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Acidifying Soil       Bookmark and Share

Growing acid-loving plants in our area can be a challenge because the soils here tend to be more alkaline than most plants like. In fact, the vast majority of ornamental plants prefer a soil pH of 6.0 to 6.5, slightly acidic. To complicate the matter, the water we use in our gardens tends to be alkaline. Often municipal water systems buffer the water to a pH as high as 8.0 to help our pipes last longer. That’s great for the pipes, but can be hard on our plants. Ideally, we would all grow acid-loving plants in naturally acid soil and alkaline loving plants in alkaline soils. But that’s so limiting, we have learned some ways to change the soil to suit the plant.

Some plants can be sensitive to soil pH. While most plants prefer slightly acid soil, most plants are also very tolerant of a range of 6.0 to 7.5 pH. There are, however, some plants that require acid (or alkaline) soil. When the pH is too high for a plant, it limits the plants ability to make use of many nutrients. In soils that are too alkaline, plants cannot make use of the boron, copper, iron, manganese and zinc even if there is enough in the soil.

To find out the pH of your soil the best way is to send a sample to a laboratory for testing. There are home kits, but they aren’t quite as accurate and can be influenced by the pH of the water you use in testing. Locally, soil can be tested at the soil labs at the University of Minnesota. Testing forms are available at the Bachman’s information desk.

What exactly does the number indicate? The pH scale ranges from 1 to 14. A pH of 7 is considered neutral. Anything below 7.0 is acid and anything above is alkaline. While it may not seem like there is much difference between a pH of 5.0 and 6.0, there is. The values are logarithmic like the Richter scale for earthquakes. That means soil with a pH of 5.0 is ten times as acid as soil testing 6.0.

There is a difference in requiring and preferred acid soil. Azaleas, rhododendrons blueberries and pin oaks all require a cid soils. Probably the most common plants in the landscape that prefer it are river birch, gray birch, magnolias, hydrangeas, serviceberries, spruce, pines, mountain ash, clethra and holly. (Whitespire birch would like it slightly acid too, but they are more tolerant of higher pH.) There are also perennials that prefer acid soil, some of the more common perennials that prefer acid soils are ferns, most woodland plants, lupines, garden lilies, butterfly weed and primroses. If you aren’t sure what pH your plants prefer, ask one of our horticulturists.

Whenever possible, it is best to acidify the soil before planting. But even if the soil pH is adjusted before planting, it will need to be monitored and may need acidifying on a yearly basis. To acidify the soil before planting will depend on the soil type. If you are lucky enough to be starting with a well-drained soil, it can be acidified by adding large amounts specific organic materials that acidify as they break down, such as peat moss and composted oak leaves. If the soil has lots of clay and is poorly drained, adding lots of organic material can make the problem worse by holding more moisture. In that situation, it is better to acidify the soil using elemental sulfur or iron sulfate. Apply the recommended amount of sulfur or iron and incorporate it into the top several inches of soil.

To safely acidify soil where you already have plants applying elemental sulfur is the best way. Sulfur is very slow acting (it may take as long as 3-4 months to be effective). Applied at recommended rates, it’s safe for your plants. Iron sulfate is good, too. Iron sulfate acts more quickly (it takes about 2-3 weeks to affect the pH) but it takes a lot more of the chemical compared to using elemental sulfur. Be especially careful with iron sulfate. It will leave rusty stains on your clothes, sidewalks and patios. It’s even a good idea to wash your gardening clothes separately from your other laundry after spreading iron sulfate.There are other options for acidifying the soil. There are several soil supplements that will help acidify, such as cottonseed meal and ammonium phosphate, but the amounts needed make them impractical for large areas. To add fertilizer with the acidity you can use a water-soluble fertilizer such as Miracid This will help keep the pH problem from getting worse, but they are not strong enough to lower the pH. Another option would be to fertilize with ammonium sulfate, but it is strong and can easily burn plants if not used carefully. Cottonseed meal can be used as an acidifying fertilizer.

There is no easy way to know how much of an acidifying product to apply. A soil test from the University of Minnesota will tell you how much of what ingredients you need. Unfortunately the instructions on packages of  products such as elemental sulfur seem vague or confusing. That’s because application rates vary widely depending on soil type as well as the initial pH. If you don’t feel confident after reading the instructions, check with one of our horticulturists. They’ll be glad to help.

Additional Bachman's Information

Packaged Soils and Soil Amendments
Understanding Soils
Soil pH
Plants for Acid and Alkaline Soils

Recommended Products
Soil Testing Kits and Information;
Baled Peat Moss
Elemental Sulfur, Liquid and Dry

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