Filed under: How to!, Tips & Tricks. Tagged as: Gardening, Plant Health Care.
Soil pH made Easier
Soils in the Willamette Valley, Oregon generally have a pH range somewhere between 4.5 and 5.5 which means our soil is very acidic. A pH of 7.0 is considered “neutral”. Soils with a pH lower than 7.0 is an acidic soil and one with a pH higher than 7.0 is alkaline. It seems that soils in wet environments tend to be acid (west of the Cascades) and those in dry environments (east of the Cascades) tend to be more alkaline. Western Oregon’s very acidic soil is good for our conifers; Douglas fir, pine, spruce and our broad leaf evergreens like Rhododendron, Azaleas. But, for our other garden favorites; roses, fruit trees, vegetables, and lawn, this is a disadvantage.
It is always a good idea to test your soil for pH. The best time for your testing is in the fall. You will need to apply any soil amendments in the fall to allow time for them to act on soil pH before the next growing season. You can buy an inexpensive pH test kit at most nurseries, or garden centers. Most kits will have booklets about interpreting your result.
Raising the pH: When soil is too acidic, the most common material used to raise pH is ground limestone, found in most garden centers. Lime will raise the soil pH in a few weeks to several months, depending on how finely ground the lime is, and soil conditions. Wood ashes from your fire place can also be used to raise the soil pH. Wood ashes do not work as fast as limestone, but with consistent use, they can raise the pH value. Care needs to be taken when using Wood ashes. Avoid using large amounts of wood ashes at one time. Spread just a thin layer during the fall. If added in the spring, they can cause damage to germinating seeds, and young plants. If you decide to use wood ashes, be sure to do a soil test for pH every year.
Lowering pH: When soil is too alkaline, the two materials commonly used for lowering the soil pH are aluminum sulfate and sulfur. Both of these can be found at nurseries and garden centers. Aluminum sulfate will change the soil pH almost instantly because the aluminum produces the acidity as soon as it dissolves in the soil. Sulfur takes about a year to lower soil pH since certain bacteria are required to complete the process. Both materials should be worked into the soil after application. If these materials come in contact with plant leaves, they should be washed off immediately or leaf burn may occur. Be sure to not over-apply either aluminum sulfate or the sulfur. Other options include pine needles, shredded leaves, sawdust and peat moss. Pine needles are a great source of acid and mulch, especially under Rhododendrons and Azaleas. This will often correct iron chlorosis showing in yellowing leaves of broad leafed evergreens. Again, test your soil pH on a yearly basis, if you use these options.
Raising pH produces pink hydrangea flowers. (Lime or wood ashes)
Lowering pH produces blue flowers. (Aluminum sulfate, sulfur, pine needles