What is Soil pH and Why It Matters
Soil pH is a measure of the hydrogen ion concentration in soil, expressed on a scale from 0 to 14. A pH of 7 is neutral, values below 7 indicate acidity, and values above 7 indicate alkalinity. Soil pH significantly influences nutrient availability, microbial activity, and overall soil health, directly affecting plant growth and productivity.
Most plants thrive in a pH range of 6.0 to 7.5. Outside this range, essential nutrients like nitrogen, phosphorus, potassium, iron, and manganese can become less available or toxic, leading to nutrient deficiencies or toxicities that stunt plant growth or cause poor yields.
Causes of Unbalanced Soil pH
1. Parent Material and Soil Formation
The mineral composition of the soil’s parent rock affects its baseline pH. Soils derived from limestone tend to be alkaline, while those from granite or sandstone are often acidic.
2. Acid Rain and Environmental Factors
Pollution-generated acid rain can increase soil acidity over time, especially in areas with poor buffering capacity.
3. Fertilizer Use
Excessive application of ammonium-based fertilizers (e.g., ammonium sulfate) acidifies soil, while repeated use of lime or alkaline materials raises pH.
4. Organic Matter Decomposition
Decomposition of organic matter releases organic acids, which can lower soil pH, particularly in poorly drained or compacted soils.
5. Irrigation Water Quality
Water high in bicarbonates or sodium can increase soil alkalinity, while acidic irrigation water may lower pH.
Effects of Acidic vs. Alkaline Soil on Plants
Acidic Soil (pH < 6)
- Nutrient Deficiencies: Phosphorus, calcium, magnesium, and molybdenum become less available.
- Aluminum and Manganese Toxicity: Toxic levels of soluble aluminum and manganese can damage roots.
- Reduced Microbial Activity: Beneficial microbes such as nitrogen-fixing bacteria are less active.
- Symptoms: Yellowing leaves (chlorosis), stunted growth, poor flowering.
Alkaline Soil (pH > 7.5)
- Nutrient Lockout: Iron, manganese, zinc, and phosphorus deficiency are common.
- Impaired Nutrient Uptake: Calcium and magnesium may be abundant but inaccessible.
- Symptoms: Interveinal chlorosis, leaf scorching, poor root development.
How to Test Soil pH Accurately
1. Field Test Kits
Available at garden centers, these kits provide quick, approximate pH readings using color-change indicators. While inexpensive and convenient, they are less precise and can be affected by soil color and moisture.
2. Electronic pH Meters
Portable pH meters offer more accuracy than kits. Proper calibration with standard buffer solutions is essential before use. Always clean the electrode after each test to maintain accuracy.
3. Laboratory Soil Testing
Sending soil samples to a professional lab is the most accurate method. Labs provide detailed reports including pH, nutrient levels, organic matter content, and recommendations for amendments.
Testing Best Practices
- Collect soil samples from multiple spots within the garden at root zone depth (6-8 inches for most plants).
- Mix samples thoroughly to create a composite for testing.
- Test soil during the growing season for best relevance.
- Retest every 2-3 years or after major soil amendments.
FAQs
Q1: How often should I test my soil pH?
Test soil pH every 2-3 years, or annually if you frequently add fertilizers or soil amendments that affect pH.
Q2: Can I adjust soil pH myself?
Yes. To raise pH (reduce acidity), incorporate agricultural lime (calcium carbonate). To lower pH (reduce alkalinity), use elemental sulfur or acid-forming organic matter like peat moss. Always apply amendments based on soil test recommendations.
Q3: What plants tolerate acidic or alkaline soils?
Azaleas, blueberries, and rhododendrons prefer acidic soils (pH 4.5-5.5). Lavender, lilacs, and clematis prefer slightly alkaline soils (pH 7-8).
Q4: Does soil pH affect soil microbes?
Yes, beneficial microbes such as nitrogen-fixing bacteria and mycorrhizal fungi thrive in near-neutral pH soils. Extremes in pH reduce microbial diversity and activity.
Key Takeaways
- Soil pH impacts nutrient availability and plant health significantly.
- Causes of unbalanced pH include parent material, fertilizers, organic matter, and environmental factors.
- Acidic soils risk aluminum toxicity and nutrient deficiencies; alkaline soils cause nutrient lockout.
- Accurate soil pH testing involves proper sampling and either field kits, electronic meters, or laboratory analysis.
- Adjust soil pH carefully using recommended amendments based on soil tests.
References
- Brady, N.C., & Weil, R.R. (2016). The Nature and Properties of Soils. Pearson.
- University of Minnesota Extension. (2020). Soil pH and Liming. Retrieved from https://extension.umn.edu/soil-management-and-health/soil-ph-and-liming
- Royal Horticultural Society. (2021). Soil pH. Retrieved from https://www.rhs.org.uk/advice/profile?pid=305
- USDA Natural Resources Conservation Service. (2018). Soil pH and Plant Nutrient Availability. Retrieved from https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/health/biology/?cid=nrcs142p2_053865
