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When to Use Compost, Topsoil and Mulch

There’s a lot more to gardening than simply sowing your seeds, giving them water, and sitting back to watch your garden flourish. 

For your garden to truly thrive, you need to use a variety of tools and soil amendments. These tools include fertilizer, mulch, compost, and topsoil to ensure that your soil is supporting optimum plant growth. 

In fact, mulch, compost, and topsoil are like the ‘holy trinity’ of gardening. Each of these helps fortify soil and ensure that plants are protected, fed, healthy, and happy. 

Perhaps because each is connected to improving the quality of soil, some gardeners tend to use each substance interchangeably, especially compost and topsoil. 

However, if you want to be sure to have the best gardening results, take some time to understand the differences between compost and topsoil. 

This article will help you make an informed decision about whether you need to be using compost and topsoil in each of your gardening endeavors. 

What is Compost?

Compost is organic matter that has been broken down to its smallest particle. Compost is used as a soil amendment or soil conditioner, as it enriches the soil with nutrients, allowing plants better access to the nutrition they need to thrive. 

In addition to this, compost also improves soil’s structure, impacting the way it retains water. The substance is known as ‘black gold’ in gardening circles. Compost is a time-tested way to improve the quality of your soil; it was even used in the ancient Roman Empire! 

How is Compost Made? 

As well as being highly beneficial for your garden, compost is also great for the environment as a whole. 

This is because you can use all organic waste to make compost, which saves it from ending up in a landfill. Compost can be made from almost any organic waste from food scraps to manure to wood chips. 

Compost can be either homemade or commercial. Homemade compost is made using cold or hot composting. 

Meanwhile, commercially made compost can be made using a variety of methods, including in-vessel composting, windrow composting, and static pile composting. 

Other methods of composting are composting toilets, which process human waste into compost and vermicomposting, which uses worms to break down organic matter. 

What is Topsoil?

Soil, which is a complex mixture of air, water, gases, and organic matter, is the foundation of every garden. 

In a natural garden, the soil has several different layers; the topsoil, followed by subsoil, followed by rock. 

The topsoil is the top layer of soil in the earth – typically the first five to ten inches of soil. It has more nutrients than all the other layers of soil and is what plants’ roots reach for when they need nourishment. 

Before the agricultural and industrial revolution, topsoil was naturally rich and nutrient-packed. 

However, because of intensive farming, climate change, land clearing, and natural erosion, the quality of topsoil across the world has deteriorated. In most places, when you’re establishing a garden, it is necessary to add topsoil.

There are three main types or grades of added topsoil: 

  • Economy grade: This topsoil is the cheapest variety and is made of ‘unscreened’ soil, which is soil that has been dug from areas where topsoil is rich and then packaged. Because it’s unscreened, the composition and, therefore, the quality of the topsoil is not certain. Additionally, it’s not ‘pure’ soil; it can contain other organic matter like roots, sticks, and stones. 
  • General-purpose topsoil: This topsoil is ‘screened’ once or twice to remove larger stones, roots, and other bulky organic matter. However, this soil will still contain matter other than soil like seeds, smaller roots, and small stones. 
  • Premium topsoil: Premium topsoil is completely screened to remove all seeds, roots, twigs, and anything which is not soil. Premium topsoil may also be enriched with added nutrients to improve its quality. 

While premium topsoil is the best one to use, it tends to be much more expensive; it is not feasible to use it for all your gardening projects. There are occasions where you’ll find that it is perfectly alright to use general-purpose or even economy grade topsoil. 

How is Topsoil Made?

Topsoil is naturally created over a span of a hundred years; bedrock is broken down and mixed with decaying organic matter, like leaves, roots and animals to create nutrient-dense soil. 

However, as the demand today for topsoil does not usually allow for the time the natural process needs, it can also be made industrially. Manufactured topsoil is made by combining soil with compost, sand, and biosolids. 

While it is possible to make topsoil at home, it is much harder than creating your own compost at home. Topsoil needs a delicate balance of elements like phosphorus, potassium, and iron, as well as nutrients. 

It may be too technical and complicated to work out these levels for yourself; you’re much better of buying store-bought topsoil!

