Preferred Soil pH

Preferred Soil pH for Many Garden Plants

by Rod Smith

© 2016-19 Rodney A. Smith
All rights reserved.

pH is a measure of how acidic or basic (alkaline) water is. Distilled water has a pH of 7, which is neutral. Below 7 is acidic. Above 7 is basic or alkaline.

Soil pH usually stays with the range of 4 to 10. High rainfall and high organic matter produces acid soil. Low rainfall and high lime or sodium produces alkaline soils. Soils in the western Oregon valleys naturally have a pH between 4.5 and 5.5. Soils in eastern Oregon commonly have a pH above 8.

Soil pH affects how well plants grow because it affects how available nutrients are. When the soil is very acid or very alkaline, most nutrients are tightly bound to soil particles and not available to be absorbed by plant's roots. Most nutrients are the most available at a pH of 6-7, slightly acid to neutral. See the Nutrient Availability Chart below.

However, iron is more available as soil is more acid. Acid loving plants, such as camellias, azaleas and rhododendrons, need to grow in an acidic soil in order to get enough iron.

Micro nutrients are also more available when soil is more acid. Many plants get the micro nutrients they need when soil pH is 5.8 to 6.2. Some micro nutrient efficient plants, such as geraniums, marigolds and rosemary, get the micro nutrients they need when the soil pH is 6.0 to 6.4. Micro nutrient inefficient plants, such as basil, grow best at a soil pH of 5.6 to 6.0. The preferred soil pH for many garden plants is listed at the end.

Nutrient Availability Chart

Soil pH can be checked with a pH meter, or soil laboratories will test soils for nutrients as well as pH and organic matter. A local soil testing lab, A & L Laboratories, is in Tigard on Nimbus Ave. They can also provide a chart telling how much lime to add to soil depending on what is grown on it.

Soil can be made less acid by adding lime (calcium carbonate) or dolomite lime, which also has magnesium. Lime now comes in a pelletized form which is ground very fine, then pressed into tiny pellets so it is easier to spread and less dusty than ground limestone. The pellets disintegrate when they get wet and the lime is released.

Soil can be made more acid by adding sulfur or aluminum sulfate. Aluminum sulfate is especially helpful to produce good blue color on hydrangea flowers. Adding lime produces pink hydrangea flowers.

The best way to apply lime or sulfur is to mix it in with the soil so soil pH is uniform throughout the root zone. Fall is the best time to apply lime to lawns since winter rains help wash lime deeper into the soil. Another good time to apply lime to lawns is immediately after aerating. Earthworms also help move lime deeper into the soil. Lime will raise the soil pH in a few weeks or several months depending on how finely ground the lime particles are. Sulfur takes about a year to lower soil pH since bacteria are required to complete the process.

Two lengthy articles from Oregon State University on soil ph and treating alkaline soils can be found at:
Applying Lime to Raise Soil pH for Crop Production (Western Oregon)
Acidifying Soil for Crop Production (Eastern Oregon)

provided by Kel Instruments Co., makers of professional quality pH meters,
with additional crops provided by Oregon State University

Bent	  	    5.5-6.5
Bermuda		    6-7
Blue, Annual 	    6-7
Blue, Kentucky 	    5.5-7.5
Buffalo		    6-7.5
Clover		    5.5-7
Fescue, Red	    5.5-6.5
Fescue, Tall	    6.5-7.5
Pastures            5.5-8.2
Rye, Annual	    5.5-8
Rye, Perennial	    6-7
Seed fields         5.5-8.2
St. Augustine	    6-7.5
Zoysia		    4.5-7.5

Fruits and Vegetables
Alfalfa		    6.5-8.4
Apple		    5.5-6.5
Artichoke	    6-6.5
Asparagus	    6-9
Barley		    7-7.5
Beans		    5.5-8.1
Beets		    6-7
Blackberry	    5.5-6.5
Blueberry	    4.5-5.5
Broccoli	    6-6.5
Brussels Sprouts    6-6.5
Cabbage		    6-6.5
Cantaloupe	    6-6.5
Carrot		    5.5-6.5
Cauliflower	    5.5-6.5
Celery		    6-6.5
Cherry		    6-6.5
Chive		    6-6.5
Corn		    5.5-8.4
Cranberry	    4.5-5.5
Cucumber	    6-6.5
Eggplant	    5.5-6.5
Fruit Trees         6-8
Garlic              6.5-8.4
Grapes		    5.5-6.5
Grapefruit	    6-6.5
Lemon		    7-7.5
Lettuce		    6-6.5
Lima Bean	    6-6.5
Mushroom	    6-6.5
Oats		    5.5-6.5
Onion		    6-8.4
Orange		    6-6.5
Parsley		    5.5-6.5
Peach		    6-6.5
Peas		    6-6.5
Peanut		    5.5-6.5
Pear		    6-6.5
Pepper		    5.5-6.5
Plum		    6-6.5
Potato		    5-8.3
Potato, Sweet	    6-6.5
Pumpkin		    5.5-6.5
Radish		    6-6.5
Rhubarb		    5.5-6.5
Rice		    5-6.5
Rye		    5.5-6.5
Soybean		    6-6.5
Spinach		    6-7
Squash		    5.5-6.5
Strawberry	    5-5.5
Sugarbeets          6-8.5
Turnip		    5.5-6.5
Tomato		    5.5-6.5
Watermelon	    5.5-6.5
Wheat		    6-7.5

