Gardens with Acidic Soil

To learn how plants that thrive in acid soil fit into their natural environment, explore a natural forest. The same forces that shape and nurture a forest create acid soil. Copious rain that filters through the trees and soaks the ground below leaches alkaline minerals, such as limestone sediment, from the soil, neutralizing it. At the same time the natural mulch of evergreen needles or the tannin-filled leaves of trees like oaks, which blanket the forest floor, contribute acidity to the soil as they begin to break down. So, unless a forest happens to be sitting atop a hefty deposit of crumbled limestone, the natural pH of its soil will be in the acidic range.

One look at a forest proves that acidic soil can support a wide range of plants. Many trees like magnolia and holly prefer acid soil, as do forest understory shrubs like azaleas and rhododendrons, and low, creeping ground-covers like ferns, bugleweed, and sweet woodruff. Indeed, acid soil is such welcome news to these and many other plants that if your soil is neutral to slightly acid, growing a woodland-type garden in acid soil requires little effort on your part.

Expanding Your Choices

But not everyone wants a woodland in his or her yard. If you want to nurture colorful flowers and shrubs that aren't native to the woods, there's good news. The vast majority of favorite garden plants, from petunias to roses, grow best in soil that is only slightly acidic or "near neutral," meaning that it has a pH between 6.2 and 6.8. Fortunately, this is easily achieved by cultivating beds, working in ample compost to neutralize the pH, or by adding garden lime to over-acidic soil or adding garden sulfur to alkaline soil as directed on the package.

Made from crushed limestone, lime is a very simple mineral soil amendment. The preferred type for most gardens, dolomitic limestone, enriches the soil with the plant nutrients calcium and magnesium while raising the soil pH. Unlike most fertilizers, lime usually persists in the soil for several years, until repeated rains wash it away. It is seldom necessary to lime a bed more than once every 3 years or so, but the only way to know for sure is to test your soil's pH annually, preferably in the fall.

You can add lime to your soil in any season. Most gardeners lime in either spring or fall, when they are digging new beds or renovating old ones. Because lime dissolves slowly, it must be thoroughly mixed into the soil. Lime that is simply dusted onto the soil's surface takes months to benefit plants' deep roots. This is fine for lawn grass, and it may be your only option when adjusting the pH for roses and other well-established plants.

Dense, heavy clay soils require more lime than their light-textured, sandy counterparts, so there is no set application rate. However, a good starting point is 50 lb (22.7 kg) per 1000 sq ft (93 m). If you check your soil's pH before working in this amount and then recheck it 6 weeks later, you will have a good idea of how much change has resulted. If the soil is still very acidic, you can work in more lime the next time you dig the bed.

Although lime is nothing more than finely pulverized rock powder, wear gloves when handling it, and avoid breathing the dust. In its pure state, lime is sufficiently potent to dry your skin and irritate lung tissues. Some stores sell lime in a granular (pelleted) form that is easier to handle.

Garden sulfur, also known as wettable sulfur, is a fine powder consisting mostly of ground-up sulfur rock. It is used as a soil acidifier. Manure, peat moss, and acidic mulches like chopped oak leaves and pine needles will have a mildly acidifying effect on the soil. The effects of these acidifiers are short-lived, so if your soil is alkaline, you should check the pH each season and add amendments as recommended by the test results.