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Gardening Columns

Woodland natives like this flame azalea thrive
in soil with a pH of 5.5 or less.

It's Time to Lime

First published on on December 7, 2006, by Suzanne DeJohn

Soil pH plays a critical role in how well plants grow, and fall is a perfect time to check it and make necessary adjustments. In regions with relatively high rainfall such as ours, soils tend to be acidic. Adding lime to the soil in autumn "sweetens" the soil over the winter in anticipation of planting next spring. So, before you wrap up your gardening season, plan to check your soil's pH.

What is pH?
The pH scale runs from 0 to 14, with 7 being neutral. Numbers lower than 7 indicate increasing aciditiy; higher than 7, increasing alkalinity. The origin of the term pH is murky, but you can think of it as standing for "potential hydrogen" since it represents the concentration of hydrogen ions (H+) in a solution. The pH scale is logarithmic, meaning that a change of one digit indicates a tenfold change in H+ concentration. So, a soil with a pH of 5.0 is 10 times more acidic than a soil wtih a pH of 6.0, and 100 times more acidic than a soil with a pH of 7.0.

Why Does pH Matter?
A soil's pH affects plants' ability to absorb nutrients; plant nutrients must be in a specific form to be available for uptake by roots. Soil pH affects whether a nutrient binds with other elements or organic matter and therefore rendering it unavailable. For example, as soils become more alkaline, iron and boron become less available to plants; in acid soils, phosphorus and calcium become less available. Most common garden plants do best in a slightly acidic soil, with a pH of 6.5 to 6.8. Rhododendrons prefer a more acidic soil with a pH of 4.5 to 5.5; lilac prefer a pH of 7.

pH also affects soil microorganisms, and this, in turn, affects nutrient uptake. For example, phosphorus becomes more available to plants as microbial activity increases.

Finally, in acidic soils aluminum can become toxic to plant roots. Raising the pH neutralizes the aluminum.

Soil pH in Our Region
Generally, acidic soils are common in areas that receive abundant rain, and alkaline soils are more common in arid regions. Soils in our region tend to be acidic. Even if a soil test has indicated a neutral pH in the past, soil acidity changes over time. The decomposition of organic matter and the application of some types of fertilizer add to soil acidity. Rain leaches calcium from the soil. So it's useful to monitor it by testing your soil every few years.

Soil Tests
You can purchase inexpensive home soil test kits, and for yearly monitoring these may be adequate. But it's useful to have a professional soil test done every few years. Most county Cooperative Extension offices provide soil test kits for free or for a modest price, usually under $10. The kit comes with detailed instructions on how to gather and prepare your soil sample for the most accurate results. You mail in the sample, and in a few weeks you'll receive the results, which usually include soil pH plus the amount of several important plant nutrients. You'll also get instructions on how to improve your soil based on the test results.

Adjusting pH
Adding lime (ground limestone) raises the pH of acidic soils; adding sulfur lowers the soil pH. Lime is available in several forms. Ground limestone is the least expensive; however, the fine powder can be messy to apply. Pelletized limestone is easier to apply and often takes effect a bit faster. However, both forms are best applied in the fall, preferably incorporating the lime into the top few inches of soil. Avoid using hydrated lime, which is very fast-acting and can harm plants. Sulfur is generally applied as a powder.

Adjusting your soil's pH can make a world of difference in how well plants grow. Lime is relatively inexpensive, and using it can reduce the amount of fertilizer you need to use. Proper soil pH maximizes the soil's ability to support plant life by maximizing nutrient availability and providing a hospitable environment for soil microbes.

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