Many beautiful ornamental plants grow their finest in acidic soils, those with a pH less than 7.0. Mountain laurel, rhododendron, blueberry and gardenia are a few examples of those that prosper in acidy ground, a pH range of 5.0 to 6.5. Changing the soil's pH can be intensive, especially if done on a large scale. However, the application of sulfur can have a profound effect on making soil acidic and the continual application of mulches and soil amendments that are acidic help create a rich, fertile soil with a low pH value.
Application of Inorganic Compounds
Apply granular sulfur to the soil and incorporate it to a depth of 6 to 12 inches. Sulfur, sometimes sold as "flowers of sulphur" will lower the soil pH once water reacts with it in the soil. Rainfall and irrigation water will leach the sulfur molecules lower into the soil profile. Sulfur is slow-acting, taking weeks or months to change soil pH and may need reapplication every two to four years to maintain a specific soil acidity.
Use aluminum sulfate or ammonium sulfate to quickly lower the soil's pH into an acidic range. These products more quickly release the molecules of sulfur into the soil once wet. Consult the dry fertilizer product label or contact your local cooperative extension office for specific information about their use and application in the soils of your region.
Mix water-soluble fertilizers into water for frequent application to plants. Some products are touted for their ability to make soil acidic and are labeled "for acid-loving plants." They contain higher concentrations of sulfur molecules than mainstream garden fertilizers and are recommended for frequent use on plants around their root zones. A good solution for localized plants that are in containers, using these fertilizers on a large scale is time and money consuming.
Application of Organic Materials
Incorporate peat into your topsoil. Peat naturally is acidic in pH and mixing this organic soil amendment with your native soil can lower the pH to varying degrees. Peat also improves the texture of the soil and its ability to retain moisture.
Spread acidic mulches across garden beds. Certain plant materials degrade and release their acidity into the soil. Pine bark, conifer evergreen needles, coffee grounds and oak leaves are all widely used as mulches and soil top-dressings around plants that appreciate acidic soil.
Repeatedly incorporate compost into your soil, particularly compost derived from materials listed in Step 2. Oak leaves and pine bark can take one to two years to break down into a friable compost, but pine needles and coffee grounds more quickly degrade. However, this compost is formed naturally if these materials are first broadcast as a mulch in the garden, providing added benefit and additional leaching of their acidity into the soil.
About this Author
James Burghardt has written for The Public Garden, Docent Educator, numerous non-profit newsletters and for Learn2Grow.com's comprehensive plant database. He holds a Master's degree in Public Horticulture from the University of Delaware and studied horticulture and biology in Australia at Murdoch University and the University of Melbourne's Burnley College.