Flowers, Trees & Plants
Abelia		    6-7.5
Acacia		    6-7
African Violet	    6-7
Ageratum	    6-7
Alder 		    6-7
Allamanda	    5.5-6
Alyssum		    6-7
Amaryllis	    5-6
Andromeda	    5-6
Anemone		    6-7.5
Arborvitae	    6-7
Aster		    5-6
Ash		    6-7
Aspen		    4.5-5.5
Azalea		    4.5-5.5
Bachelor Button	    6.5-7.5
Bayberry	    4.5-6
Beech		    6-7
Begonia		    5.6-6.5
Birch		    4.5-6
Bottlebrush	    6-7.5
Bougainvillea	    4.5-5.5
Boxwood		    6-7
Buttercup	    6.5-7.5
Butterfly Bush	    6-6.5
Caladium	    5.5-6.5
Camellia	    4-5
Canna		    6-7.5
Carnation	    6-7.5
Cedar		    5-7
Chestnut	    5-6.5
China Aster	    5-7
Chrysanthemum	    5.6-6.5
Cotoneaster	    6-7
Cottonwood	    6-7.5
Crapemyrtle	    5.5-7
Croton		    5-7
Cypress		    5-6
Daffodil	    6-8
Dahlia		    6-7
Daphne		    5.5-6.5
Delphinium	    6-7.5
Deutzia		    6-7
Dogwood		    6-7
Elm		    6-7
Eucalyptus	    6.5-7.5
Euonymus	    6-7
Fern		    5-6
Fir		    5-6
Firethorn	    5.5-7
Forsythia	    5.5-6.5
Fuchsia		    5.5-6.5
Gardenia	    5-6
Geranium	    6-7.5
Ginkgo		    6-7
Gladiolus	    5.5-6.5
Hawthorn	    6-7.5
Hemlock		    5-6
Hibiscus	    5-6.5
Holly		    5-6
Honeysuckle	    6-7
Horsechestnut	    6-7
Hyacinth	    6-7
Hydrangea, Blue	    4.5-5.5
Hydrangea, Pink	    6.5-7.5
Ilex		    5-6
Iris		    6-6.5
Ivy		    6-7
Juniper		    5-6
Lantana		    5.5-6.5
Larch		    6-7
Laurel		    5-6
Ligustrum	    5-6.5
Lilac		    6-7
Lily		    5-6
Lily of the Valley  5-6
Linden		    6-7
Locust		    6-7
Lycoris		    5.5-6.5
Magnolia	    5-6
Maple		    6.5-7.5
Marigold	    5.5-7
Mulberry	    6-7
Myrtle		    6-7
Narcissus	    6-6.5
Nasturtium	    5.5-7
Oak		    5-6
Oleander	    6-7.5
Orchid		    4.5-6
Palm		    5.5-7
Pansy		    5-6.5
Peony		    6-7.5
Periwinkle	    6-7.5
Petunia		    5.5-7
Phlox		    5-6
Pine		    5-6
Pittosporum	    5-6.5
Plane		    6-7
Podocarpus	    5-6.5
Poinsetta	    6-7
Poplar		    6-7
Privet		    6-7
Pyracantha	    6-7.5
Quince		    6-7
Rose		    5.5-7
Rhododendron	    4.5-5.5
Snapdragon	    6-7
Spirea		    6-7
Spruce		    5-6
Sweetgum	    6-7
Sycamore	    6-7.5
Tulip		    6-6.5
Tupelo		    6-7
Tuliptree	    6-7
Verbena		    6-7.5
Violet		    6-7.5
Viburnum	    6-7
Walnut		    6-7
Weigela		    6-7
Willow		    5-7
Wisteria	    6-7.5
Yew		    6-7
Yucca		    6-7
Yaupon		    5-6.5
Witchhazel	    6-7
Zinnia		    5.5-7

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