A soil's acidity is measured by its pH, or potential hydrogen, level. Some plant species thrive in highly acidic soils, but most prefer a neutral or alkaline pH. When a plant is grown in compatible soil, it is healthy, green and, if species appropriate, readily produces flowers and fruit.
Basics of pH
The pH measurement is based on the available hydrogen ions in a solution and is defined on a numerical scale. It defines the level of acidity or alkalinity in a substance. Acidic solutions have a higher number of hydrogen ions than basic solutions. Distilled water hovers around a pH of 7 and is considered a neutral solution. Household vinegar, which is mildly acidic, has a pH of about 3. Lemon juice is even more acidic, coming in at around 2. Seawater comes in at 8, making it slightly alkaline.
The pH scale runs from 0.0 to 14.0. Acidic soil is considered sour, while alkaline soil is called sweet. Soils that are acidic have lower pH numbers. Seven is considered neutral, a balance of sour and sweet. The pH scale uses the same measurement increments as the Richter scale does for earthquakes. A soil sample measuring 5.0 is 10 times more acidic than one measuring 6.0.
Soil pH Affects Plant Nutrient Availability
Highly acidic soils make it difficult for many plants to attain needed nutrients. Calcium, phosphorous and magnesium are bonded to other molecules and are unavailable to the plants. Aluminum and manganese are released into the soil and too much of these are toxic to plants. The nutrient absorption issue is also true for highly alkaline soils. Alkaline soils in the western United States, for example, often prevent the absorption of iron by plants, even though there may be abundant iron in the soil.
Acidity Affects Organic Material Decomposition
Strongly acidic soils inhibit the decomposition of organic materials. The microscopic bacteria that break down organic matter aren't tolerant of the acidity and basically shut down or die. This is how peat is formed. Without the organic matter breakdown, certain nutrients, such as nitrogen, aren't processed into a form that can be used by plants.
Symptoms of Plant Stress in Highly Acidic Soils
- High levels of aluminum can damage the root system, making the roots appear swollen and shorter.
- An overabundance of manganese can cause stunted leaf growth. The leaves often look crinkled and have white or gray spots, indicating a lack of chlorophyll in some areas. Since chlorophyll is the green substance that helps a plant make its food through photosynthesis, the blotchy leaves can lead to plant starvation and death.
- In parts of the country, such as Hawaii, acidic soils are deficient in available calcium. This causes stunted growth throughout the plant. In some cases, the younger leaves don't open up all the way. Instead, the tips or margins die off.
- The lack of phosphorous in acidic soils causes older leaves on plants to turn from green to a purplish hue. Photosynthesis ceases in the turned leaves.
Iron Chlorosis and Soluble Salinity
Iron chlorosis is often found in acid loving plants growing in highly alkaline soils. The first stage causes the leaves to turn yellow, while the veins retain their green. As the deficiency persists, the leaves turn a paler yellow or go nearly white, with the veins only showing a bit of green. Brown spots sometimes appear between the veins, while the leaf edges turn brown. On certain evergreens, the needles turn from green to yellow.
Soluble salts increase pH, sometimes beyond what plants can handle. Salts inhibit a plant's ability to absorb water, which reduces its growth rate. Excessive amounts of salt can injure the plants at the cellular level, causing withering.
- University of Vermont Extension, Department of Plant and Soil Science: pH for the Garden, Leonard Perry, Extension Professor
- University of Arizona, College of Agriculture: Soil pH and Salinity
- ESF: Environmental Information Series: Soil pH: What it Means
- University of Hawaii at Manoa: Acid Soils in Hawaii: Problems and Management
- Colorado State University Extension: CMG Garden Notes: Iron Chlorosis
- pH, Acids and Bases
- Plant Stress.com: Impact of Salinity Stress:Dr. Rana Munns, CSIRO Division of Plant Industry