Gardening: Some plants, such as blueberries, spruce, grow best if you acidify soil

Now that we can honestly say it’s spring, it’s time to fertilize plants to get them off to a good start for the growing season.

While most plants do just fine with ordinary fertilizers, some plants require a fertilizer that creates and maintains an acidic soil base for them to grow well.

If your high school chemistry has been long forgotten, here’s an explanation about what this all means. Soils in general are classified as alkaline, neutral or acidic based on the pH scale. The pH scale is a 14-point logarithmic scale in which seven is neutral and numbers below that indicate acidity while those above are show alkalinity. In general terms, the scale expresses a series of chemical reactions that make nutrients available or unavailable to the plants. In the in the Spokane region, our soil tends to have a pH of six to seven, which means its slightly acidic to neutral. Most plants grow well in this range, but a few of our popular plants need a lower pH or acidic soil to grow well.

Plants such as blueberries, rhododendrons, hydrangeas, dogwood, hollies and conifers like hemlock and spruce fall into this category and need special attention to do well. Signs that the plants aren’t doing well vary and are caused by the unavailability of certain chemicals. In rhododendrons the lack of iron availability results in yellowing leaves with green veins and weak flower production. In hydrangeas, if the pH is not acidic enough, bigleaf hydrangeas (Hydrangea macrophylla) produce pink flowers instead of blue. In blueberries, the plant doesn’t grow robustly and has few berries. In dogwood, hollies and conifers, their growth may be stunted. In dogwoods, flower production will be sparse.

Creating acidic soil where there is a prominence of neutral soil is a challenge because of the shear mass of soil involved, but it is not impossible. It does take some planning and monitoring to get good results. Soil amendments like sphagnum peat moss, ammonium sulfate, iron sulfate, acidic fertilizer (rhododendron fertilizer), elemental sulfur, and aluminum sulfate can all be used to acidify soil. Peat moss should be mixed into the soil at planting while the others can be added as top dressings according to the package directions. Each of these has a specific effect on a plant so read the directions carefully and use only the one that fits your situation.

Sphagnum peat moss is best added to the soil at planting at the rate of 4 inches thoroughly worked into about 8 inches of soil.

Blueberries are best planted into raised beds filled with free draining soil and compost and amended with peat moss, conifer sawdust and elemental sulfur and fertilized in the spring with rhododendron fertilizer.

Aluminum sulfate added to the soil will turn bigleaf hydrangeas from the pink tones usually found here to bluish tones over time.

Rhododendrons need a spring application of iron sulfate and rhododendron fertilizer to produce dark green leaves and a showy display of flowers.