Dirt and its Garden Relatives

A Soil is Not a Compost is Not a Mulch

Text and Photos by Robert Weaver

My horticulture instructor at Meramec had a favorite saying when a student would refer to the stuff we grow things in as “dirt”: “It’s only dirt if you track it onto the carpet. Otherwise, it’s SOIL!” No matter whether you call it dirt or soil, the whole topic of soils…and compost and mulch, can be a little confusing, especially for new gardeners. Can I use top soil to make a raised bed or for growing plants in containers? What’s the best mulch to use? What’s the difference between compost and mulch? Knowing the answer to those questions may spell the difference between a successful garden and a frustrating one. So here’s a primer on dirt and its kin.

Soil

At its essence, soil consists of 3 minerals: sand, silt and clay, classified by their grain size. Sand is the largest, followed by silt, and clay being the smallest. The ratio of the three minerals will determine characteristics of your soil, such as moisture retention and drainage. The classic soil triangle that every horticulture student will be familiar with illustrates how soils can be classified based upon this ratio. Soil high in sand content will have greater porosity and less ability to hold moisture and nutrients. Soil higher in clay content (hello St. Louis gardens) will be more compact and have greater moisture and nutrient retention.

Okay, that’s soil in the lab or on Mars. But real soil also has organic matter (dead residue of living things like plants and animals), and living microorganisms (worms and much, much smaller, invisibleto-the-eye squiggly things). For gardeners (and lawn lovers), this is what gives your soil it’s growing ability. Organic matter helps improve the holding capacity of sandy soils, helps open up heavy clay soils, and adds nutritional value. (This is a primer, it’s all so much more complicated than this, with lots of fungi, bacteria and other things getting’ down and dirty in the soil. But you get the idea.)

So, now that you know the basics of soil, what are all the products that say soil on the bag, and which is right for you?

Let’s start with Top Soil. Top soil in nature is, as the name suggests, the top layer of soil, which contains most of the organic matter found in soils. It is usually screened to remove sticks and other debris. There are no industry standards for percentage of organic matter, so when choosing top soil it is best to buy a name brand product as opposed to a commodity brand. Top soil is a good choice for filling in holes or low spots in a lawn, changing elevation or slope in a landscape, or serving as a base/filler in raised beds. But in garden beds it will need to be supplemented with some sort of organic matter, either yard-waste compost, cotton burr compost or other compost/manure mixture. Typically the ratio of top soil to organic matter should be 1:1 or 1:2.

Many bagged soils say “garden soil” on the bags. These products are typically a mixture of top soil and organic matter such as compost, peat moss or other organic amendment. They may also include a charge of synthetic or natural fertilizer. They can be used for raised beds as well.

If you have a large project, you might save money by buying your soil in bulk. Suppliers such as St. Louis Composting have a variety of soil options that homeowners can either pick up with a truck or trailer, or they will deliver for a fee (they also sell bagged versions of some of their products). Their Raised Bed Mix is a blend of topsoil, sand, pine bark fine and their Black Gold compost for ideal growing conditions in raised beds. Their Rain Garden Mix is a blend of soil and compost for bioretention and rain garden applications. Topsoil Plus is a 70/30 blend of river-bottom topsoil and compost they suggest for a base for sod or as a garden bed restorative. Their Garden Mix is a 50/50 blend of topsoil and compost for enhancing raised beds or other planting areas.

Compost

When you see bagged compost, it can be many things. It might be composted (decomposed) yard waste (leaves, sticks, etc.), manure compost (a mixture of animal manure and straw or sawdust, cotton bur compost, or other organic composted matter). All are useful amendments to mix into soil to improve aeration, soil porosity and moisture control, and nutrition in new or existing gardens. They are also beneficial for top-dressing lawns. As with soils, compost can also be purchased in bulk from suppliers like St. Louis Composting.

Mulch

Like compost, mulch can be many things, from composted yard waste (called leaf mulch or leaf mold) to chipped or shredded wood (oak and other trees, shredded pallets or waste from woodproduct manufacturing) pine needles (pine straw), straw, or nonorganic mulches like gravel or rock. What you use may depend on personal aesthetic preference, but more appropriately, should be chosen for the plants you are mulching.

Rock and gravel are ideal for plants that like a dry, hot environment (rock gardens), but for most shrubs, perennials and annuals, these materials tend to reflect and retain the sun’s heat, often causing plants to stress in hotter months.

Leaf mulch, or leaf mold, is partially decomposed yard waste, and is ideal for use around annual and perennial gardens. (And to answer the question posed in the opening paragraph, the difference between compost and leaf mulch is simply the degree of decomposition.)

It is the preferred mulch at the Missouri Botanical Garden for most of their ornamental beds. It suppresses weed growth, helps retain moisture in the soil, and adds nutritional value to the soil as it decomposes. It also decomposes quickly, reducing the risk of mulch piling up over subsequent annual applications. It is not as commonly used as wood mulch, so is not as readily available. It can be purchased from suppliers like St. Louis Composting, and loving plants, or by transplanted Southerners who are accustomed to the mulch more commonly used in Southern states. It is not readily available in this area, but Zick’s Great Outdoors in Wildwood and Effinger’s Garden Center in Belleville are two suppliers carrying pine straw bales. is also sometimes available for free to homeowners in various communities. Check with your City Hall.

Pine straw is sometimes used around evergreens or other acid-

Wood mulch—ground or double-ground—is the most commonly used mulch, due to homeowners’ preference and the preponderance of folks driving around offering it inexpensively. It is fine for most annual, perennial and woody (tree and shrub) plants. It doesn’t decompose as quickly as leaf mulch, so may not need to be reapplied annually, and in fact, if done so, may build up to levels that inhibit oxygen exchange in the soil. If annual reapplication is desired, some or all of the previous year’s mulch should be removed so mulch levels remain at 4” or less. Also, because it doesn’t decompose as quickly, it doesn’t release as much beneficial goodies into the soil.

Finally, straw mulch is often used in vegetable gardens to suppress weeds and disease, and control moisture. Be sure to buy straw, and not hay, or you will be visited by an abundance of grassy weeds. So, choose according to your preference, hopefully with some consideration for the plants’ preferences as well. And take as you wish one garden designer’s opinion on its use: “Mulch should be the underwear of the garden, not the party dress!”