I’ve killed a lot of plants in my time. The list is long and includes garden aristocrats like delphinium and lupines but also tough customers like yarrow, penstemon and hardy rock-garden plants that would seem to grow almost anywhere.
The longer I gardened, the more I saw a pattern. After briefly thriving, the plants shrank, and by Year 3 had vanished entirely.
Was it something I was doing? Or was the problem where I was trying to grow them? The soil in my yard near the Mississippi River is rich, black, moisture-retentive silt. The plants that were disappearing prefer sandy, coarse soils or sharp drainage. Their roots were rotting in my wet yard.
Soil isn’t something a lot of gardeners think much about, but we should. Soil characteristics can determine what thrives and what dies in our yards. Whether a soil is sandy or clay, compacted or loose and acid or alkaline affects everything from drainage to fertilization to how easy it is for plants to take root and absorb nutrients and water.
While many garden and landscape plants are tolerant of a range of acidic or alkaline soils — something called soil pH — if you want to grow acid-loving plants like blueberries, azaleas and rhododendrons, you may need to amend your soil. Soil pH affects the availability of nutrients that plants need to do well. A pH of 7 is neutral, a number below 7 is acid, and a number above 7 is alkaline or “sweet.”
Most plants do well in slightly acid soil with a pH of 6 to 7. Blueberries, though, need a pH of 4 to 5, azaleas and rhododendrons a pH of 4 to 5.5.
The starting point is to get a soil test. Home soil-testing kits are cheap but often unreliable; consider spending $17 to get an accurate test from the University of Minnesota’s Soil Testing Laboratory on the St. Paul campus.
If that seems expensive, it’s peanuts compared with the thousands of dollars homeowners spend on plants, trees and turf over the years. A basic soil test at the U lab will analyze what type of soil you have and how much organic matter is in the soil, and will include fertilizer recommendations, depending on whether you are growing grass, vegetables or flowers. It will check to see if the soil contains lead and whether salts are present from excess fertilizer or road salt.
Find much more information here: soiltest.cfans.umn.edu/
According to the university, many Minnesota soils already have significant amounts of lime in them. But if soils are too acid for the plants that are to be grown there, pH can be raised by adding lime or wood ash. Wood ash is less effective than lime at making the soil sweeter, and too much can damage roots and new plants. Use lime only if a soil test recommends it.
In the Twin Cities, it’s more common for gardeners to want to make soil more acid for plants such as blueberries, azaleas and blue hydrangeas like Endless Summer. Again, a soil test will determine how much acidifying material is needed. Sphagnum peat moss dug into the garden acidifies soils, but the effect lasts only five to 10 years. Elemental sulfur is one of the cheapest ways to lower pH, but it takes time to act and should be mixed with soil a year before planting.
Aluminum sulfate is sometimes recommended for blue hydrangeas, but it needs to be used with caution because it can be toxic to roots. Iron sulfate is a better option, though it is more expensive.
If you want to read more about modifying your soil’s pH, this is a nice summary: extension.umn.edu/garden/landscaping/implement/soil_ph.html
Improving your soil
If all that seems complicated, remember that most plants do quite well in most Minnesota soils without manipulating the pH. To prepare an area for a garden, the best thing you can do is dig in well-rotted compost. It will add organic matter, improve drainage as well as the water-holding capacity of soil, break up hardened soils and allow water to penetrate the soil surface.
Never till or dig in wet soils — the clumps of dirt that result will harden to rocklike consistency. And avoid adding material from bags that are simply labeled “top soil.” You may get some beautiful loam, but it’s more common to get chunks of hard dirt containing sticks and stones. I’ve had better luck incorporating bags of composted manure.
If you order black dirt for your yard or garden, don’t be afraid to ask to visit the landscaper to see what you’re ordering. Again, quality varies widely.
If you’re growing plants in pots, always buy sterilized potting soil. Garden soil tends to compact in pots, with a hard crust that won’t absorb water. You’ll get better results with commercial mixes, or a potting soil you mix yourself that has some peat and vermiculite or perlite in it.
Mary Jane Smetanka is a Minneapolis freelance writer and a Master Gardener.