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Your First Garden: What You Need to Know About Topsoil

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Your First Garden: What You Need to Know About Topsoil

September 6, 2018

Welcome to Your First Garden. If you have questions about the basics (as in: what is topsoil?), you’re like me. For newbies like us, I’ll answer them one at a time—with advice from experts—twice a month. Our gardens are going to love this.

Here’s the dirt on dirt. I’ve got some myself, a quarter acre of it that my husband and I acquired when we moved from Brooklyn to the suburbs four years ago, full of hope and excitement for our family of four. Buh-bye alternate-side parking and piles of dog poop on pavement. Hello, fresh air and flowers. That first summer, at a plant sale organized by the local gardening club, I eagerly bought whatever struck my fancy—astilbe, peonies, lavender, sedum—without much forethought or consideration. So what if much of my garden was in the shade? Or if that new sedum might clash with the coral bells? I was drunk with the idea of being a landowner.

Not surprisingly, about two-thirds of what I planted that season withered. By the second year, aside from weeding and watering, I had given up on gardening. Today the perennials that made it through that first summer come back year after year, but they’re like grumpy teenagers. They are leggy, blemished, and tired-looking, as if they were woken up too early by spring.

This is not the garden I was meant to have. This is not the garden you are meant to have. Which brings me to topsoil. I’ve been wondering if the problem could be the state of the soil (which was already there, so not my fault!). Maybe I need to add more nutrient-rich soil? And what is topsoil, anyway? Read on.

N.B.: Featured image by Britt Willoughby Dyer, from Walled Gardens: An Organic and Picturesque Plot at Old-Lands in Wales.

Q: What is topsoil?

Above: Natural topsoil (as opposed to store-bought topsoils) is the nutrient-rich layer of soil just under the surface. It is darker and less compact than the layers below it because of all the biological activity that goes on in that layer. Photograph by Nicole Franzen, from Garden Visit: A Modern CA Garden Inspired by the Classics.

A: A soil scientist—such as Dr. Stephanie Murphy, the director of Rutgers Soil Testing Laboratory in New Brunswick, New Jersey, who answered my many questions for this story—would say that real topsoil is different from what you purchase in bags in stores: it’s the mineral-dense top layer of earth.

It’s “characterized by an accumulation of organic matter (living, dead, and decaying organisms), which gives it a darker shade than the soil below it,” she says. “Real topsoil is the most valuable layer of soil.”

How is store-bought topsoil different? There are two types you can buy: blended topsoil (a mixture of mineral material, usually excavated from a construction site, and organic matter such as compost) and organic topsoil (containing matter such as shredded wood, moss, and peat).

Q. Which kind of store-bought topsoil is best?

Above: Before you bring home blended topsoil from the store, check the stockpiled soil for unwanted materials like large stones, broken glass, paint chips, and plastic. In addition, there should be less than 10 percent gravel content. Photograph by Michelle Slatalla, from Hydrangeas: How to Change Color from Pink to Blue.

A: Dr. Murphy cautions against buying organic topsoils. Unfortunately, that’s also what’s most commonly sold in stores. “Rarely have I seen a bagged soil contain any mineral component at all,” says Dr. Murphy. Some stores, though, do have stockpiles of blended topsoil for bulk purchase and those are better than bagged organic topsoil. And you can always “ask topsoil suppliers for a copy of a recent soil test. Reputable suppliers perform quality-control testing regularly.”

Q: How is store-bought topsoil different from garden soil and potting soil?

Topsoil to die for; lucky spring onions. See more at Walled Gardens: An Organic and Picturesque Plot at Old-Lands in Wales. Photograph by Britt Willoughby Dyer.
Above: Topsoil to die for; lucky spring onions. See more at Walled Gardens: An Organic and Picturesque Plot at Old-Lands in Wales. Photograph by Britt Willoughby Dyer.

A: Potting soil is designed for use in containers only and actually has no soil in it at all. Instead, it’s often a mixture of peat moss and other organic materials such as composted sawdust. “Its physical characteristics are often more important than the fertility,” explains Dr. Murphy “Potting soil is usually lightweight (organic matter being much less dense than soil minerals) and should have high water-holding capacity but drain excess water rapidly. Any nutrients from the decaying organic matter is quickly depleted or leached, and the plants rely mainly on continual input of fertilizer nutrients.”

Garden soil, on the other hand, tends be made up of topsoil and a mixture of other materials and nutrients (compost and fertilizer) designed for specific types of plants. It tends to be more expensive than topsoil. In general, use potting soil for container gardening, garden soil for planting in flower or vegetable beds (you have to mix it with existing soil), and topsoil for larger projects or if your lawn or garden is in need of greater soil volume (for instance, if there’s been soil erosion from strong winds or heavy rains).

Q: How do you add topsoil?

One good reason you may need to buy topsoil: to replace soil lost to erosion. Photograph by Matthew Williams, from Hardscaping \10\1: Erosion Control.
Above: One good reason you may need to buy topsoil: to replace soil lost to erosion. Photograph by Matthew Williams, from Hardscaping 101: Erosion Control.

A: Ideally, you should till the soil that’s already in your garden (especially if it’s compacted) and add a 3-inch layer of blended topsoil, tilling it all together again to create a 6-inch-deep surface layer; this will best mimic real topsoil. “You do not want to simply lay a three-inch layer on the surface of a soil and attempt to plant into that layer,” advises Dr. Murphy. “Creation of artificial layers almost inevitably creates drainage problems. Roots may proliferate in that layer and never grow deeper into the mineral layer.” Spring or fall, when rains keep soil moist and encourage biological activity and decomposition, are the best times to add topsoil.

Q: Do you need to add anything else to store-bought topsoil?

See more at Walled Gardens: An Organic and Picturesque Plot at Old-Lands in Wales. Photograph by Britt Willoughby Dyer.
Above: See more at Walled Gardens: An Organic and Picturesque Plot at Old-Lands in Wales. Photograph by Britt Willoughby Dyer.

A: “Probably not compost, since most blended topsoils and bagged topsoils already have or are composed of compost,” says Dr. Murphy. As for fertilizers, you’ll need to have your soil tested to find out what nutrients your soil could use more of.

Q: Is it necessary to buy topsoil at all?

Above: Amending your existing topsoil with compost is a more sustainable and better option than hauling in store-bought topsoil. Photograph by Jim Powell, from Composting: Are You Obsessed?

A: If you simply need to increase soil volume—whether it’s to replace soil that has eroded or to even out property or to create a garden from scratch—then yes, buying topsoil is an inexpensive way to do it. But if you’re interested in encouraging a healthier, more fertile topsoil, according to Dr. Murphy, amending what you have with compost is preferable to buying topsoil. “The more we can re-create natural topsoils, the less fertilizer input is needed—and long-term sustainability can be achieved. For example, it has been shown that very old lawns require less fertilizers and less irrigation,” she says. “In most cases, improve what you have with soil amendments, unless you specifically need greater soil volume.”

Do you have a question about your first garden? (Or your second or third or fourth garden)? Let us know what they are in comments section below, and I’ll try to answer them in upcoming installments of Your First Garden. In the meantime, find more beginner gardening lessons here:

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