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Soil and Compost – Selecting the Right One

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.

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.

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.

References:

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

 

Robert Pavlis
Editor of GardenFundamentals.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Fundamentals an educational site that helps you understand your garden better.

8 Responses to 'Soil and Compost – Selecting the Right One'

  1. Brynne Jones says:

    Wow, I didn’t realize that there were so many types of soil. I found it interesting that you adding garden soil to the potting mix allows your soil to hold moisture longer. My grandmother has been wanting to grow fresh vegetables in her backyard. Looking into what soil would be best for her garden could be beneficial to her.

  2. Nancy Carmickle says:

    My soil is clay. How do I get it right? It is hard as a rock when dry and sticks to the the bottoms of my shoes when wet, very hard to work in. If I don’t till it, I can’t dig. What do you suggest?

    • I have always gardened in clay. When making a new bed I incorporate a couple of inches of course sand. Some people believe this will harden the soil but it did not have that result in my soil. Try a small area first. Also use lots of organic matter – compost, manure – whatever you can get cheaply. then mulch with wood chips. The clay will get better over time, but it is not a fast process.

  3. Peter van Rens says:

    New to your posts but very much enjoying them.

    By happy accident I started my new veg bed by digging in a lot of leaves and grass/clover, more or less as you suggest. I understand that we should not disturb the soil structure too much but is soil compaction an issue and if so, how do we remedy that? My dad’s garden desperately needs organic material and is incredibly hard packed once you get 3-4 inches down. I guess you wouldn’t agree with his rototiller approach to gardening.

    Thanks for the info.

    • A rototiller can be used to make a new garden bed – after that it should never be used again. Rototilling destroys soil structure and speeds up the decomposition of organic matter. Even farmers are now moving to no-till farming where possible, for this very reason.

      Compaction also destroys soil structure. Best thing you can do is not walk on the soil. Second best is to always walk on paths and stay out of planting areas. Mulch with organic matter which over time will improve compaction.

      Since a veg bed is planted every year, mixing in organic matter is not all bad if the soil is very low in organic matter. Try to do this with as little digging as possible.

  4. Inger Knudsen says:

    I made a special bed 20 years ago for Rhododendrons. I put 5 large bags of peat moss on top of the soil, watered it well and planted the Rhodos directly into the peat The Rhodos do not grow much, in fact not at all but they survive and give me a small show every year. I sowed the Rhododendrons myself so I am a bit nostalgic about them My soil has high pH and I know the Rhodos would not have survived with out the peat. I tried to measure the pH of the peat many years ago and it was not particularly acid around 6 I think

    • Peat moss is a lot less acidic than people think and adding it to alkaline soil does not change the pH very much.

      Add some sulfur to the soil to lower the pH. You can get it at Sharpe.

  5. Anushka says:

    Very useful and logical!