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Rock gardening: What you need to know about soil acidity and alkalinity

by Timber Press on February 16, 2017

in Gardening, Regional

The peaty soil at Inverewe provides perfect conditions for choice, acid-loving plants like primulas, rhododendrons, and Himalayan blue poppies. Photos by the author.

With this excerpt from Joseph Tychonievich’s Rock Gardening, you can decode the acidity of your soil to pick the best plants for your plot.

Aside from the physical structure and drainage of the alpine bed, there are chemical considerations, most importantly the pH, a measure of the acidity of the soil. The pH level is measured on a scale ranging from 0 to 14, with 7 being the neutral point (neither acidic nor alkaline), 0 being the most acidic, and 14 being the most alkaline. The pH level in the soil determines a lot about how a soil behaves chemically, most importantly how available certain elements are for plant roots to absorb. In alkaline soils, for example, iron is tightly chemically bound to the soil particles, so plants can’t access it, leading to iron deficiencies. In highly acidic soils iron is very available but other elements less so, and aluminum becomes so freely available that it can result in aluminum toxicities. Plants that are native to different soil types have evolved to thrive in those soils, dealing efficiently with shortages of some elements and handling potentially toxic excesses of another. Most plants are fairly adaptable to a reasonable range of different soil pH and the vast majority of plants will grow best in soil that is neutral to slightly acidic, but some species that have adapted to extreme soil pH will demand similar conditions to thrive in the garden.

Rhododendrons and Japanese maples make a quiet corner for a Japanese stone garden.

PLANTS FOR ACID CONDITIONS

alpine snowbells (Soldanella species)
bluets (Houstonia species)
Chinese gentian (Gentiana sino-ornata and related hybrids)
heaths (Calluna species)
heathers (Erica species)
rhododendrons, dwarf species and cultivars (Rhododendron)

PLANTS FOR ALKALINE CONDITIONS

baby’s breath (Gypsophila species)
blue broom (Erinacea pungens)
candytufts (Iberis species)
chalk milkwort (Polygala calcarea)
saxifrages, silver-leaved species (Saxifraga)
woodruffs (Asperula species)

Bluets in a bog garden.

In nature, two main factors determine the acidity of a soil. Rainfall is a big one. Water moving through the soil leaches out minerals like calcium and tends to make soil increasingly acidic. So places like rain forests and wetter climates in general tend to have acidic soils. Drier climates don’t have the leaching effect of rain so tend to have alkaline soils. The other big factor is the bedrock that is the source of the soil to begin with. I grew up in Ohio where soils are mostly quite acidic, but there is a little strip of limestone bedrock running through the state producing very alkaline soils. In the United Kingdom, another rainy climate that would generally produce acidic soils, there is also a lot of alkaline bedrock—most famously chalk (which is what makes those cliffs of Dover so white), producing alkaline soils. The preferences of rock garden plants in terms of acidic or alkaline soil are varied. True alpines will be adapted to either alkaline or acidic conditions depending on the chemical composition of the particular mountains they evolved on, while the many excellent rock garden plants that come not from mountaintops but from semi-arid steppe climates around the world will tend to be more alkaline adapted.

Saxifraga sempervirens.

In a regular garden you are more or less stuck with the soil that your local bedrock and rainfall patterns have given you, particularly if you have highly alkaline soils. Rock gardens, on the other hand, are endlessly customizable, largely because of their small size. Trying to dig a big enough bed and filling it with peat to keep a large rhododendron happy when you have limestone bedrock is a massive logistical challenge; however, building a small raised bed or filling a container with a soil mix that has just the right acidic soil for a collection of heaths is quite easy. So research the soil preferences of the plants you want to grow, and consider building different beds—some with limestone, some without—to accommodate the full range of plant preferences.

Joseph Tychonievich studied horticulture, plant breeding, and genetics at the Ohio State University and was the nursery manager at Arrowhead Alpines, a premier rock garden nursery in Fowlerville, Michigan. He spent a summer working at Shibamichi Honten Nursery in Japan and has been a repeat guest on public radio’s food show The Splendid Table. Organic Gardening Magazine called him one of “six young horticulturalists who are helping to shape how America gardens.”

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Click the image below for a look inside this book.

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the peaty soil at Inverewe provides perfect conditions for choice, acid-loving plants like primulas, rhododendrons, and Himalayan blue poppies. Photos by the author.

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