I’ve heard many conversations between couples in the garden section of a hardware store that follow this format:
“What should we get: compost or garden soil?”
“I don’t know. What about composted manure?”
“Isn’t that the same as compost?”
“I don’t know. Let’s just get what’s cheapest. It’s all dirt.”
Labels can be confusing. If you’re not familiar with the lingo, choosing the right materials for your garden or landscape can be frustrating and discouraging.
In very general terms,
- Topsoil is the layer of soil that we walk on.
- Compost is an amendment; meaning it’s an ingredient in healthy soils or soil mixes.
- Topsoil and compost are the ingredients in garden soil.
Topsoil and compost both play important roles in healthy gardens, but they serve different purposes. Before you can decide which one you need, you must first understand the differences between the two.
Topsoil vs. Compost
Soil is divided into layers, or horizons, that define exactly what soil properties are present (source).
The O Horizon may or may not be present, but it represents a layer of dead organic material on top of the soil. Thatch or dead leaves are considered to be part of the O Horizon.
The A Horizon is topsoil. It is either just under the O Horizon (if present), or it is the visible top layer of soil. This is where earthworms, insects, and bacteria live.
This is where most root structures spread out to find water and nutrients. It is composed of the organic material that falls onto it, roots that have died while growing in it, and the parent material that the soil rests on.
A healthy topsoil has a loamy texture, which you can’t control, mixed with organic matter, which you can control.
Basically, the A Horizon is topsoil and the O Horizon is compost. The best A Horizons have O Horizons on top of them. However, an A horizon could be entirely sand or clay with almost zero organic matter. This is where you must be careful.
For a complete understanding of soil horizons, structure, and texture, read What is Soil and Why does it Matter?
Topsoil is not synonymous with healthy soil
Topsoil is purely the top layer of any soil. There are many different soil structures, which lead to a variety of soil textures. Adding topsoil does not mean you are improving your situation. (source)
If you live in Texas, you may be trying to establish a lawn in clay soil. If you order topsoil from a local garden center, you will get soil with a similar texture as the soil in your current lawn. In other words, if you live in an area with clay, the topsoil in the surrounding area will also be clay.
However, many garden centers source their topsoil from areas with a high amount of organic matter. This means that the texture may be the same, as in clay, but the structure will be superior due to the addition of organic matter. (See What Is Soil? for more information on texture and structure)
What does all of this mean?
It means that topsoil is only as good as its organic matter.
Organic matter occurs naturally when dead organic material decomposes. When you create a pile of dead organic material and encourage it to decompose, the end result should be a nice, crumbly, dark compost.
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Here’s how it breaks down:
- Topsoil is the top layer of soil, for better or for worse
- Organic matter is an essential ingredient in high-quality topsoil
- Compost is organic matter
Therefore, healthy topsoil is, essentially, diluted compost.
When To Use Topsoil
In general, topsoil adds bulk, and compost adds quality.
- If you are establishing something new, you almost always need topsoil.
- If you are amending something that already exists, you almost always need compost.
Soil can have a variety of structures that are useful in their own way even if they are not suitable for growing plants. For example, if you are building a new garage and need to level the site before pouring concrete, you may need to add fill dirt. In this scenario, a loose, sandy soil would not work. Neither would a dark, crumbly, loamy garden soil.
For a foundation, you need a soil that doesn’t erode easily or settle. Clay fits part of this profile, but not all of it. The best mixture for this application is a clay/sand mixture. This sounds counterintuitive, but if you mix clay, sand, and water, you get concrete (which is why adding sand to clay will not improve drainage).
However, this mixture would be exactly the opposite of what you would want in the lawn area surrounding your new garage, so you would want to import different soil for this application.
Establishing A New Lawn
If you are modifying or establishing an area primarily for plants, you will have to have some measure of organic matter. How much you need will depend on the soil you are trying to improve or replace.
If your existing soil is extremely poor (texture, fertility, or otherwise), you will need to import high-quality topsoil.
Why? Because the problem is much larger than a nutrient deficiency. Extremely poor textures or neglected soils need an infusion of organic matter and a supporting structure in order to support healthy plant growth. In this scenario, if you are establishing a large growing area (like a lawn) in extremely poor soil (like clay or sand), it is best to bring in high-quality topsoil with a high organic matter content.
A 3”- 6” layer of topsoil should be enough to establish a new lawn on most poor soils.
Calculating topsoil for a new lawn
Bulk soil, compost, and mulch orders will be measured by the cubic yard, which is 3’x3’x3’ in volume.
