Ericaceous (Acid-Loving) Compost
Azaleas, rhododendrons (genus Rhododendron), blueberries, cranberries (Vaccinum), and other plants in the plant family Ericaceae tend to prefer acidic soil. Ericaceous compost tends to be more acidic than other finished composts, and it may lower the pH (a measure of acidity) of soil over time.
The ingredients in ericaceous compost aren’t that surprising when you consider the environment where these acid-loving plants thrive. For instance, in Great Smoky Mountains National Park, rhododendron thickets can be found under groves of oaks (Quercus) and beech (Fagus) trees and beneath needle-bearing conifers like hemlocks. In fact, needles, oak leaves and beech leaves can lower compost pH.
To make your own ericaceous compost at home, try mixtures of the following ingredients. Each of these should be easy to find in your own landscape or at your local garden center, nursery, co-op or box store.
Oak and/or beech leaves are a good source of carbon needed to create compost. Both oaks and beeches are in the same plant family – Fagaceae – and their foliage can acidify the soil as it decomposes. In the forest, it takes about two years for these leaves to turn into leaf mold. You can speed up the process at home by chopping up the leaves into smaller sections, keeping the pile moist, turning regularly, and adding a source of nitrogen like grass clippings, food scraps or manure.
Pine needles are increasingly popular as garden mulch, which means that bales of pine straw can be purchased from many garden retailers. Although fresh, red needles are most desirable as an attractive mulch for garden beds, newer needles can take quite some time to decompose. Dingy and gray bales of pine straw are unattractive as mulch, but great for composting. Retailers may be willing to sell older bales at a discount.
Coffee grounds, according to some sources, are a great source of nitrogen (20:1 carbon-to-nitrogen ratio) and can even lower soil pH. The bacteria in your compost pile need nitrogen to break down carbon in autumn leaves and pine straw. It may be worthwhile to include a hearty dose of coffee grounds when adding other food scraps to the pile. Not a coffee drinker? Many coffee houses will gladly give away spent grounds for gardening and composting projects.
Peat moss, when amended into the soil around ericaceous plants, can promote healthy growth and improve soil pH. However, peat is a finite resource. It takes the plants in peat bogs about 1,000 years to grow 3 feet. Most peat for sale in the U.S. is harvested from Canada, which means that it has been shipped long distances and can be somewhat expensive. Some recipes of ericaceous compost may call for peat moss. However, you should be able to satisfy the pH needs of your acid loving plants without this ingredient.
Most unfinished compost tends to be acidic, so it’s best to wait to test compost pH after it’s finished and has been mixed into the soil. You may still need to lower soil pH with amendments like sulfur for the first few years, but over time your ericaceous compost should make the soil environment better for acid-loving plants.