The ideal growing environment for plants is soil in the 6.0 to 7.0 pH range of acidity and alkalinity. A pH level over 7.0 is considered alkaline soil. Plants such as geranium, bottlebrush, cabbage, cauliflower and thyme thrive in alkaline soil. Most plants like slightly acidic soil below 7.0. Acid and alkaline conditions are easily corrected through soil amendments and compost.
Acidity and alkalinity are measured in pH units expressed in a scale from 0 to 14. Acidity is associated with an increase in hydrogen ions and alkalinity is connected to an increase of hydroxyl ions. The differences in this chemical balance affect how molecules react and interact in soil. A change of one number on the scale increases or decreases it by a factor of 10. A pH of 8.0 is 10 times more alkaline than pH 7.0.
Soil pH is created by the rock material at its base and the natural processes of weather, vegetation and time. Alkaline soils have less organic matter and therefore fewer nutrients available to plants. Areas of drought and desert conditions tend toward alkalinity. Traditional farmers smelled and tasted soil to determine its pH, which was called “sweet” and “sour” soil. Alkaline was said to have a sweet taste or smell.
Solubility of nutrients is affected by acid or alkaline environment of soil. Plants cannot absorb certain nutrients under overly alkaline conditions. “The availability of most trace elements is reduced. Phosphorus availability becomes limited above pH 7.5,” according to soil biologist John Howell of University of Massachusetts Amherst. Plants need phosphorus to manufacture DNA, build cell membrane walls and to produce energy.
Phosphorus must dissolve before it is a usable nutrient for plants. It's an essential nutrient ingredient in vegetables and in human nutrition and helps create bone strength. A pH level between 6.0 and 7.0 allows phosphorus to dissolve into a form plants can use. Adding phosphorus soil amendment without the correct pH level will not increase its availability to plants.
Correct the pH
Soil test kits are available at garden centers. Soil tested below 6.0 pH can be corrected by adding dolomite lime or wood ashes. Apply wood ash sparingly at a rate of 2 pounds per 100 square feet every other year. Apply lime at a rate of 5 pounds per 100 square feet. Soil that is too alkaline is corrected by adding acidic materials such as pine needles or shredded leaves. Compost brings most soils to the ideal pH of 6.0 to 7.0.