March/April 2000

SmartGarden™

SmartGarden™- 
Soil pH Understanding soil chemistry and how it affects plants

Most gardeners understand that plants generally grow best in a soil with a pH between 6.0 and 7.0. But why is the pH of the soil so critical to plant health? A brief foray into the world of soil chemistry will explain its significant influence on plant growth and SmartGarden™ practices. 

pH is a measurement of the degree of acidity or alkalinity of a solution as determined by the concentration of hydrogen ions. The pH scale is a continuum that rates solutions from most acidic-0-to most alkaline-14-with 7 being neutral. 

What Determines Soil pH? 
The inherent pH of a soil is determined by many factors, such as the type of rock from which the soil originated, the amount of precipitation, and the type of vegetation growing in it. Most plants can survive in soils with a fairly wide pH range-from about 4 to 9-but they may not thrive at the extremes of that range because mineral elements important to plant growth become chemically bound in the soil (see chart below). 

Deficiencies of these essential nutrients damage plants and make them more susceptible to pests and diseases. In alkaline soils, elements such as copper, iron, and manganese become less available. In acidic soils, phosphorus, potassium, and calcium are locked up. Additionally, acidic soils inhibit the survival of certain beneficial organisms, including earthworms, mycorrhizal fungi, and many bacteria. 

These organisms are responsible for the decay of organic matter and help plants obtain nutrients. While most plants grow best in a nearly neutral soil, there are exceptions. 

Members of the heath family (Ericaceae) such as blueberries, azaleas, rhododendrons, and camellias grow better in a soil with a somewhat lower pH-usually between 4.5 and 6.0. When grown in neutral or slightly alkaline soils, these plants often display symptoms of iron deficiency such as yellowing of leaves between the veins. 

Another indirect effect of soil pH is its influence on the control of certain disease-causing soil microorganisms. For example, the pathogen that causes common potato scab, Steptomyces scabies, is not active below a pH of 5.4. 

By maintaining a pH below this level for growing potatoes, the disease is effectively controlled with a cultural practice rather than a pesticide. how to adjust SOIL pH. 

Before planting a garden, test your soil to determine its pH. Several types of soil test kits are available for home gardeners, but a professional soil test laboratory will provide the most reliable results. These labs, often associated with state universities, can provide an accurate assessment of your soil type and its pH as well as recommendations for adjusting the pH and nutrient levels for the plants you want to grow. 

Selecting plants suited to your soil's existing pH minimizes or eliminates the need for adjustments. However, if your soil pH restricts you from growing the plants you desire, you will need to amend the soil and monitor it to keep it within the desired range. Limestone-usually ground or dolomitic-is applied to raise the pH, or decrease the acidity. 

Sulfur or aluminum sulfate are the most commonly recommended supplements for lowering the pH-or increasing the acidity. The amounts needed to produce the desired pH level will vary depending on the texture of your soil-the relative quantities of sand, silt, and clay-and the amount of change needed. 

Modifying the pH of your soil takes time; it may require repeated applications of limestone or sulfur over several seasons to achieve the correct level. 

You will benefit most from soil amendments that are thoroughly worked into the soil. Even in an established garden, it's advisable to monitor your soil pH regularly; it may fluctuate with the decomposition of organic matter, the assimilation of nutrients by plants, and the leaching of elements beyond the root zone. 

Maintaining an appropriate soil pH is an important part of creating a healthy, well-balanced environment for your plants-and it is a key element of the SmartGarden™ program. 

Rita Pelczar, Associate Editor