Acid-Loving Plants and pH of the Soil

Harold Sweetman
Jenkins Arboretum. Devon, Pennsylvania

Why is soil Ph important?
pH is important to us and it is important to our plants. For our plants, the pH of the soil permits or blocks the uptake of nutrients.

The vast majority of plants prefer a Ph range between 6 to 7. Most plants are somewhat tolerant of pH. pH determines acidity or alkalinity of soil. While pH is an old name, we're stuck with it because it has been in use for a long time. To visualize the effect of pH, you may be familiar with the color of hydrangeas changing with the ph of the soil from blue with an acidic soil to pink with an alkaline soil.

pH really results from interaction with water. H2O or water breaks into ions with either a positive or negative ion. For example an h-plus breaks into a hydroxide ion. There is a logarithmic scale to measure pH which runs from 1 to 14. A 7 is neutral. Because it is logarithmic it is not a straight 1 to 14, each number changes by a factor of 10, so that as the level changes from 1 to 7 there is a separation by a factor of 1000 to 10,000.

Regions with acidic soils tend to have more rain; those with alkaline soils tend to be dry. However, even in areas of abundant rain, if the soil is on top of limestone, then you would have alkaline soil.

Comparisons
Stomach acid is about like battery acid. It is 1 on the ph scale.
Sodas and coke are 2.8 to 3.5--you can remove rust stains with coke. It is Very acidic, about 10,000 times more acidic than 7.
Peat moss tends to be 3.5 acidic. It is used for potting mixes and the like.
Natural products like honey, but there is an acid base reaction with a level 3.6 Its sweetness masks the acidity.
Vinegar and lemon juice are about 4.0.
Tonic water is 4.0; normal rain is 5.6;
Acid rain is more complicated; it is a whole other matter.
Pure water is 7.0, but it is hard to find naturally.
Blood is 7.35 to 7.45 and should be, slightly on the alkaline side; but if it deviates to 7.6 you are near death. This can make you sick or if it goes to the acidic range; if you go to 7.0 you are dead.
(Saliva is about 7.5)
Milk is 6.5---put a tooth in milk and you may save the tooth.
Baking soda is 8.0
Lava soap and ammonia is 9 (10k more than 7).
Liquid plumber, is 12 alkaline on ph scale. It is very caustic.
LYE is 13.5.

Use hydrangeas as an example of plants tolerance of pH--there are different colors depending on the soil pH-- pink, blue and white. They don't care about pH. Grow everywhere in the landscape--pH tolerant; the color shift has to due with aluminum and it's solubility that affects the color. Blue is acidic; so to turn a hydrangea blue add sulfur or aluminum sulfate.

Silica and the third is aluminum, but locked up.
We separate plants into: pH tolerant plants; and pH demanding (need acid).

Plants that like acidic soil, that is soils that are a bit below the 6 on the scale include:
Shade loving or aricace (heath family) Mountain laurel, blueberries, sourwood, hollies, camellias, magnolias, pin oaks, conifers, spruces, birches and the like.

Perennials and wildflowers need acidic soils. Primroses, primulas ferns, lilies,
Iron in soil becomes available at 5.5 acidity. Azaleas and rhododendrons need iron. So if they show signs of iron deficiency, check the pH. The outward symptoms are yellow leaves with green veins called chloritic state. Adding iron won't help as iron is bound unless it is in the presence of acid.

Solutions to create more acidic soil:
Add organic matter as it creates acidic residue.

Elemental sulfur; sulfur chips or dust--ground sulfur will breakdown faster.
Ferrous sulfate, a salt is another. It becomes instantly available to the plant. You can put it on the leaf or into the ground and will perk up the plant within 2 wks and entirely green in a month. Use about a tablespoon per gallon, no more.

Peat moss (3.5 ph) add this to the soil about 1/2 to a 1/3 mix it in the ground. If it is dry it stays dry. So wet it first.

Aluminum sulfate is a salt, and you can overdo the application.
Vinegar can be added to water to acidify soil.
Do not use spent mushroom soil it has a pH of 7 or 8 ph, which won't work for acid loving plants.

To make a soil more alkaline you can add lime to soil or compost (which is usually neutral).

Plant Chart of pH levels
and plants that do well at various levels of acidity

Below a pH of 6.0 (acid: some nutrients such as nitrogen, phosphorus and potassium are less available.
Above a pH of 7.5 (very alkaline): Iron, manganese, and phosphorus are less available.

Very acid
(pH 5.0 to 5.8)

azalea
blueberry
celeriac
chicory
crabapple
cranberry
eggplant
endive
heathers
huckleberry
hydrangea
Irish potato
lily
lupine
oak
raspberry
rhododendron
rhubarb
shallot
sorrel
spinach
beet
spruce
wild strawberry
sweet potato
watermelon
white birch

Moderately acid
(pH of 5.5 to 6.8

bean
begonia
Brussels sprouts
calla
camellia
carrot
collard greens
corn
fuchsia
garlic
lima bean
parsley
peppers
pumpkin
radish
rutabaga
soybean
squash
sunflower
tomato
turnip
viola

Slightly acid (pH 6.0 to 6.8)

asparagus
beet
bok choy
broccoli
gooseberry
grape
kale
kohlrabi
lettuce
mustard
muskmelon
oats
okra
onion
pansy
peach
, peanut
pear
peony
rice
spinach
Swiss chard

Very alkaline
(pH 7.0 to 8..0)

acacia
bottlebrush
cabbage
cauliflower
celery
Chinese cabbage
cucumber
date palms
dusty miller
eucalyptus
geranium
oleander
olive
periwinkle
pinks
pomegranate
salt cedar
tamarisk
thyme

Plant Preferences for pH

Houseplants and their pH preferences pH 4.5 to 5.5

Achmines
Adiantum
African violet
Aloe
Amarylis
Aphelandra
Aurucaria
Norfolk Pine
Azalea
Begonia
Caladium
Calathea
Crossandra
Cyclamen
Dieffenbachia
Epiphyllum
Gardenia
Hydrangea
Impatiens
Maranta
Pellaea - a fern
Peperomia
Pilea
Polypodium
Primula
Rechsteineria
Saxifraga
Scindapsus
Streptocarpus
Syngoniurn
Zygocactus

Acid-loving plants (acidophilic or ericaceous) e.g. blueberries, rhododendrons, azaleas
low pH ( pH 4-5.5 ) trace elements such ~ AI, Fe, Mo, Cu become more available
Al is toxic to most plant roots but not to ericaceous plants(within reasonable amounts).