As you generally need large amounts of topsoil for each project, it’s available in kilogram bags. These tend to be fairly costly, but it helps if you buy and store them in bulk – soil doesn’t spoil! 

Do I Need Compost or Topsoil? 

Once you are armed with a better knowledge of the compositions and uses of topsoil and compost, you’ll be able to understand better why topsoil is needed in some gardening projects, and compost is used in others. 

When to use compost 

There are a variety of different ways you can use compost to help your garden thrive. 

Improving the quality of soil 

Depending on where you live, there are a variety of different types of soil you may have to grow plants in. 

Loam soil and silty soil are the best types of soil to work with, while chalky, clay, and sandy soils are more challenging. Chalky soil and sandy soil retain very little water, which results in them having fewer nutrients. 

On the other hand, clay soil holds on to too much water and can get waterlogged, drowning out nutrients. Clay soil also tends to run cold in winters, which harms plants’ roots. 

When you mix compost into each of these soils, you make them stronger and more suited to supporting plants. 

Adding compost to loam and silty soil will help make them even easier to work with. Meanwhile, different ratios of compost can be added to other soils to address each of their unique problems. 

For instance, adding compost to clay soil can keep it warmer and improve its capacity to drain water. 

Growing and maintaining flower beds

Compost can be a helpful tool in establishing and nourishing your annual and perennial flower beds. 

Adding an inch of compost to your flower beds after planting seeds or bulbs will help encourage them to grow. 

Meanwhile, adding compost to the beds in fall will help protect the flowers’ roots from the cold and conserve water. 

Maintaining vegetable beds

Adding compost to vegetable beds in fall will help protect vegetables from the winter cold and from droughts caused by a lack of rain. 

Meanwhile, adding half an inch of compost monthly to the base of your vegetable plants every month will help your vegetables grow stronger and faster and improve the quality of your produce – and the taste of your veggies!

Maintaining and planting trees and shrubs

Compost is also necessary to help bigger plants thrive. For bigger plants, like trees and bushes, compost acts as the replacement for the layer of organic matter that is scattered on forest and jungle floors. 

So, after you have planted a sapling, work the compost into the top two inches of soil surrounding the sapling. 

Be sure not to fill the hole you have planted the sapling in with compost as this will discourage the roots from spreading to find nutrients and eventually interfere with the tree’s foundation. 

 As the sapling grows, gently rake the compost into the soil around the tree twice a year. You can also spread compost over the soil around the tree to make sure moisture isn’t lost from the soil and to ensure the roots don’t freeze over in winter. 

Planting and refreshing lawns

If a vibrant green lawn is what you’re after, compost can help. When establishing a new lawn, till about ¼ inch of compost into loose soil before planting the grass seeds. This will protect the soil and prevent moisture from escaping. 

You can also use compost as a top dressing on an already established lawn; simply sprinkle fine particles of compost over your lawn and allow it to seep through the grass blades into the soil. 

In potted plants 

Mix compost with your potting soil before you plant houseplants to give them an extra boost of nutrition. Once you have planted, you can add compost as a top dressing to your potted plants every month. 

Compost as mulch

Mulch is a layer of material spread over soil to protect the soil from pests and weeds, stop it from losing moisture in hot and dry climates, and helps keep the soil warm in cooler areas. Compost can act as mulch when it is used as a top dressing or side dressing. As well as protecting the soil, compost will improve its quality. 

If you have both mulch and compost and you’d like to know when to use compost and or vs. mulch we’ve covered this topic as well.

When not to use compost 

Compost is great for a variety of purposes, but it’s important to remember that compost should never be used as a soil replacement. It’s a soil amendment, and it should always be mixed with soil; whether it’s the soil that naturally exists in your garden or it’s soil you are adding like topsoil or gardening soil. 

Simply using compost as soil can stop your plants from growing as their roots will never reach past the compost into the soil. 

As a result, the plants’ growth will be stunted, as there’ll be only so much compost they can find. Pure compost can also cause water retention problems as it doesn’t drain as well as natural soil. 

So, water won’t be able to reach the plants’ roots, preventing them access to the nutrients they need. 

