Master gardener Paul James of HGTV's Gardening
by the Yard
Q. How big a deal is soil pH?
A. It's a very big deal, especially if it's too low or too
high. For example, most landscape plants, vegetables and fruits prefer a slightly
acidic or near neutral soil pH, somewhere in the neighborhood of 6.8 to 7.2. When the soil
pH drops below or rises above those levels, plants can suffer due to their
inability to take up certain nutrients. Be careful—symptoms of pH imbalance can mimic other plant
problems.
For example, in alkaline soils—those with a high pH—iron may
not be available to the plant, and its leaves may begin to yellow. Too low of a pH
may release toxic levels of manganese, which can also cause yellowing or browning of
leaves. That's why it's a good idea to test your soil's pH once a year or so. Home test
kits work pretty well and are cheap to use. Just make sure you sample several areas
throughout your landscape as pH can vary from one place to the next.
To lower a soil's pH or make it more acidic, sulfur works
well, whether applied to the soil surface or gently worked into the top six inches of
soil. To raise a soil's pH or make it more alkaline, use lime. In both cases, before
making an application, make sure you follow the package instructions. Finally, realize that
certain fertilizers, notably synthetic forms that contain ammonium sulfate or ammonium
nitrate, can lower pH, and those that contain potassium nitrate or calcium can raise
pH.