Urban Farming: How to Find the pH Level of the Soil

The sweetness (alkalinity) or sourness (acidity) of your soil is measured by a term called pH. This term is used a lot in farming circles. It’s not necessary to understand the chemistry behind this “measure of hydrogen ion concentration.” What you really need to know is the pH number of your soil.

The pH scale runs from 1 to 14, with 1 being the most acidic and 14 the most alkaline. You’ll likely never see soils at the extremes of this range. Most soils lie between 5 and 9, and most plants grow best with a pH between 6 and 7.

Knowing your pH is important. Plants can’t absorb certain nutrients if the soil’s pH isn’t in the proper range for them. For example, blueberries love a highly acidic soil in the pH range of 4 to 5. If the pH is higher than that, the plants can’t take up nutrients well, the leaves turn yellow, and the plant becomes stunted.

You can raise the pH by adding lime, and you can lower it by adding sulfur. In general, areas of the country that have high rainfall amounts, such as the East and Pacific Northwest, tend to have acidic soils, and drier areas, such as the Southwest, tend to have more alkaline soils. Clay soils and soils high in organic matter tend to buffer the pH, keeping it around neutral (a pH of 7). Sandy soil and soils low in organic matter, on the other hand, are more susceptible to swings in the pH level.

A soil test tells you the pH level of your soil so you know how much lime or sulfur (if any) you need to add. You can conduct a home test or you can hire a professional to do it for you. Just keep in mind that a home test is inexpensive but not as detailed or reliable as a professional test.

The key to getting a useful soil test result from a laboratory or a home kit is to take a proper soil sample. Follow these steps:

  1. Remove the sod or top vegetation and dig down 4 to 6 inches into the soil to get a soil sample.

  2. Take six to eight soil samples from various locations in your garden.

    Do separate tests from various crops. For example, collect samples for your vegetable garden in one test and samples for a lawn in another test.

  3. Mix all the soil samples for a single test in a clean bucket and take a sample of the sample.

    Place two cups of the soil sample in a plastic bag to send to the laboratory, or use the sample for your home kit testing.

After you know how much lime or sulfur you need to apply, you’re ready to spread it. In the small gardens often found in urban areas, spreading these materials by hand is the easiest. Spread the lime or sulfur at least a few months before planting; the minerals need time to take effect. To hasten the effects, work the lime or sulfur into the top 6 inches of the soil.

Lime and sulfur come in two forms: powder and pellet. Powdered lime and sulfur react quickly with the soil to change the pH. However, they’re dusty and can irritate your lungs. Avoid spreading these materials on windy days, and wear gloves and a mask when spreading. People with respiratory issues should avoid using powdered forms of these amendments.

Pelleted lime and sulfur are easier to use, but they can be more expensive and harder to find.

In urban areas especially, salts from de-icers used on snow and ice can build up in the soils and become toxic to plants. A simple solution if you’re concerned about salt build-up in your soil or if a soil test indicates you have high salt levels, is to flush your soils in spring with water to remove the salt from the upper layers.

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