In order to calculate how much topsoil you need to order, use the following formula:
- (length of lawn) x (width of lawn) = square footage
- (square footage) x (depth of topsoil*) = cubic feet of topsoil needed
- (cubic feet of topsoil needed) / 27cu’ (1 cubic yard) = cubic yards of topsoil needed
*You’re working with square footage, so a 6” layer of topsoil will be a depth of .5 feet
- Lawn is 25’ x 50’, so it is 1,250sq’
- I want a 6” layer of topsoil, so: 1250sq’ x .5’ = 625cu’
- I can either buy 625 bags of garden soil, or purchase cubic yards in bulk, so: 625/27 = 23.15 yards of topsoil
This formula also works for compost. The only variable is the measurements of the area and the depth of the topsoil or compost to be added.
Establishing A New Garden Plot
If you are establishing a new area to grow vegetables, fruit, or flowers, you will need to pay closer attention to the health of the soil. Turf grasses are forgiving of some soil failures, but fruits and vegetables are not.
If your existing topsoil is adequate for growing a lawn, you can mix in compost to create a high-quality garden bed.
Why? If your lawn seems to handle the native soil well, then it has a good baseline for growing more needy plants. You can clear an area, till up the existing soil, and mix in 3”-5” of compost over the area to improve fertility, drainage, and texture.
If your existing topsoil is not adequate for growing a lawn, you will need to bring in a larger quantity of high-quality topsoil and mix in compost.
Why? Because if your grass is struggling, there is no way your vegetables will be happy. Compost is too potent to create a garden bed as the sole ingredient (plus it’s expensive), so the best option here is to rip out the lawn, till up the area, then mix in topsoil and compost.
This will help establish a more healthy soil texture long-term, and will also help to prevent a hardpan.
Garden soil is a mixture of compost and topsoil. You can use pure garden soil for a new garden bed, although it may not be cost effective.
Establishing A New Landscape Bed
If you want to transform some of your existing yard into a landscape bed, you should first decide which plants will be installed in the area. Almost all fruits and vegetables need loamy, fertile soil, so you can prepare a garden bed high in organic matter and plant almost any food crop with success.
Landscape plants, on the other hand, have a wide range of soil preferences. Some prefer moist, well-drained, fertile soil. Others prefer poor, sandy, dry soil. Still others prefer “wet feet”, or soggy soil.
First, plan which plants will grow in your new landscape bed, and ensure they all have roughly the same soil preferences. Then, find the soil that matches those requirements.
In general, if you want to plant native landscape plants (which is almost always more efficient and successful), your soil will need little adjustment.
Native plants thrive in native soil, so as long as your soil isn’t too compacted, you should be able to build landscape beds and fill them with local topsoil if you need to add bulk to the area.
If you want to plant non-native landscape plants, you will need to create a more specialized landscape bed.
It is much easier to create a landscape bed that holds more moisture in a dry climate than a landscape bed that drains well in a wet climate.
For example, if you want to use water-loving plants in an arid climate with sandy soil, you would need to import a topsoil with high organic matter and add compost to make up for a loose or clay soil texture. This bed would hold water better than the surrounding, native soil, but it could still drain easily to prevent oversaturation.
However, if you wanted to plant succulents or cacti in a wet climate, you will have a very difficult time building a landscape bed that drains properly. You can build a bed full of sand, but the underlying soil will not allow for proper drainage.
This would require the use of underground drainage systems and large amounts of imported topsoil from a supplier with very different soil options than local garden centers. The end result would be expensive, and the plants would still struggle due to the consistent rainfall.
The most cost-effective way to build a landscape bed is to plant native plants, and then fill the beds with native soil. This reduces the amount of irrigation and fertilization necessary to have a healthy landscape.
When To Use Compost
Compost adds organic matter into the topsoil. In most situations, this is beneficial. However, some landscape plants prefer poor soils. Weeds are generally just native plants that thrive in the local soil, meaning as the soil improves, the weeds lose their footing. Therefore, if your landscape is native, “improving” the soil may cause problems instead of solving them.
Amending an existing lawn
If you have a relatively healthy lawn, and you are practicing a healthy maintenance schedule, then an annual application of compost should be sufficient for resilient turf.
Topdressing twice per year with ¼”- ½” of compost can help prevent a deep thatch layer, increase aeration, and help the soil to retain nutrients. As long as your lawn has no major compaction or erosion issues, compost should meet the needs of yearly maintenance.
