How to Lower
Your Soil pH
How can I amend alkaline soil?
--Maureen Stewart, Denver
In a nutshell: Your best option is to add
organic matter and grow plants that do well in alkaline soils.
The whole story: Technically, all soils with a
pH higher than 7.0 are alkaline, or "sweet," but most garden plants
tolerate a pH up to about 7.5. Alkaline soils are generally composed of basic
(high pH) parent materials such as limestone (calcium carbonate). High-pH soils
are more prevalent in arid climates, because rainfall does not leach the
calcium and other basic materials out of the soil. Soil pH affects nutrient
availability for plants, and in soils with a pH over 7.8, iron, zinc, and
phosphorus deficiencies are common.
Lowering soil pH is a slow and challenging process.
"The truth is, lowering soil pH is hard, because the limestone in the soil
continually dissolves," says Jessica
Davis, Ph.D., a professor of soil science at Colorado State University.
Davis
recommends getting your soil tested to determine its pH, lime content, soil
texture, and mineral and nutrient content. If the test results indicate a
medium or high lime content, she recommends not trying aggressive measures to
lower the soil pH.
The most aggressive way to lower soil pH is to apply
pure sulfur or flowers of sulfur. You should follow your soil test's
recommendations, but in general, if you are applying sulfur to an area that
does not currently have any plants growing in it, such as a new garden bed, you
can apply it at a higher rate than in areas with growing plants. You can also
add organic matter, such as compost or composted manure, to your soil annually
and mulch with acidic organic mulches, such as pine needles. Adding organic
matter slowly lowers your pH over time, while increasing microbial life and
improving the structure of your soil.
Regardless of how, or if, you decide to amend your
soil, Davis
recommends growing plants that do well in alkaline soils, such as linden trees
(Tilia spp.). "Gardeners who have their heart set on growing an
acid-loving plant like a blueberry should grow it an container," Davis says. "That
way, you only have to change the pH of a small amount of soil."
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