Early spring has many of us rushing out to buy topsoil (landscape soil/garden soil) either to fill/add to flower beds, build up vegetable patches, or to use on our lawns. But do we really know what we are getting?
Most gardeners believe they can judge good topsoil by sight (dark), feel (crumbly) and smell (sweet smelling). But many US Extension soil experts say it’s impossible to judge topsoil quality just by looking at it.
The problem with topsoil
Did you know that there are no official or legal definitions of topsoil in USA and Canada? There aren’t any official standards for topsoil!
Technically topsoil is anything found on top. That “topsoil” could be anything. In eroded areas it could be very infertile, high salt subsoil. Or beach sand. Or old agricultural soil filled with pesticides!
Topsoil bags don’t have labels. We have no idea of the pH or the texture of the soil. Nor do we know the organic matter and nutrient content of the soil. We don’t know if the soil contains stones or even more alarmingly high levels of salt, weed seeds, or toxins. We don’t even know if plants can grow in the soil!
During last week’s #groundchat, Jeavonna Chapman, a Baltimore Master Gardener, shared her experience in buying topsoil, “I bought a bag of topsoil, the very last time. Almost 40% sticks and rocks by weight.”
Solutions for topsoil
Without legal standards, growers have banded together form the Mulch & Soil Council. The Council compiled voluntary guidelines for landscape soil. Their definition of landscape soil says, and I quote,
Landscape Soil: A material, mix or blend for in-ground growing of plants, and made primarily from natural soils, bark, peat, humus, compost, and/or manure. It may include fertilizer, pesticides, and/or additives intended as soil conditioners (e.g.,perlite, vermiculite, sand, peat moss, charcoal). It shall have soluble salts ≤ 20 mmhos/cm and pH of 4.5-7.5. Suitability for use shall be determined by the testing protocol described in Appendix C.”
For those of you that are surprised that topsoil can be a mix, I offer Dr. Linda Chalker-Scott’s sage words. She says that,
“It is almost impossible to purchase native topsoil in urban areas; it is too precious a commodity. Commercially available topsoil is usually a mixture of native topsoil and a variety of inorganic and organic materials including sand, perlite, compost, peat moss, bark, sawdust, and manure. “
The growing protocol is a useful addition as it tests for soil toxicity.
The landscape soil in question is sown with radish, ‘Early Scarlet’ (Raphanus sativus); tomato, ‘Better Boy’ (Lycopersicum esculentum); and marigold, ‘Janie’ (Tagetes patula). If plants of all 3 species in the test soil produce cotyledons and the first set of true leaves, and the quantity of seedlings match the provided chart, the test soil passes the MSC Toxicity Test for Landscape Soils and Soil Amendments.
Hmm. But Mulch & Soil Council definition doesn’t give me the attributes of good topsoil, just barely adequate topsoil. Fortunately, Rutgers Agricultural Experiment Station (US Extension) has come up with a concise list of good topsoil attributes:
• Organic matter content between 1.5 to 10%
• Soil pH 4.5 to 5.9 for acid loving plants
• Soil pH 6.0 to 6.8 for most plants
• Soluble salts less than 0.5 mmhos/cm
• Soil textures: sandy loam, silt loam or loam
• Gravel content less than 10%
• Free of broken glass, paint chips, plastic
• Uncontaminated with lead (if you are growing edibles).
Notice that that good topsoil has much less soluble salts than the Mulch & Soil Council requirements. I have a feeling Mulch & Soil Council made an error in their soluble salt values. Saline soils soluble salt content start at 4 mmhos/cm. Utah State University Cooperative Extension states that the ideal topsoil has a value of less than 2 mmhos/cm. Acceptable topsoil has less than 4 mmhos/cm soluble salts. Anything above 4 mmhos/cm is unacceptable. The most saline soil clocks in at 14 mmhos/cm.
Maybe Mulch & Soil Council’s soluble salt value of 20 mmhos/cm is a typo? It should read 2 mmhos/cm.
btw dS/m (deciSiemens per metre) and mmhos/cm (milli mhos per centimeter) are both measurements of electric conductivity, which is predominantly caused by soluble salts in the soil. Both units of electric conductivity are used interchangeably in the literature. So large amounts of dissolved salts in soil translates to a higher electric conductivity reading. Fortunately 1 dS/m = 1 mmhos/cm!
I also need to point out that Mulch & Soil Council permits pesticides in their topsoil. Pesticides are a big no-no for organic growing, especially edibles. And pesticides negatively affect soil biology, a critical element in nutrient exchange between plants and soil.
Now, that we know what to “look” for in a topsoil, how do we know we are buying it?
- Get to know your supplier and ask about the source of the topsoil he or she is selling.
- If you are buying soil from a garden or landscape supply centre, ask the vendor for the product’s test data. Soil analysis, at the very least, should include pH & texture. Best texture: sandy loam, silt loam or loam and pH between 5.5 -7.5.
- If the topsoil hasn’t been tested, ask for a small sample and have it tested. Test it yourself or send it off for testing. Do a grow test to see if the soil is ok for growing. How? Use Mulch & Soil Council’s method!
- And use your nose; don’t buy topsoil with a chemical, or other off-odour!
And remember to till 3-4″ of the original soil, then mix with new topsoil, before adding the rest. Topsoil sitting as a completely separate layer on top of the existing soil is going to create drainage and growing problems. This small step creates a transitional layer, which allows water to drain and percolate up as well as facilitating root growth into the original denser soil.
To calculate how much soil you need to buy for your garden with this easy online soil calculator!
More information on topsoil myths and truths
Michigan State University Extension. The Shocking Truth about Topsoil.
Oregon State University Extension. Buyer’s beware! You could be getting just about anything when you order topsoil
In-depth fact-sheets on topsoil quality
Puyallup Research and Extension Center, Washington State University. The Myth of Soil Amendments, Part III “Healthy soil has high organic content”
Rutgers State University Cooperative Extension. Topsoil suitable for landscape use
Utah State University Cooperative Extension. Topsoil quality guidelines for landscaping