The Level of Acidity or Alkalinity of a Soil

Nutrient Availablity

• The pH level in soil affects availability of soil nutrients. Plant roots absorb mineral nutrients that are dissolved in water. Soil pH affects dissolution of minerals. Most essential plant nutrients dissolve easily when soil pH ranges from 6 to 7.5. But when soil is strongly acidic with a pH below 6, nitrogen, calcium, magnesium, boron, molybdenum, phosphorous and potassium don’t readily dissolve and are less available to plants. When soil is strongly alkaline with pH above 7.5, manganese, iron, copper, zinc, boron and phosphorous become less available.

Soil Bacteria Affected

• The pH levels also affect the activity of beneficial soil bacteria that digest organic matter and release the nutrients. Very acidic or alkaline soil inhibits beneficial microbial activity. Strongly acidic or alkaline soil also encourages growth of pathogenic bacteria and fungi that cause a number of plant diseases. Extreme acidity or alkalinity adversely affects performance of insecticides and herbicides.

Regional Variations

• Environmental factors such as temperature, vegetation and rainfall affect soil pH. In general, the Eastern and Pacific Northwest regions of the United States have moderately acidic soil. Arid areas of the West have alkaline soil. Soil in the Midwest generally is near neutral. But in home construction, native soil often is removed and may be replaced by soil from another place, so your soil pH may be markedly different from the neighborhood norm.

Testing pH

• You can test your soil pH using an inexpensive pH testing kit available at home and garden centers. Most kits direct you to dissolve a soil sample in water, let the solids settle, and pour off the water into a test tube. Then you add an indicator chemical and wait for the water to change color. You tell the pH level by comparing the water color to a pH color chart. Each point on the logarithmic pH scale is 10 times greater than the point before it. For instance, a pH of 5 is 10 times more acidic than a pH of 6; a pH of 9 is 10 times more alkaline than a pH of 8.

Correcting pH

• Soil that is too acidic or alkaline for the plants you want to grow can be corrected by use of soil amendments. The cheapest and safest way to correct excess soil acidity is to add finely ground limestone to your soil, at a rate of around 6 pounds per 100 square feet to raise soil pH by one point. Adding elemental sulfur is the safest and least expensive way to correct excess soil alkalinity. Apply it at a rate of 2 pounds per 100 square feet to lower pH by one point. Both of these amendments work slowly, over a period of several months.