients to thrive.
It depends on the types of plants and their requirements, however, I would recommend fertilizing in the home garden using organic amendments that feed the soil rather than using chemical fertilizers that feed the plants directly. This helps you foster a healthy soil system rather than focusing on the plant's needs.