As a gardener, when I find soil that’s lacking, it’s my preference to work with what I find on-site, adding amendments and mulching with some organic matter and then waiting for the worms to transform what they find below.
However, this approach can take months or years to complete itself. That’s why so many of my gardening friends prefer to just have a load of topsoil delivered. That’s an expensive fix, but a quick one in many situations, from patching bare spots in the lawn to filling a raised bed.
Or, I should say that topsoil can be a fix, depending on what the vendor delivers. As Dawn Pettinelli, director of the University of Connecticut’s Soil & Nutrient Analysis Lab, told me recently, shopping for topsoil is a definite case of “let the buyer beware.”
To begin with, Pettinelli informed me, there are no legal standards governing what a topsoil is. The common definition, that it should come from the top layer (or horizon as Pettinelli called it) of the soil, means that it will vary dramatically in composition depending on where you collect it. In a sandy field, topsoil will consist of sand; in the floodplain of a river, it is likely to be silt. And because of the lack of legal standards, unscrupulous vendors may pass subsoil harvested from a construction site through a screen to remove rocks and debris to temporarily fluff it up, and market that as topsoil.
To be safe, many gardeners will buy their topsoil from a farm, because farmers certainly know their soils. Yet even that can be problematic. If the farmer has been applying herbicides or other pesticides to his or her crops, toxic residues may be in the topsoil they sell, and that isn’t something you want to introduce into your home landscape.
TIPS FOR QUALITY TOPSOIL
After alerting me to these possible problems, Pettinelli shared some shopping tips that will help to ensure that you get good-quality, healthy topsoil.
First of all, she encourages the shopper to ask the supplier where they got the topsoil and to ask if they may inspect the product before it is delivered. I suggested that we might try to obtain a sample and submit it for analysis to a lab such as hers. Pettinelli replied that could be tricky if the vendor is sourcing his or her soil from different sites. Still, inspecting the vendor’s stockpiled soil can be revealing. It should be free of glass, plastic, and other trash, and free of large rocks. If it is dark in color, that commonly indicates soil rich in organic matter, which is desirable in most gardening situations. Check, however, that the soil was not dredged from a wetland — such soils may be dark in color but are typically very acidic and have a poor texture once they dry out.
At the least, Pettinelli told me, it would be a good idea to get whatever is delivered analyzed for lead before you spread it in the garden. Lead was, for many years, an ingredient in most types of gasoline and many paints, and it is commonly found in soils near roadways or older homes. It may be present even in rural soils, as lead arsenate used to be a popular insecticide, especially in apple orchards, throughout most of the 20th century. In fact, Pettinelli says, fully a quarter of the soil samples submitted to her lab in Connecticut test for levels of lead that make the soil inadvisable for use in growing food crops.
Selling topsoil is often an income source for farms, which commonly mix the soil with composted manure or some other decomposed organic material with their product. Aside from the chance of contamination mentioned above, such improved soils generally make a good addition to the garden.
Finally, before you make a purchase, be aware of simply spreading a layer of new soil over whatever you already have on site. The abrupt transition from one, presumably richer, soil to a poorer substrate can impede water drainage — the moisture tends to collect at the point of transition, creating a “perched” water table that will keep roots from penetrating deeper into the soil. It’s best to mix the new soil with the old by tilling or digging.
To hear more of Pettinelli’s tips on shopping for topsoil, listen to our conversation on the “Growing Greener” podcast at berkshirebotanical.org.