Long ago, a gardener’s choices of soils and amendments were simple—dirt simple, in fact. Maybe you’d enrich your soil with compost or animal manure or break it up with sand or gravel, but the options were pretty basic.
These days, most well-equipped garden centers have an enormous wall or two devoted to packaged soils. Many are things our great-grandparents never heard of—like biosolids, polymer crystals, and soilless growing medium. Some boggle the imagination: Bags of garden soil? Isn’t that what’s in your garden already? To help you navigate the confusing world of soil amendments, here are some of the most common questions—and tips that will help you get the most from your next trip to the garden center.
What’s the difference between a soil, a soil amendment, and a soil conditioner?
Soil is dirt. A soil amendment is anything you add to the soil to improve it. And so is a soil conditioner, except that savvy markers have created a variety of blends of soil amendments and slapped the label “soil conditioners” on them.
Why would you grow plants in a soilless mix?
Soil weighs down mixes. A soilless mix is lightweight and is easy to sterilize, perfect for seeds and trickier container plants.
What’s the difference between a mulch and a soil amendment?
To use something as a soil amendment, you need to work it at least several inches into the soil. Mulches, on the other hand, are spread on top of the soil to conserve moisture and suppress weeds.
Confusion occurs because some materials can be used as either an amendment or a mulch. Compost and finely chopped or well-rotted leaves, for example, can be worked into the soil or spread on top of it for mulch. Bark dust, too, can be used as either a mulch or a soil amendment (but don’t mistake the finely ground substance for bark nuggets or materials labeled simply as bark mulch). You shouldn’t work sawdust or any fresh organic material directly into the soil because in the process of breaking down, it robs nitrogen from soil. However, it’s fine to add such materials to the compost pile, where they’ll break down before you use them.
How to get your money’s worth
If this information makes your head spin, it’s not surprising. Creative marketing teams give all sorts of names to all sorts of soils and soil-like substances in bags. Actual content can vary according to company. When in doubt, read the label and talk to a knowledgeable sales assistant or manager. When shopping for bags of soil, potting mix, conditioners, and amendments, keep in mind the following:
Focus on details. Labeling laws for garden products vary by state, but the more specific a manufacturer is about what’s inside, the more likely it is the bag contains something worth boasting about. The reverse is also true. If there’s very little information about what’s inside, it’s probably not worth crowing about.
Buy volume, not weight. When it comes to potting soils, mixes, and conditioners, weight has almost nothing to do with quality. The weight could be coming from dark, heavy, low-quality peat, water, or even just plain sand. Heaviness, in fact, is more often a sign of poor quality than of good value. As a rule, go for larger bags that are lighter.
Look for key words. Buy quality. Look for words such as “professional” or “premium.” These words aren’t a foolproof assurance of quality, but they’re a good sign.
Don’t think that mix-ins are a must. Potting mixes often include fertilizers and special moisture-retaining additives. The fertilizers added to potting soils are usually short-term, so keep in mind that you’ll still need to fertilize. Also, hydrogels or polymer crystals (also called water-retaining crystals) claim to help potting mixes retain moisture so you don’t have to water as often, but there’s scant evidence that they’re truly effective. When in doubt, don’t pay extra for these mix-ins.
Feel it. Bags often break open, so if you see a ripped bag of the brand you’re thinking about buying, put some of the mix in your hand and feel it. Or ask a clerk if there’s any already open that you can feel. If you’re planning on buying several bags, ask the clerk to open a bag so you can feel it.
Stick with your favorites. Once you find a product and a brand you like, stick with it. Many a gardener has opened up a bag of garden soil or topsoil to find something disappointing. Save yourself the surprise and expense by staying with what you like. As a bonus, you’ll find yourself spending a lot less time in front of that array of soil-like products, trying to figure out what’s in the bag.
The Little Bags
In smaller bags, you’ll find the following soil amendments, some of which double as fertilizer or can alter the soil’s pH. As their smaller size suggests, they’re added to soil in smaller proportions. Read labels carefully.
Bonemeal: Made of finely ground, steamed animal bones. This phosphorous-rich material is a popular fertilizer for bulbs, but specialty bulb fertilizers are more effective. Slightly improves soil texture. Work into planting holes, or sprinkle on the soil surface and rake in. May attract animals.
Bloodmeal: Made of dried animal blood. A rich organic source of nitrogen. Slightly improves soil texture. Sprinkle lightly on the soil surface or follow package directions and rake in. May attract animals.
Gypsum: Breaks up some clay soils, but its effectiveness is limited. Spread on the soil surface and then dig in to a depth of several inches, following package directions to get the right amount.
Lime: Made of ground limestone. Raises soil pH. Should be applied only on acid soils to accommodate plants that need more alkaline conditions; don’t automatically apply every spring. Processed lime takes effect in a few weeks; dolomite takes a few months.
Sulfur: Lowers the pH of the soil, making neutral and alkaline soils better for acid-loving plants such as azaleas and hydrangeas. As with lime, use only if you know your soil’s pH.
Vermiculite: Bits of the mineral mica, popped under high heat like popcorn. Holds water like a hard sponge. Common in potting mixes. Excellent for both clay and sandy soils because it both loosens soils and helps them hold water. Work into the soil before adding to the container or into the top several inches of the soil. It’s now harder to find in garden centers because of environmental concerns.
Perlite: Made of tiny, heat-popped volcanic rock. Holds water on the outside of the tiny rock. Often used like vermiculite; in addition, it’s an ideal medium for starting cuttings so they don’t get overly wet.
Choosing garden soils and amendments
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Veronica Lorson Fowler is a garden writer in Ames, Iowa.