Plants That Need Acid Soil Conditions Exclusively
Beautiful White Flower Racemes of a Pieris japonica
Plants that Require Acid Soil Conditions Exclusively
Acid Soils Only
Many plants are listed that thrive on acid soils, but on closer inspection can also cope with neutral or even alkaline conditions. This is not particularly helpful if you're looking for plants that require an acid soil exclusively.
Here are some plants that fit that category, which should be useful either for students of Horticulture, or people that are simply interested in knowing which plants require acid soils only.
Searching for a few plants that like acid soils exclusively can be time consuming. While there are searchable databases that can be searched by specific characteristics, such as this searchable database from The National Gardening Association, the problem arises that it does not specify whether these plants exclusively require an acid soil.
A plant liking an extremely high acidity might suggest that such a plant cannot tolerate a neutral soil, for example, but it might not be the case always. For example, a search for plants on the RHS website (The Royal Horticultural Society) of Chamaecyparis pisifera 'Boulevard', shows that it likes acid soil conditions, which we would expect as it is a conifer, but also specifies that it likes neutral conditions as well. See photo for screenshot.
Searching is further frustrated by the fact, that while the RHS website provides clearly detailed information, there are many plants which it simply has no detailed information about.
The list here is for plants that are really exclusively for acid soils. Perhaps an academic exercise but interesting nevetherless!
If you have anymore that can be added, please let me know.
RHS Website Showing Plant Requirements
Which Plants Like Acid Soils Exclusively?
Calluna vulgaris 'Glenfiddich'
Chile lantern tree
Japanese andromeda, Japanese pieris, lily-of-the-valley bush
Pieris japonica 'Prelude'
Common rhododendron, pontic rhododendron
Rhododendron ponticum 'Filigran'
Vaccinium corymbosum 'Duke'
These five example are plants that specifically prefer or need an acid soil, rather being able to cope with an acid soil as well as a neutral one, such as Camellia (from the Theaceae family).
If there are any more plants that require acid soils exclusively, please let me know.
Negative anions hold onto positive cations in soils that have a low pH
Soil Acidity and Nutrition
Acid soils are sometimes called 'sour soils' and are called acid because the particles in the soil have a mainly negative electrical charge, due to the concentration of hydrogen ions. The British Isles, for example, are known to have 'sour' soils overall, due its geology and weather. This negative charge allows the soil solution to hold onto positive elements such as Potassium (K+), Magnesium (Mg+) and Calcium (Ca+) which are three major nutrients necessary for plant health.
At Altitude, the Himalayan Origins of the Rhododendron Become Apparent
Soil Acidity and Soil Structure
The ability of a soil to attract positive particles is called floculation (the joining together of particles) and is most readily achieved with humus (where there are clay particles), where the rich decayed material coats and binds mineral particles. A soil is said to have cation exchange capacity if negative particles attract positive particles, where cation and anion exchanges take place.
The pH of a soil (its measure of acidity) is therefore intrinsically linked to the structure of the soil, that is, the arrangement of particles. Soils such as clay, and humus, are therefore said to be acidic and have good cation exchange capacity, as they resist any drastic changes to their pH levels.
Clay soil for example consists of many small particles that overall have a larger surface area, as compared to sandy soils, as grains of sand are large compared to clay particles. This extra surface area due to the greater number of smaller particles allows more particles to bind, while the smaller spaces between the particles allows less water runoff. This is also referred to as the water holding capacity (WHC) of the soil - clay soils have a high WHC, whereas sandy soils have a low WHC. Note the direct link between water holding capacity and the cation exchange capacity - clay soils have a very good cation exchange capacity, resisting change to pH, and have a high WHC whereas sandy soils have a low cation exchange capacity, and do not resist changes to pH in the soil.
This is why clay soils are heavy and usually damp and difficult to change. This is also why clay soil is called a 'late' soil, as it takes much longer to warm up in spring, compared to sandy soil. This is worth thinking about for growers who wish to maximize their growing season.
How to Make a soil More Acidic
If your soil is very alkaline, incorporating bulky organic matter helps lower the pH. This can be in the way of pine needles and well decomposed compost or adding sulphur chips. Blueberries for example need a pH of 4.5-5.5, which is a very low pH, and are very particular about the soil's acidity. They will not grow well if planted in an alkaline soil. If the pH of the soil is higher than 8, it's best to plant blueberries in a container as there is little that can be done to lower such high alkalinity.
Rhododendrons will require the soil to be managed if its alkaline, staying away from enriching the soil with multi-purpose composts that often have lime in them (which has an alkalizing effect on the soil). There are plenty of composts designed specifically for acid loving plants and Ericaceous borders, called Ericaceous composts.