Soil acidity is measured using the pH scale, which ranges from 0 to 14. A value of 7.0 is neutral, with numbers below 7.0 indicating acidity and numbers above 7.0 indicating alkalinity. This scale is logarithmic, meaning a drop from a pH of 6.0 to 5.0 represents a tenfold increase in acidity, which dramatically alters the chemical landscape for plants.
Plant health and growth are directly tied to soil pH because it controls nutrient availability and the presence of toxic elements. Most garden plants thrive in a slightly acidic to neutral range, typically between 6.0 and 7.0. When the soil pH falls outside this preferred range, plants cannot efficiently absorb necessary elements for growth, even if those nutrients are present.
How Acidic Soil Affects Nutrient Uptake
When soil becomes too acidic, it interferes with the plant’s ability to take in essential macronutrients, a phenomenon often described as nutrient lockout. Low pH conditions, generally below 6.0, reduce the solubility and uptake of elements like Calcium and Magnesium. These nutrients are vital for cell structure and photosynthesis, and their deficiency hinders plant development.
A primary problem in highly acidic soil, typically below a pH of 5.5, is the increased solubility of Aluminum. Aluminum is present in soil minerals, but its toxic ionic form becomes readily available for plant absorption in low pH environments. This toxic aluminum rapidly inhibits root growth, often causing root tips to become stunted and thickened.
Root damage prevents the plant from effectively absorbing water and nutrients, leading to poor growth and decline. Furthermore, the toxic Aluminum ions interfere with the uptake of Calcium and Magnesium at the root surface, compounding existing deficiencies. Acid-intolerant plants thus suffer from both nutrient scarcity and root poisoning.
Common Plants That Prefer Neutral or Alkaline Soil
Plants that do not tolerate acidic conditions are sometimes called calciphiles, or “lime lovers.” They perform best in neutral or slightly alkaline soil environments, generally with a pH range of 6.5 to 7.5 or higher. These species require or tolerate the higher concentration of base cations and the lower availability of aluminum found in these soils.
Vegetables
A variety of vegetables prefer a pH near neutral for optimal growth. Asparagus thrives in slightly alkaline conditions, with a recommended pH range of 6.5 to 7.5. Cabbage and spinach prefer neutral to slightly alkaline soil to avoid specific diseases. Carrots and peas also grow well in neutral soil, generally favoring a pH between 6.0 and 7.0.
Ornamentals
Many popular ornamental plants suffer in acidic soil. Lilacs require a neutral to slightly alkaline pH for vibrant blooming and to prevent leaf yellowing. Clematis and certain varieties of Iris, particularly bearded irises, also prefer alkaline conditions. Lavender is an aromatic perennial that thrives in the free-draining, lime-rich environments characteristic of higher pH soil.
Trees and Shrubs
Several common trees and shrubs are poorly suited for highly acidic sites. Sugar and Red Maple trees prefer neutral to slightly alkaline conditions for healthy foliage color and growth. Boxwood is a popular evergreen shrub that performs better in neutral to slightly alkaline soil. Growing these plants in acidic soil results in poor vigor, stunted growth, and nutrient deficiencies.
Testing and Amending Highly Acidic Soil
The first step in addressing overly acidic soil is an accurate assessment of the current pH level. Home testing kits or electronic probes give a general idea, but a professional laboratory soil test provides the most accurate information. A lab test is beneficial because it includes a “buffer pH” reading, which measures the soil’s resistance to pH change, indicating how much amendment is required.
The most effective method for raising soil pH is the application of liming materials, which are compounds rich in calcium or magnesium carbonates. Agricultural lime (primarily calcium carbonate) and dolomitic lime (containing both calcium and magnesium) are the most common amendments. Dolomitic lime is chosen when the soil is deficient in magnesium.
Liming materials neutralize acidity by replacing acidic hydrogen and aluminum ions with calcium and magnesium, making the soil more hospitable to acid-intolerant plants. The process is gradual; lime dissolves slowly, and it can take several months for the pH level to adjust, requiring patience and retesting. The amount of lime needed depends on the initial pH, the desired final pH, and the soil texture, with clay soils requiring more lime than sandy soils.