Amending an existing garden bed
If you’ve been using the same plot for vegetables for a few years, then it’s likely low in nutrients and has a worn-down texture. A no-till garden space will help reduce soil erosion, and consistent applications of compost will help build up a healthy texture.
Compost can either be used as a mulch around existing plants, or as a soil amendment between plantings. Mix in a few inches each year to help maintain a spongey, dark plot, and do annual soil tests to monitor nutrient levels and pH values.
Amending an existing landscape bed
If the plants in your landscape prefer fertile, well-drained soil, then annual applications of compost will benefit the area. However, if you have a native landscape that thrives in poor soil, compost may actually hinder healthy growth.
Before amending a landscape, you need to positively identify each plant and diagnose potential soil problems. Blindly adding compost each year may be creating a wonderful, dark, fertile soil that is not suitable for the existing plants.
If your landscape has a mixture of soil preferences, then you can amend established plants or certain areas of the beds with compost, and leave others undisturbed.
How Compost Affects Soil
Compost is dead organic material that breaks down in a controlled environment. The only difference between compost and a naturally-occurring layer of organic matter is that compost was made on purpose.
Both have the same effect on soil, which is to improve texture, increase fertility, and mediate pH values.
It’s very similar to baking:
Adding compost to clay is like adding beaters to egg whites. Clay has almost no porosity, which means there are no holes in the soil that allow air and water to penetrate. As soon as you incorporate compost, clay will be able to breathe, and have a much lighter texture.
Adding compost to silt is like adding beaters to a cake mix. The ingredients for a good cake are already there, but compost keeps them bound together and operating at their full potential. Silt is floury and crumbly, so compost acts more as a binding agent and gives a spongy texture.
Adding compost to sand is like beating butter and sugar together. Sugar is loose, and could not hold its shape in any baked good without a binding agent. As you blend in butter, the sugar forms into lumpy aggregates, and now it can hold shape and take in more ingredients.
Compost is a fix-all for soil, but it’s potent. If you filled a garden bed with pure compost, plants would show signs of nutrient toxicities and many would suffer from nitrogen burn.
Compost is also expensive. Not only is it not cost-effective to fill an entire growing area with compost, but it’s also unnecessary. The inorganic, mineral component of soil is just as important as the organic, fibrous component of soil.
High-quality topsoil delivers an adequate amount of organic matter (nature’s compost) along with an adequate, inorganic soil structure. This is the most efficient way to add bulk and fertility to a growing area.
The Pros And Cons of Adding Compost or Topsoil
In almost all scenarios, adding compost to soil will be beneficial if it’s high-quality compost added in the correct ratio. The exception to this would be adding organic matter to a native landscape bed where the plants prefer poor soil.
If you are adding topsoil or compost to an area with the intention of growing plants that thrive in fertile soil, then the following pros and cons would apply:
Pros of adding high-quality topsoil
- Adds stable, fertile volume to an area
- Usually unnecessary to add in compost for the first growing season
- Cheap to buy in bulk ($10-$60/cubic yard)
- Suitable for most new projects except high-demand growing plots with extremely poor soil
Cons of topsoil
- A local garden center will provide topsoil from local sources, which means you may need to add compost to improve an extremely poor texture
- In areas with extremely poor local soil, you may need to buy bagged topsoil, which is expensive ($35-$120/cubic yard)
- Dump trucks will dump topsoil in your yard, but you will be responsible for moving, grading, and mixing in the topsoil, unless you want to pay for additional services
- Some garden centers lie about the source of their materials, so you could get soil polluted with chemicals or full of weed seeds
- It’s impossible to return extras
Pros of adding compost
- Compost is a potent, fertile, spongey, all-around beneficial soil amendment
- A yard of compost spread to ½” deep will cover a 10’x 65’ plot, and cost only $35 (bulk price)
- You can make your own!
- Compost doesn’t leach harmful chemicals into the groundwater
Cons of compost
- Some bagged compost (like composted manures) can smell after application
- Bagged compost is significantly more expensive than bulk compost
- Bulk compost may not be aged properly and may collect weed seeds from being in piles at garden centers
- The potency of compost will range depending on the ingredients, and composted manures can cause nitrogen burn if not used at the proper rates
In general, topsoil adds volume while compost adds quality. There are a few situations where creating a more spongey, fertile soil bed is not helpful, but this is uncommon.
When you order topsoil or compost in bulk, check out the materials beforehand to make sure you are happy with the product. It’s almost impossible to return a large pile of composted cow poop that was dumped in your front yard.