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Brush up on potting mix, topsoil, mulch and more

Thousands of people are flowing through checkout lines in garden centers this month. They have chosen their plants. The cashiers scan the plant tags, and often ask, “Do you have the compost (or mulch or potting mix) you’ll need?”

Those questions are more than add-on sales techniques. Sales staff or cashiers ask them because the customer probably does need some products to complete a successful planting project. Most soils are not ideal and most beds or planters aren’t ready for planting.

Most people do not even think about their soil, don’t know what the plants need to grow, and don’t know the language: What is compost, mulch, topsoil or garden soil and potting mix?

This article is intended to help you figure out what you need to help your plants thrive, whether you’re planting a vegetable or flower garden or a home landscape. When asked, you will know what to purchase to improve your soil and to help your plants grow.

Potting mix

This is a simple one, but often misunderstood. For plants in containers rather than in the ground you need potting mix, sometimes called “sterile potting mix.”

Do not use your natural outside soil in containers. That’s because real soil contains living organisms, most of them invisible (microorganisms). They consume and decompose organic matter, and produce nutrients that plants can absorb. In a pot they have no food source except your plant’s roots, and that container can’t ever be a healthy ecosystem.

Also, soil is too heavy. Use high-quality sterile stuff from your garden center.


Compost is the end product of the decomposition of organic matter. Gardeners turn compost into their soil when they’re prepping a garden or planting a tree. They spread it on top of lawns or around perennial and landscape beds.

Compost improves soil texture, moisture-holding ability, and adds life back into impoverished and compacted soil. It is the necessary additive for clay, sandy or damaged soil.

At home you can make compost from combinations of grass, leaves, branches, finished plants, weeds (preferably not the weed seeds), vegetable and fruit kitchen scraps (not meat or fats), coffee grounds, and shredded paper.

Mature compost contains humus, carbohydrates, lipids, amino acids, and living organisms that break down organic matter and release useful nutrients.

While it’s almost always great to add compost to gardens and landscape beds, all compost is not alike and equally good.

The wrong compost can introduce weed seeds, so consider the source. Ideally composts should contain a living community of microorganisms that are well matched to particular plant needs, so for large landscape or vegetable garden projects you should get in-depth analysis and seek composts intended for specific plant groups.

For most practical purposes you are not going to be able to analyze which compost is most perfect for you. Your choice will be: bagged or bulk compost and how much to get?

Bagged compost comes from many sources, near and far, much of it wonderful and some of it … I’m just not sure. Reading the ingredients may not help much – manures, cottonseed meal, peat, shellfish products – how can you tell what’s good?

I advise you to ask experts – in your nursery or landscape company for example: Why do they sell the product they are offering? Testimonials from repeat buyers and their own experiences will be informative. Then try the product.

Several WNY garden centers sell products from Coast of Maine Organics, including Bumper Crop Compost, and results are extremely positive from all reports. It is one of several fine possibilities.

Bulk compost is available from a few area suppliers, and many sell a compost-topsoil mix to the satisfaction of their customers. I always suggest trying a small amount before ordering a truckload.


Manure from many animals is used in most composts, and finished (aged) manure is an important source of nutrients for plant growth. Unfinished (hot) manure will burn plants but you can trust it when thoroughly composted.

Many farmers and gardeners use straight manure to add organic matter and nutrients (especially nitrogen) to the soil. Some manures may include weed seeds, depending on animal feed and bedding, so observe the results.

Topsoil, garden soil

There are lots of problems with the term “topsoil” and what it is. It is not legally defined or regulated, and some people end up buying the dead, compacted soil (even subsoil) from building sites.

Topsoil can appear dark and fine but can be mostly clay in fact. Some soil can introduce weed seeds, including invasive plants, to your property. Use respected companies for bulk topsoil purchases, try out some of the soil, and I suggest you request a compost-topsoil mixture.

A less familiar term has entered the landscaping and gardening vocabulary: “Garden soil.” It is used in particular by the Big Yellow Bag company for a product line, “Black Garden Soil,” that is distributed in our region by Lakeside Sod.

It is not compost, but contains 50 percent organic matter including composted manure. People are using it in raised beds by itself, or more typically mixed with existing soil, with good results including my own experience.

Its other winning feature is the delivery method: A generous cubic yard is dropped in your driveway in a neat bag that you can tie up until you can spread it all – no messy piles, no runoff.


Important distinction: Mulch is organic material that you use on top of the soil, with the purpose of blocking weeds, holding in soil moisture, or for decorative purposes. Mulch should not be turned into the soil – it’s not decomposed yet. (Mulch may decompose over time to contribute to soil life.)

On the other hand compost may be used on top of the soil as mulch or turned into the soil. Typical mulches are shredded bark, wood chips, straw, aged leaves (leaf mold) and cocoa shells. Paper and cardboard function as mulches too.

It’s time to get into gardening, but first: “Do you have the compost or mulch or potting mix you’ll need?”

Sally Cunningham is a garden writer, lecturer and consultant.

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