Soil pH measures the acidity or alkalinity of the soil on a scale from 0 to 14. A value of 7 is neutral; numbers below 7 indicate increasing acidity, and numbers above 7 indicate increasing alkalinity. The scale is logarithmic, meaning a drop from pH 6 to pH 5 represents a tenfold increase in acidity. Understanding soil pH is a practical first step to explaining the presence of certain plants, including weeds, in your garden or lawn.
How Soil Acidity Affects Weed Growth
Weeds do not typically prefer acidic soil, but they possess a high tolerance for the harsh environment it creates for most cultivated plants. When soil pH falls below 5.5, it severely restricts the availability of essential nutrients like phosphorus, calcium, and magnesium. This low-pH environment also increases the solubility of elements such as aluminum and manganese, which can become toxic to the roots of sensitive, desirable plants.
Weeds flourish by capitalizing on the reduced competition from less-tolerant plants, which struggle with nutrient deficiencies and metal toxicity. Their robust root systems and adaptive biology allow them to scavenge scarce resources more effectively than typical garden or lawn plants, giving them a competitive edge.
Indicator Weeds That Thrive in Low pH
The presence of certain weeds can serve as a simple diagnostic tool, suggesting that the soil pH is acidic before an official test is performed. These plants handle the chemical challenges of low-pH soil better than the majority of cultivated plants.
Common Acidic Soil Indicators
The following weeds are often reliable indicators of low-pH soil:
- Sheep Sorrel (Rumex acetosella): Often found in soils with low fertility and a pH below 6.0. This weed accumulates calcium and phosphorus, signaling a need for soil correction.
- Broadleaf Plantain (Plantago major): Tolerates compacted and low-fertility soil alongside its preference for high acidity.
- Moss: Proliferates on the surface of acidic turf where grass struggles to grow due to the unfavorable pH level, often indicating poor drainage or compaction.
- Horsetail (Equisetum arvense): A reliable indicator of low-pH soil, especially in areas that are poorly drained or heavily compacted.
- Dandelions (Taraxacum officinale): Highly adaptable but thrive in acidic conditions. Their deep taproot helps pull up minerals like calcium from the subsoil, which can then enrich the topsoil.
Practical Steps to Adjust Soil Acidity
The most reliable method for confirming and correcting acidic soil is to perform a professional soil test. This test determines the current pH level and the soil’s buffering capacity—its natural resistance to a change in pH—which dictates how much material is needed. Home pH kits can give a general reading, but they cannot accurately calculate the required application rate to neutralize the acid.
The primary approach to raising soil pH is by applying liming materials, most commonly agricultural lime (finely ground calcium carbonate). Dolomitic lime is an alternative recommended if the soil test indicates a magnesium deficiency, as it is a blend of calcium and magnesium carbonate. These materials work by neutralizing the active hydrogen ions in the soil, effectively reducing the acidity.
Lime should be applied several months before planting or during the spring or fall to allow sufficient time for the material to react, as the process is not instantaneous. For established areas, lime is typically spread on the surface, but incorporating it into the top few inches of soil through tilling provides the quickest results. Ongoing monitoring with a soil test every two to three years is necessary to ensure the pH remains in the optimal range of 6.0 to 7.5 for most plants.