Soil and Compost – Selecting the Right One

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Robert Pavlis

You are in the market for some soil or compost and you visit the local nursery or big box store. There are so many products to choose from. Which one is the right one? Should you buy soil, or triple mix, or compost? Or is potting soil the right product to buy?

In this post I will try to sort out this confusion and show you which product to use for different types of jobs.

Planting trees - Soil profile showing top soil (layer O + A)
Planting trees – Soil profile showing top soil (layer O + A)

Different Types of Soil

In my last post, Topsoil, Compost, Triple Mix – What’s the Difference? I discussed the differences between topsoil, triple mix, compost, potting soil, black garden soil, peat moss and garden soil.

To simplify things, you never need to buy peat moss, unless you are making your own potting or seedling mix. It can be added to the garden, but compost and manure are better options.

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Soil for Containers

The soil you use in containers should be light and airy. This is what plants want and it makes it easier for you to move the container. The best soil for this job is potting mix or potting soil – they are the same thing.

The problem with potting soil is that it dries out quickly. To reduce watering I like to add some garden soil or some topsoil to the potting mix so that it hold moisture longer. This also reduces the cost.

Soil for Garden Beds

Landscape beds or garden beds are the common beds used for growing shrubs, perennials or annuals. A lot of people buy top soil, or garden soil and add it to these beds each year. I have never understood this practice. You already have soil – why add more soil?

To better understand this, let’s look at what your soil needs. Most garden soil does not have enough organic matter in it. Organic matter feeds the plants, makes the soil more friable, adds more air to the soil, and feeds the all important microbes. Unless you have been adding organic matter to soil for years, your soil probably needs more.

Instead of buying soil which you already have, buy organic matter. Compost and manure are great choices for this. Do not dig this into the soil. Use it as a mulch and layer it on top of the soil. Nature will move it into the soil for you.

Stop buying soil for your garden beds.

Soil for an Existing Vegetable Bed

Assuming the vegetable bed already exists, this garden is no different than any other garden bed. What it needs is organic matter and both compost and manure are good choices.

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Even for the vegetable bed it is best to add the organic matter as a mulch and leave it on top of the soil. Digging it into the soil destroys soil structure, increases weeds, and speeds up the decomposition of organic matter (ie it does not last as long).  Dig as little as possible in your vegetable bed.

Another great option is straw. Use it as a mulch by itself or in addition to compost and manure. Cover everything with a layer of straw. You will have fewer weeds and the moisture will be retained in the soil longer. Straw slowly decomposes adding organic matter to soil.

Soil for a New Vegetable Bed

If you are building a new vegetable bed that is not raised you still do not need to buy soil. Buy organic matter as described above and add that to the soil.

When making the bed for the first time it is OK to dig in the organic matter as part of your preparation process. But do this just once when you first make the bed. In future years, disturb the soil as little as possible.

Soil for a Raised Bed

This situation is different than the ones discussed above. In this case you do not have enough soil and you do need to buy more. Many people will buy triple mix for this job. This seems to make sense. Triple mix is a combination of soil, peat moss, and compost. It is a great soil for growing things.

Triple mix also has a problem. Since 2/3 of the mix is organic matter, which decomposes over time, the level of the soil will go down each year. You will be constantly adding more soil to keep the level up. This is not a big problem in a vegetable bed or one that is used for growing just annuals, but it is not good for perennials and shrubs. In no time at all perennials and shrubs will be planted too high as the soil around them shrinks.

It is much better to make the bed using only top soil. Even this will settle over time, but not nearly as much as triple mix. When the bed is finished, plant and mulch with some organic matter. Over time the organic matter will be incorporated into the soil.

Soil for a New Lawn

Triple mix is the common product that is used to lay a new lawn. The grass, either seed or sod, will grow well in it. A thin layer of an inch or two is not a problem.

Adding more than a couple of inches will cause problems. Over time the soil shrinks as discussed about. As it shrinks the lawn gets lower. After a few years you will notice that the lawn is lower than the driveway or the sidewalk. This is caused by too much organic matter in the soil laid down before adding grass.

It is much better to use top soil under the grass and then top dress the lawn with organic matter on an annual or bi-annual basis.

Selecting the Right Soil

The first think to do is to figure out what problem you are trying to solve. Do you need to raise the level of the existing soil to make it higher? In that case add top soil. It is the one product that will maintain the desired level, but even it settles a bit.

If you want to add organic material then use compost or manure. Don’t add soil or a product that contains soil.

Containers are a special situation – use potting soil for them.

Things like triple mix, black garden soil, peat moss and garden soil are products that you should not be adding to the garden.

More Information About Soil

What is the Best Manure for Your Garden

Soil for Raised Beds – Which One is Best?

Topsoil, Compost, Triple Mix – What’s the Difference?

References:

  1. Photo source; US Department of Agriculture, public domain

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

39 thoughts on “Soil and Compost – Selecting the Right One”

  1. Hi. What a helpful post! I have just removed my driveway and dug down about 12” to an old layer of gravel and clay. I am looking to create a bed for perennials – a mix of plants and shrubs. I am not sure from your post if I should use mostly topsoil with a layer of compost or something different? Also, I plan to have stepping stones along the garden – is there something different I should put underneath those areas? many thanks!

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