The Homeowners Column

The Homeowners Column

Understanding acid-loving plants and soil pH

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

Gardeners are edgy. We walk to the edge of the cliff of good gardening, hang our toes over the precipice, and look out over the horizon. Our eyes are drawn to the one plant growing on a nearby pinnacle. After we admire its beauty, we quickly ponder, "I wonder if I could grow that?" We are always attempting to grow plants on the edge; on the edge of their cold hardiness, heat tolerance, or tolerance to water, drought, shade, or sun. Maybe it's the challenge or maybe it's the undying optimism of being a gardener.

Our overwhelming desire to grow acid-loving plants is a good example of our compulsion to garden on the edge. Acid lovers include rhododendrons, azaleas, hollies, and blueberries. But what does acid-loving really mean? It's a story of love, desire, and chemistry.

It's all about the soil pH (potential Hydrogen). A pH of 7 is neutral; below 7 acidic; and above 7 alkaline. A one-point difference reflects a big change. A pH of 5 is ten times more acidic than pH of 6 and pH 5 is 100 times more acidic than pH of 7. Most plants including flowers, vegetables, shrubs, and trees grow best where the soil pH ranges from 5.8 to 7.0. We are pretty lucky here in central Illinois since our native soils are in that range.

However pH can change through a variety of activities: fertilizing (especially over-fertilizing); adding too much wood ash; or irrigating with alkaline city water. Also construction activities can raise the pH of surrounding soils such as limestone driveway additions, or through debris from drywall or concrete. Soils can be changed in construction due to inversion or removal of topsoil.

Traditionally lime is used to raise pH and sulfur is used to lower pH. A common misguided activity is the yearly addition of lime to gardens or landscapes. Lime is a common addition to agricultural land since the fertilizer of choice, anhydrous ammonia, lowers the soil pH. However since we don't use anhydrous ammonia on our landscapes, a yearly addition of lime on lawns or gardens is not recommended unless a soil test reveals the need. In other words soil pH can and does change due to our intended or unintended activities.

Soil pH is a crucial part of plant growth since it affects the availability of plant nutrients and microbial activity. Plant nutrients are held in different forms largely dependent on the soil pH. Most nutrients are in their plant available forms at slightly acidic to neutral pH. However nutrients of iron, manganese, zinc, and copper are more available in acid soils. The plants listed as acid-loving are really nutrient-loving.

At extreme ends of the pH scale plants struggle to obtain the proper nutrients or in especially acid soils some nutrients such as aluminum and manganese can reach toxic levels. In many parts of the world soils are so acidic nutrient toxicities have left the land unusable for growing crops. Also at pH extremes the microbial activity necessary for nutrient cycling in the soil ceases. The wrong soil pH produces a pantry-full of canned food, but the plant has lost its microbial can opener.

Too high a pH for acid-loving plants reveals itself most commonly as stunted growth and yellow leaves (chlorosis). Iron deficiency or iron chlorosis is typically seen as yellow leaves, but the leaf veins remain green.

As with all good stories of love and desire you will have to wait until next week's column to read about how dramatic changes in lowering soil pH require the addition of sulfur, how aluminum sulfate should only be used on hydrangeas to lower pH, and how some fertilizers such as iron sulfate, ammonium sulfate, and sulfur-coated urea can lower pH in small increments. Stay tuned.

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