Rhododendrons are very tolerant of acid soils and grow best in soil with a pH between 4.5 and 5.5.
Photo credit: Uwe Fischer Flickr CC BY-NC-SA 2.0
|Influence of soil pH on the relative availability of plant nutrients in a typical mineral soil.
Image source: Chart of the Effects of Soil pH on Nutrient Availability
|Surface application of lime to a field. Plants should be irrigated after lime application to ensure the foliage is free of the amendment.
Photo credit: Dwight Sipler Flickr CC BY 2.0
Soil pH is a measure of the acidity or alkalinity (basicity) of a soil, and is reported as a value between 0 and 14. A soil test for pH measures the concentration of hydrogen ions in the soil solution.
A pH of 7.0 is considered neutral. A pH value below 7.0 indicates that the soil is acidic, with lower values representing increasing acidity. A pH value above 7.0 indicates that the soil is alkaline (basic), with higher values representing increasing alkalinity.
The pH scale is logarithmic, so a change in 1 pH unit reflects a 10 fold change in acidity or alkalinity. On average, garden and landscape plants grow best in soils with pH values between 6.0 and 7.2.
Soil acidity can be caused by a number of factors:
Soil acidity can be ameliorated and the pH of the soil increased by the addition of lime/limestone (calcium carbonate) and similar compounds that have been ground fine for use. Types of lime-like amendments include:
Each lime-like amendment has its benefits and drawbacks, such as effectiveness, price, and purity. Lime is most effective at neutralizing acidity when it is incorporated/tilled into the soil to the full depth of the plow layer or root zone.
Lime treats acidity by combining with carbon dioxide gas, water, and hydrogen ions to form free calcium ions and carbonic acid (weak acid). The carbonic acid then dissociates to form carbon dioxide gas and water, ridding the soil of hydrogen ions. Liming is also effective at accomplishing the following:
Other lime-like amendments neutralize acidity, but may follow different reaction paths. For instance, calcium oxide combines with water and hydrogen ions to immediately form free calcium ions and water.
The amount of lime needed to treat the soil acidity depends on the following:
The SMP, Adams-Evans, and Mehlich buffer methods are used to determine the lime requirement of soils. These different methods were developed for distinctly different soil types. Other methods, such as titratable acidity and reactive aluminum, may also be used to determine soil lime requirements. A soil sample can be submitted to an analytical laboratory to determine the lime requirement, which will be given in the results report.
Lime should be applied a few months before planting, approximately one time per year. Lime applied to turf should be irrigated after application to wash any lime off the leaves. Lime should not be applied if a soil test indicates that liming is not necessary; similarly, caution must be taken to avoid over-liming. A soil with too high of a pH poses a whole new set of problems.