Soil pH is a measure of the acidity or alkalinity of the soil, determining the chemical environment where plants absorb nutrients. Lime, typically composed of calcium carbonate, is added to soil to raise the pH and counteract natural acidity. When too much lime is applied, the soil becomes overly alkaline. This highly alkaline condition disrupts the delicate balance of soil chemistry, hindering plant growth and overall soil health. Addressing this issue requires a targeted approach using specific materials to reduce the high pH back to an optimal range.
Understanding the Impact of Excessive Liming
Nutrient lockout is the primary consequence of excessive liming, reducing the availability of essential elements to plants. High soil alkalinity decreases the solubility of several micronutrients, causing them to bind to other compounds in the soil. Even if these nutrients are physically present, they are rendered inaccessible for plant uptake.
This lockout most commonly affects iron, manganese, and zinc, leading to visible deficiency symptoms in plants. Iron deficiency often manifests as interveinal chlorosis, where the leaves turn yellow while the veins remain green. Excessive alkalinity can also reduce the availability of the macronutrient phosphorus, which precipitates into insoluble forms when the pH rises too high.
The soil’s biological health is also compromised by a highly alkaline environment. Most beneficial soil microorganisms, including the bacteria responsible for converting nitrogen into a usable form, thrive in a near-neutral pH range of 6.0 to 7.5. An overly high pH level can slow down the activity and reduce the diversity of these microbes, which are necessary for organic matter decomposition and nutrient cycling.
Actionable Steps to Lower Soil pH
The most direct way to correct overly alkaline soil is by introducing acidic materials that chemically react to lower the pH. The choice of material depends on the severity of the problem and the desired speed of correction. Elemental sulfur is considered the most effective long-term treatment due to its high acidifying potential.
Elemental sulfur works by relying on soil-dwelling bacteria, specifically Thiobacillus species, to convert it into sulfuric acid. This biological process is slow, typically taking several months to a year to show significant results, and requires warm, moist soil conditions to be active. Due to this slow reaction, application should be limited to no more than 20 pounds per 1,000 square feet annually.
For the sulfur to be most effective, it must be thoroughly incorporated into the top six to eight inches of the soil where the plant roots reside. If the soil is a heavier clay or has a high buffering capacity, it will require a greater quantity of sulfur to achieve the same pH reduction compared to a sandy soil. In cases where the soil contains a high concentration of free calcium carbonate, such as calcareous soil, the required amount of sulfur may be so massive that acidification becomes impractical.
A faster-acting alternative for minor pH adjustments is ammonium sulfate, a common nitrogen fertilizer. The use of ammonium-based fertilizers acidifies the soil as microbes convert the ammonium into nitrate, releasing hydrogen ions in the process. This material provides the added benefit of supplying nitrogen to plants, which can help alleviate nutrient deficiencies that may have been caused by the high pH.
Ammonium sulfate should be used cautiously and in small, frequent applications to avoid providing excessive nitrogen, which can harm plants. It is generally reserved for gradual corrections or maintaining a lower pH level. For a milder, temporary effect, organic amendments like Canadian sphagnum peat moss can be incorporated. Peat moss has a naturally low pH (3.0 to 4.5), but its effect is limited to the immediate surrounding area, requiring substantial amounts to influence overall soil pH significantly.
Soil Testing and Prevention Strategies
Correcting an over-liming issue must begin with a comprehensive professional soil test to determine the exact degree of alkalinity. The soil test report will provide the current soil pH. Crucially, the report should also include a measure of the soil’s buffering capacity, sometimes referred to as buffer pH.
The buffer pH reveals the soil’s resistance to a change in pH, which is influenced by the amount of clay and organic matter present. Soil testing laboratories use this buffer value to calculate the precise amount of acidifying material needed to reach the desired target pH for the specific plants being grown. Attempting to correct the pH without knowing the soil’s buffering capacity often leads to either under-application or, worse, a new over-correction.
The most effective prevention strategy is to only apply lime or other amendments when a soil test indicates a need. Instead of guessing, gardeners should calculate the exact required amount based on the soil test recommendations. Once the pH is corrected, ongoing management involves retesting the soil every two to three years to monitor the pH level and ensure long-term stability. This calculated approach ensures that the soil environment remains within the optimal range for nutrient availability and biological activity.