When to use topsoil 

When used correctly, compost can be the ‘jack of all trades’; it can help you in a collection of gardening endeavors. Meanwhile, topsoil is used for more specific purposes. 

Before using topsoil, do a little ‘homework’; test the pH levels of the current soil in your garden and try to match it to the topsoil you are purchasing. 

You should also identify which type of soil your garden has (loamy, or sandy, for instance) and get topsoil, which has been designed specifically for that particular type of soil. 

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Understanding the ingredients of the topsoil, you’re buying and matching it as closely as possible to your current soil will help the new topsoil blend seamlessly into the older soil, making it more effective. 

Creating raised beds 

Topsoil is most commonly used when you want to establish raised gardening beds, which are enclosed sections of your garden that are ‘raised higher’ by adding more soil on top of the already existing soil. 

Gardening beds are great for growing flowers and vegetables as they’re more effective in keeping away pests, weeds and tend to have healthier soil. 

To create a raised bed, use topsoil, adding it to the top of the ground until it is a few inches higher than its surroundings. Essentially, you are creating an ‘artificial’ ground, and the topsoil will provide plants with what they need to grow. 

Use premium grade topsoil for creating raised beds. 

For vegetable gardens 

Even if you aren’t creating a raised bed, you should still use premium grade topsoil when growing vegetables. Topsoil is nutrient-packed and will improve the quality of your produce. 

Rake topsoil over the ground and ensure that the soil is loose and damp before sowing your vegetable seeds. 

When laying turf 

Use topsoil before you lay down turf to keep your lawn healthy and thriving. Topsoil provides the layer of nutrients that turf needs and also soaks up water, preventing your turf from getting waterlogged and soggy. 

Lay down at least four inches of topsoil, patting it firmly into the ground and then roll out the turf. General grade topsoil will work beautifully for this!

To improve soil drainage 

If there’s a part of your garden that tends to hold on to water, add topsoil to the existing soil to help improve drainage. This will help ensure that water is being filtered down into the plants’ roots. 

When not to use topsoil 

Topsoil works best outdoors; it is not advisable to use it as potting soil. Topsoil tends to be heavier than potting soil as it contains matter other than soil like seeds and stones. 

Meanwhile, potting soil has been specially designed for potted plants’ unique needs; it doesn’t rapidly compact, is light, and drains water more rapidly. 

If you’re growing indoor plants, don’t use topsoil. However, if you are transferring potted plants into your outdoor garden, then lay down a layer of general grade topsoil before planting them as it will make it easier for them to adapt to their new outdoor life!

Mixing compost with topsoil 

If you have both topsoil and compost on hand, you can also look at combining them to get the best of both worlds! 

Compost will enrich your topsoil, making it even more nutritious. As a general rule of thumb, the ratio of compost to topsoil should favor the topsoil – you should combine less compost with more topsoil. 

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Mix compost and topsoil when you’re nurturing nutrient hungry plants like flowers, vegetables, and fruit trees. 

But, if you don’t have both compost and topsoil hand, then which should you use? 

There are several occasions when topsoil can be highly beneficial, from laying turf to creating raised beds. However, topsoil can be very expensive, and it is not cost-effective to use it for all your gardening. 

Instead, you can use compost to improve the quality of your already existing soil – creating a substitute for topsoil. 

Because compost is a soil amendment, it’s very versatile. 

Compost’s versatility means you can blend it with decent quality soil; in most cases, this will result in soil that is perfectly adequate to support and encourage plant growth. 

However, if your soil is weak or the garden’s natural topsoil layer has been eroded, then it may be necessary to invest in general grade or premium-quality topsoil. 

You should also use topsoil when creating raised gardening beds and vegetable gardens, as high-quality soil is necessary. 

Final Thoughts

This guide can help you decide when to use topsoil and when to use compost. In addition to this, talking to experts in your local gardening stores and farmers’ markets can also be very helpful. 

Local experts will understand the unique needs of the soil in your area and also be able to make suggestions based on what you’re hoping to grow. 

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However, instinct and past experiences should also guide your decisions. Experiment with what works best in your garden and keep refining your technique. You’ll soon be able to judge whether you need compost or topsoil for yourself!