Nursery Notes 32(3):21; May-June 1998.
Soil pH: crucial for nursery crop production
by M. Ray Tucker, Agronomist
Adjusting the pH should be the first step in preparation for growing nursery crops. A proper pH is essential for providing a good nutritional environment for container and field grown nursery plants. Nutrient availability, the effects of toxic components such as hydrogen (H) and aluminum (Al) and microbial activity are dramatically influenced by pH. If the pH is too low or too high, it can have adverse effects on plant production. For example, when soil pH is high micronutrients such as iron and manganese are less available. When soil pH is low, acid components restrict root growth and nutrient absorption. Furthermore, a proper pH is required for microorganisms to convert nutrient compounds to forms that can be utilized by plants.
pH is a measurement that indicates the degree of acidity or alkalinity of the growth media. The scale for measuring pH ranges for 0 to 14. A pH of 7.0 is neutral, below 7.0 is acid, above 7.0 is basic. Native pine bark would be classified as extremely acid with a pH ranging from 3.0 to 4.5.
As pH declines, the amount of acid in the soil increases and can reach a level where it becomes toxic to plants. As pH increases, soils become more basic and can "tie up" certain micronutrients causing deficiency symptoms to appear. This is particularly true for iron. Most nursery crops grow better under moderately acidic conditions (pH 5.5 to 6.0). Some nursery crops prefer a pH at the lower end of this pH range (azaleas and rhododendrons).
If the soil pH is too low, it can be easily corrected. Lime applications neutralize soil acidity and raise the pH. In addition to neutralizing soil acidity, dolomitic lime provides enough calcium and magnesium to sustain growth for the entire growing period.
Although pH does not measure the nutrient content directly, it can be used to indicate levels of calcium and magnesium. For example, soils that have a low pH (less than 5.5) will also have low levels of calcium and magnesium. Soils to which dolomitic lime has been applied will have a higher pH and higher levels of calcium and magnesium. Calcitic lime is equally effective in raising pH and supplying calcium but contains little or no magnesium. Calcium and magnesium applied from commercial fertilizers have no liming capability and cannot be used as a substitute for lime to raise the soil pH.
Nursery crops are commonly grown in mixtures of pine bark and sand. Because this media contains little or no aluminum, crops can be safely grown at a pH of 5.5. Aluminum, which is a natural component of mineral soils, can be toxic to some plants if the pH is 5.5 or less. Acid-loving plants such as azaleas and rhododendrons are more tolerant to aluminum than other nursery stock and grow well in acid soils.
Due to problems asssociated with aluminum toxicity, field-grown nursery stock requires a higher pH than container-grown stock. Field stock performs better when the pH is in the range of 5.8 to 6.0.