Potting Soil for Acid-Loving Plants

Many flowers, like azaleas, require acidic soil.

Many flowers, like azaleas, require acidic soil.

Many varieties of houseplants, fruit trees and vegetables prefer to grow in acidic soil. Acidic soil has a pH lower than 7.0. For example, a soil with a pH between 4.5 and 5.0 is classified as strongly acidic. Home gardens often do not have the type of soil acid-loving plants need to thrive. Growing plants in containers allows the gardener to control the acidity of the soil. Amendments, such as coffee grounds and peat moss, also help raise soil acidity.

Acidic Potting Mix

Many commercial potting mixes lack the ingredients that acid-loving plants need. Acid-loving plants can grow in an all-purpose potting mix, if necessary, but they will probably not thrive as they would if grown in the proper soil type. African violet potting mix is ideal for most acid-loving plants. It contains perlite, lime and sphagnum peat moss for extra acidity. Gardeners who prefer to make their own potting mix can combine 4 parts organic matter with 1 part perlite, vermiculite or sand.

Coffee Grounds

Coffee grounds usually get tossed into the trash, but they have several uses in the garden. Coffee grounds increase the acidity of garden soil and potting mix, and as an added benefit, they also repel slugs. Before use, gardeners should spread coffee grounds out on a paper towel or newspaper to allow them to dry. The grounds stay fresh when sealed in a plastic bag. Coffee grounds can be added to the bottom of a container before filling it with potting mix. For plants in small containers, one or two spoonfuls of coffee grounds helps boost acidity. Plants in larger containers do best with 1 or 2 cups of coffee grounds lightly worked into the soil.

Peat Moss

Peat moss develops in peat bogs found in Canada and the northern United States. Peat moss retains moisture very well and allows adequate air circulation for plant roots. It makes an excellent soil amendment for acid-loving plants, but it is usually too acidic when used alone. Potting mixes that contain a blend of peat moss, perlite, vermiculite and pine bark will usually satisfy many acid-loving plants.

Sulfur and Ferrous Sulfate

Ferrous sulfate monohydrate and sulfur, chemicals available in many garden centers, quickly increase the acidity of soil. These chemicals work much faster than organic amendments, such as peat moss and coffee grounds. Too much of either chemical can cause root burn, resulting in plant death. Some gardeners use aluminum sulfate to raise soil acidity, but this practice can actually poison certain plants, such as azaleas and rhododendrons.

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