Acid soil
A soil that has a high activity of hydrogen ions, which gives the soil a pH value below 7 and produces a yellow, orange or red reaction when in contact with universal indicator solution (see pH). Acid soils are suitable for growing ericaceous or calcifuge (lime-hating) plants.
Adelgids
Aphid-like insects of the family Adelgidae that are found exclusively on conifers. They feed on the sap of the host tree and cause cone-like growths, which can stunt or kill twigs and, in severe infestations, disfigure ornamental trees.
Aerate
To loosen soil in order to relieve compaction, allowing improved drainage and movement of air.
Aerial roots
Aerial roots grow on the stems of plants, such as ivy. These may be used for support or the uptake of air and moisture.
Air layering
This technique encourages the production of roots from a stem in order to propagate a plant. Usually involves making a shallow cut in the stem and surrounding it with a sleeve of polythene, which is taped above and below the cut and packed with sphagnum moss or peat.
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