Rhododendron

Azaleas at Callaway Gardens

Soil pH is an indicator of the relative abundance of hydrogen (H⁺) ions and hydroxyl (OHˉ) ions in garden soils (Table 1).  These ions play a very important part of soil fertility and a plant’s ability to absorb nutrients from the soil. The pH scale runs from 1 (highest acidity) to 14 (highest alkalinity). Soil pH of 7 is neutral.

Fourteen (14) of seventeen(17) essential plant nutrients are obtained from the soil. Most nutrients that plants need are readily available when the pH of the soil solutions ranges from 6.0 to 7.5, or slightly acid to neutral.  Phosphorus is a major plant element that becomes less available below pH 6.0 and above 7.0.

Plants drink (absorb) nutrients that are dissolved in the soil water.  Some minerals are more soluble in the company of H⁺ ions, and other minerals are more soluble in the company of OHˉ ions.  Acid soils hold more H⁺ ions, and iron, manganese and phosphorus are more soluble in these soils.  Alkaline soils have more OHˉ and  calcium, magnesium, potassium, and molybdenum are more soluble in these soils.

Table 1: pH range and Nutrient Availability

Acidic Soil (below 7)                      Neutral = 7                                       Alkaline Soil (above 7)

More H⁺                                           Distilled (Pure) Water                   More OHˉ

Key Nutrients Most Available below pH 6:              Key Nutrients Most Available above pH 7:

Iron                                                                                                 Calcium

Manganese                                                                                    Magnesium

Aluminum                                                                                      Potassium

Molybdenum

Most (not all) nutrient elements are available between pH 6.0 to 7.0. As soil pH changes up or down, some elements becomes more soluble and available to the plant. Extremes in soil pH range, below 4.5 or above 7.7, may contain toxic nutrient levels and injure plant roots.

Certain plants have specific nutritional needs.  Acid-loving plants (in the Ericaceous family) such as rhododendrons, azaleas, and blueberries are some examples.  These plants don’t “love” acidic soil, but absorb higher amounts of iron compared to other plants.  Ericaceous plants can not find enough soluble iron in neutral or alkaline soils where it is scarcer. They are also very efficient in absorbing phosphorus at a lower acidic pH.