Does pH Up or pH Down Work for Container Plants?

Lowering pH

• Before attempting to change soil pH, perform a pH test. These are available at many nurseries or your local office of a university’s cooperative extension program. Plants often display symptoms of soil that’s too alkaline, including yellowed leaves with dark green veins and stunted growth. When soil pH is too high, plants can’t absorb iron, zinc and manganese. If your container medium is alkaline, add some fertilizer containing ammonium-N, ammonium sulfate or sulfur-coated urea. Make sure the soil is moist, aerated and warm, and give the fertilizer time to work.

Raising pH

• If your soil pH level is too low, which can be caused by a lack of oxygen in the soil or runoff from bark mulch, raise it using an application of nitrite-based fertilizer. Give the treatment at least seven days to work. If levels don’t rise to a satisfactory level, sprinkle dolomitic lime or pelletized agricultural lime on top of the soil. Test the soil two, four and six weeks after treatment. Other pH-raising treatments include soaking the soil with potassium bicarbonate or flowable lime.

Plants for Acidic Soil

• Some container plants grow best in soil with a pH below 5.0. These include azalea (Rhododendron spp.), heather (Erica spp.), Japanese pieris (Pieris japonica), many junipers (Juniperus spp.), loropetalum (Loropetalum chinense) and white ageratum (Ageratum houstonianum). Other plants grow best in moderately acidic soil with pH from 5.0 to 5.5. These include American holly (Ilex opaca), creeping phlox (Phlox stolonifera), fringed bleeding heart (Dicentra eximia), gayfeather (Liatris spp.), wintergreen (Gaultheria procumbens) and yellowroot (Xanthorhiza simplicissima).

Plants for Alkaline Soil

• Few container plants prefer alkaline soil with pH between 7.0 and 8.0. Exceptions include barberry (Berberis thunbergii), bush cinquefoil (Potentilla fruticosa), clematis (Clematis spp.), creeping juniper (Juniperus horizontalis), hills of snow hydrangea (Hydrangea arborescens), honeysuckle (Lonicera spp.), salvia (Salvia nemorosa) and yarrow (Achillea spp.).